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Section 1
Chromosomes
Section 1
Focus
Objectives
Overview
Before beginning this section
review with your students the
objectives listed in the Student
Edition. In this section, students will
learn that cells must divide in order
to grow, replace worn-out cells, or
reproduce asexually. It is vitally
important that each new cell receives
the proper set of chromosomes to
function normally. Students will
study the structure of chromosomes,
the role of chromosomes, and learn
that each organism has a characteristic number of chromosomes.
Bellringer
Write the words haploid, diploid,
and zygote on the board. Ask students to write a single sentence that
reflects an understanding of each
term. (Answers will vary. For example:
During fertilization, haploid gametes
combine to form a diploid zygote.) Bio 6E
Motivate
Demonstration
GENERAL
Show the class two pictures or
photos. One picture should show a
person without any visible genetic
disorders. The second picture
should show a person with recognizable physical abnormalities due
to a genetic disorder caused by the
presence or absence of a chromosome (such as Klinefelter’s or
Turner’s syndrome). Discuss with
students how a single chromosome
contains thousands of genes that
code for proteins involved in
determining how a person’s body
develops and functions. LS Visual
TAKS 2 Bio 6C; 6F
pp. 118–119
Student Edition
TAKS Obj 2 Bio 4B
TAKS Obj 2 Bio 6A
TEKS Bio 4A, 4B, 6A, 6B, 6E, 6F
Teacher Edition
TAKS Obj 2 Bio 6A, 6C
TAKS Obj 3 Bio 4D
TEKS Bio 4D, 6A, 6C, 6E, 6F, 11D
118
● Identify four examples of
cell division in eukaryotes
and one example in
TAKS 2
prokaryotes.
4A 4B 6E
●
●
●
●
Formation of New Cells
by Cell Division
About 2 trillion cells are produced by an adult human body every
day. This is about 25 million new cells per second! These new cells
are formed when older cells divide. Cell division, also called cell
Differentiate between
reproduction, occurs in humans and other organisms at different
a gene, a DNA molecule,
times in their life. In Figure 1, the cells of the fawn that is growing
a chromosome, and a
and developing and the cells in the wound that is healing are underchromatid.
6A 6E TAKS 2
going cell division. The type of cell division differs depending on the
Differentiate between
organism and why the cell is dividing. For example, bacterial cells
homologous chromosomes,
undergoing reproduction divide by one type of cell division. Eukaryautosomes, and sex TAKS 2
otic organisms undergoing growth, development, repair, or asexual
chromosomes.
6A 6E
reproduction divide by a different type of cell division. And the forCompare haploid and
mation of gametes involves yet a third type of cell division. Gametes
6E
diploid cells.
are an organism’s reproductive cells, such as sperm or egg cells.
Predict how changes in
Regardless of the type of cell division that occurs, all of the inforchromosome number
mation stored in the molecule DNA (deoxyribonucleic acid) must
or structure can affect
be present in each of the resulting cells. Recall from Chapter 3 that
6C 6F TAKS 2
development.
DNA stores the information that tells cells which proteins to make
and when to make them. This information directs a cell’s activities
Key Terms
and determines its characteristics. Thus, when a cell divides, the
gamete
DNA is first copied and then distributed. Each cell ends up with a
binary fission
complete set (copy) of the DNA.
gene
chromosome
chromatid
centromere
homologous
chromosome
diploid
haploid
zygote
autosome
sex chromosome
karyotype
Figure 1 Cell division
The cells of these organisms are undergoing some type of cell division.
Repair
Growth and development
118
Planner CD-ROM
• Reading Organizers
• Reading Strategies
• Basic Skills Worksheet
Length, Area and Volume
• Occupational Applications
Medical Sonographer GENERAL
Chapter 6 • Chromosomes and Cell Reproduction
INCLUSION
Strategies
• Learning Disability
• English as a Second
Language
Ask students to label three sheets of notebook
paper with the titles of each section of the
chapter, Chromosomes, The Cell Cycle, and
Mitosis and Cytokinesis. Have students take
notes as they read each section. The notes
should be divided into three parts, key terms,
notes, and questions. In a small group or with
the teacher, students may share what they
learned in each section and ask for clarification on their questions.
Prokaryotic Cell Reproduction
A prokaryote’s single DNA molecule is circular and is attached to the
inner cell membrane. Prokaryotes reproduce by a type of cell division
called binary fission. Binary fission is a form of asexual reproduction
that produces identical offspring. In asexual reproduction, a single
parent passes exact copies of all of its DNA to its offspring.
Binary fission occurs in two stages: first, the DNA is copied (so
that each new cell will have a copy of the genetic information), and
then the cell divides. The prokaryote divides by adding a new cell
membrane to a point on the membrane between the two DNA
copies. As new material is added, the growing cell membrane
pushes inward and the cell is constricted in the middle, like a long
balloon being squeezed near the center. A new cell wall forms
around the new membrane. Eventually the dividing prokaryote is
pinched into two independent cells. Each cell contains one of the
circles of DNA and is a complete functioning prokaryote.
Real Life
Escherichia coli cells
can produce 1 million
new cells in less than
7 hours.
A variety of E. coli known
as O157:H7 is sometimes
found in raw or undercooked meat. When such
meat is eaten, this bacteria
can cause life-threatening
intestinal bleeding and
kidney failure. Thorough
cooking is necessary
to destroy the bacteria.
Finding Information
Research outbreaks of E. coli
O157:H7 in your
community or
state.
Eukaryotic Cell Reproduction
The vast amount of information encoded in DNA is organized into
units called genes. A gene is a segment of DNA that codes for a protein or RNA molecule. A single molecule of DNA has thousands of
genes lined up like train cars. Genes play an important role in determining how a person’s body develops and functions. When genes
are being used, the DNA is stretched out so that the information it
contains can be used to direct the synthesis of proteins.
As a eukaryotic cell prepares to divide, the DNA and the proteins
associated with the DNA coil into a structure called a chromosome,
as shown in Figure 2. Before the DNA coils up, however, the DNA is
copied. The two exact copies of DNA that make up each chromosome
are called chromatids (KROH muh tihdz). The two chromatids of a
chromosome are attached at a point called a centromere. The chromatids, which become separated during cell division and placed into
each new cell, ensure that each new cell will have the same genetic
information as the original cell.
Supercoil within
chromosome
Further coiling
within supercoil
DNA and
proteins
DNA double
helix
119
Chapter Resource File
• Lesson Plan GENERAL
• Directed Reading
• Active Reading GENERAL
• Data Sheet for Quick Lab
GENERAL
READING
SKILL
BUILDER
Interactive Reading Assign
Chapter 6 of Holt Biology Guided
Audio CD Program to help students achieve greater success in
reading the chapter.
Real Life
Answer TAKS 3
Bio 4D; Bio 11D
Answers will vary by community.
The strain was first identified in
1982. Between 1992 and 1998,
30 outbreaks each year were
reported in various communities,
including areas in the Pacific
Northwest; Alpine, Wyoming;
Milwaukee, Wisconsin; and
Indianapolis, Indiana.
Demonstration
Figure 2 Chromosome
structure. A chromosome
consists of DNA tightly
coiled around proteins. The
chromosomes are formed as
a cell prepares to divide.
Centromere
Chromosome
(made of 2
chromatids)
Teach
Transparencies
TT Bellringer
TT Chromosome Structure
TT Chromosome Number of
Various Organisms
TT Karyotype
Use a microprojector or microscope
with a video camera to show the
class a cross section of an onion
root tip. The root grows because
cells in the root tip undergo
repeated mitotic divisions. Point
out chromosomes that are visible.
Tell students that before each root
tip cell divides, the chromosomes
are replicated and then divided
equally into the two resulting cells.
Explain that chromosomes consist
of genes, which are segments of
DNA. LS Visual TAKS 2 Bio 6A; Bio 6E
READING
SKILL
BUILDER
GENERAL
Writing
Reading Organizer Have
students create a reading
organizer to compare reproduction
in bacterial cells and eukaryotic
cells. A description of reproduction
in eukaryotic cells is given in
Sections 2 and 3. LS Verbal
BIOLOGY
• Unit 4—Cell Reproduction:
Topics 1–4
This engaging tutorial introduces students to principles of chromosome
replication and cell division
Chapter 6 • Chromosomes and Cell Reproduction
119
How Chromosome Number and
Structure Affect Development
Teach, continued
continued
Teaching Tip
GENERAL
Missing Homologue Ask students to hypothesize what might
happen if a human sperm or egg
cell did not contain one member of
each homologous pair. (The resulting zygote would not have a full set
of chromosomes. The zygote might
fail to develop. If the zygote did
develop, the individual would not be
normal because its cells would lack
the important information contained
in the missing genes.)
www.scilinks.org
Topic: Chromosomes
Keyword: HX4042
Sets of Chromosomes
Each of the 23 pairs of chromosomes consists of two homologous
(hoh MAHL uh gus) chromosomes, or homologues (HOH muh logs).
Homologous chromosomes are chromosomes that are similar in
size, shape, and genetic content. Each homologue in a pair of homologous chromosomes comes from one of the two parents, as
shown in Figure 3. Thus, the 46 chromosomes in human somatic
cells are actually two sets of 23 chromosomes. One set comes from
the mother, and one set comes from the father. A human chromosome is shown in Figure 4.
TAKS 2 Bio 6C; Bio 6E
SKILL
BUILDER
Each human somatic cell (any cell other than a sperm or egg cell)
normally has two copies of 23 different chromosomes, for a total of
46 chromosomes. The 23 chromosomes differ in size, shape, and
set of genes. Each chromosome contains thousands of genes that
play important roles in determining how a person’s body develops
and functions. For this reason, a complete set of all chromosomes
is essential to survival.
GENERAL
Figure 3 Fertilization
Math Skills Prokaryotic chromosomes are hundreds of times longer
than the cell that contains them.
For example, if a chromosome in
an E. coli bacterium were fully
extended, it would measure about
1 mm in length. The cell itself is
only about 0.002 mm in length.
Have students calculate how much
longer E. coli’s chromosome is than
the cell itself. (1 mm/0.002 mm 500 times longer than the cell)
When haploid gametes fuse, they produce a diploid zygote.
LS Logical Bio 4A
Using the Figure
Fertilization
GENERAL
Use Figure 3 to point out how
fertilization restores the diploid
number of chromosomes. Ask students why sexually reproducing
organisms possess an even number
of chromosomes in the diploid
state. (If students do not realize that
2n must be an even number, then ask
them to calculate the haploid number
in each gamete as if the diploid number in humans were 47 and not 46. It
cannot be done without a “half”
chromosome.) LS Visual Bio 6E
pp. 120–121
Student Edition
TAKS Obj 2 Bio 4B
TAKS Obj 2 Bio 6A
TEKS Bio 4B, 6A
Teacher Edition
TAKS Obj 1 Bio/IPC 2C
TAKS Obj 2 Bio 6C
TEKS Bio 3D, 3F, 4A, 6C, 6E, 6F
TEKS Bio/IPC 3C
120
Egg cell
n = 23
Sperm cell
n = 23
Zygote
2n = 46
120
Cultural
Awareness
Mormons and Gene Mapping
The Human Genome Project is an effort to
map the over 100,000 genes in human cells
to each chromosome and to sequence each
gene. Scientists throughout the world are
now engaged in this effort. One group has
traced genetic markers through three generations of 60 Mormon families living in Utah.
Their work with the Mormon families has
resulted in the mapping of almost 500 genes.
Bio/IPC 3C
Chapter 6 • Chromosomes and Cell Reproduction
IPC Benchmark Mini Lesson
Biology/IPC Skills TAKS 1 Bio/IPC 2C Organize,
analyze, evaluate, make inferences, and predict
trends from data. Activity Have students imagine
several possible uses for the information gained by
the Human Genome Project. Have students write an
essay describing their ideas and how they feel about
research on the human genome.
All of the cells in the body, other than gametes, are somatic cells.
When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid (DIHP loyd). Unlike somatic cells,
human gametes contain only one set of chromosomes (23 total).
When a cell, such as a gamete, contains one set of chromosomes, it
is said to be haploid (HAP loyd). Biologists use the symbol n to represent one set of chromosomes. The haploid number in a human
gamete can be written as n 23. The diploid number in a somatic
cell can be written as 2n 46. The fusion of two haploid gametes—
a process called fertilization—forms a diploid zygote, as shown in
Figure 3. A zygote (ZY goht) is a fertilized egg cell, the first cell of
a new individual.
As seen in Table 1, each organism has a characteristic number of
chromosomes. The number of chromosomes in cells is constant
within a species. Fruit flies, for example, have only eight chromosomes in each cell. Although most species have different numbers
of chromosomes, some species by chance have the same number.
For example, potatoes, plums, and chimpanzees all have 48
chromosomes in each cell. Many plants have far more chromosomes. Some ferns have more than 500. A few kinds of organisms—
such as the Australian ant Myrmecia, the plant Haplopappus
(a desert relative of the sunflower), and the fungus Penicillium
(from which the antibiotic penicillin is obtained)—have only one
pair of chromosomes.
Magnification: 12,542ⴛ
Teaching Tip
Figure 4 Human
chromosome. As many as
500 chromosomes lined up
end to end would fit in a
0.2 cm space—about the
thickness of a nickel. The
chromosome above has
replicated and consists of
two identical chromatids.
Bio 6E
Activity
Table 1 Chromosome Number of Various Organisms
Organism
1–4
Saccharomyces (yeast)
18
Mosquito
6
Housefly
12
Garden pea
14
Corn
20
Adder’s tongue fern
1,262
Frog
26
Human
46
Orangutan
48
Dog
78
GENERAL
Karyotypes Obtain copies of
normal human karyotypes from the
library or on-line resources. Enlarge
them so that each chromosome is no
smaller than 2.5 cm (1 in.) in length.
Paste the copies of the karyotype on
a poster board. Cut out each chromosome and mix up the pieces.
Allow the students to sort the chromosomes into pairs using size,
length, position of centromere, and
banding patterns. Have them compare the karyotypes with each other
and with the karyotype shown in
Figure 5.
English Language
Number of chromosomes
Penicillium
Prefixes Have students use the
prefixes in the words haploid and
diploid to help them remember the
meanings of these words. Have
them relate the words to the algebraic terms n and 2n. Haploid (n)
represents one set of chromosomes,
as seen in gametes. Diploid (2n)
represents two sets of chromosomes, as in somatic cells. Tell
students that the prefix haplmeans “single” and that the prefix
dipl- means “double.” Add that the
prefix poly- means “many.” Then
have students predict the nature of
a polyploid cell. (It would have
multiple sets of
English Language
Learners
chromosomes.)
LS Logical
Learners
Bio 6F
121
REAL WORLD
CONNECTION
Invite a genetic counselor to speak to the
class. Before the speaker attends, have the
students research chromosomal abnormalities that result from too few or too many
chromosomes. Have the students prepare a
list of questions based on their findings. Ask
the speaker to bring sample karyotypes of
the abnormalities the students researched, if
possible. LS Interpersonal Bio 3D, 6F
HISTORY
CONNECTION
In the 1920s, a biologist reported the diploid
number in human cells to be 48. For many
years following this report, textbooks described
the normal diploid number of human chromosomes as 48. It was not until 1956 that J. H.
Tjio and A. Levan showed 46 to be the correct
number. These two scientists worked out a
procedure to culture white blood cells. Their
methods increased the number of cells undergoing mitosis and improved the spreading of
the chromosomes, which allowed the scientists
to accurately count the chromosomes.
Bio 3F, Bio/IPC 3C
Chapter 6 • Chromosomes and Cell Reproduction
121
Sex Chromosomes
Teach, continued
continued
SKILL
BUILDER
GENERAL
Math Skills Combined, the 22
somatic chromosomes and one
sex chromosome consist of
3,000,000,000 DNA nucleotide
pairs. Ask students how many total
nucleotides are in a human diploid
cell? (6,000,000,000) LS Logical
The word chromosome
is from the Greek chroma,
meaning “color,” and
soma, meaning “body.”
Chromosomes were so
named because they
absorbed a colored dye
that made them more
visible under a microscope.
TAKS 2
6C, 6D
On the Trail of a Bio
(grade 10 only)
Chromsomal Deletion
Teaching Strategies
Researchers have found that life
threatening defects in the aorta
can result when a patient has
two defective copies of the gene
Tbx1. Patients with one normal
and one defective copy of Tbx1
usually experience less serious
abnormalities.
Discussion
How did researchers narrow
the search from 25 to 15 genes?
(They found out that the chromosome in mice with the same
disorder only contained 15
genes.)
Researchers now know that the
gene Tbx1 controls other genes.
What types of genes might it
control? (those involved in development of the heart, blood, face
and immune system)
Of the 23 pairs of chromosomes in human somatic cells, 22 pairs
are called autosomes. Autosomes are chromosomes that are not
directly involved in determining the sex (gender) of an individual.
The sex chromosomes, one of the 23 pairs of chromosomes in
humans, contain genes that will determine the sex of the individual.
In humans and many other organisms, the two sex chromosomes
are referred to as the X and Y chromosomes. The genes that cause a
fertilized egg to develop into a male are located on the Y chromosome. Thus, any individual with a Y chromosome is male, and any
individual without a Y chromosome is female. For example, in
human males, the sex chromosomes are made up of one X chromosome and one Y chromosome (XY). The sex chromosomes in human
females consist of two X chromosomes (XX). Because a female can
donate only an X chromosome to her offspring, the sex of an offspring is determined by the male, who can donate either an X or a Y.
The structure and number of sex chromosomes vary in different
organisms. In some insects, such as grasshoppers, there is no Y
chromosome—the females are characterized as XX and the males
are characterized as XO (the O indicates the absence of a chromosome). In birds, moths, and butterflies, the male has two X chromosomes and the female has only one.
Change in Chromosome Number
Each of an individual’s 46 chromosomes has thousands of genes.
Because genes play an important role in determining how a person’s
body develops and functions, the presence of all 46 chromosomes
is essential for normal developFigure 5 A human karyotype
ment and function. A person must
Karyotypes are used to examine an individual’s chromosomes.
have the characteristic number of
To prepare a karyotype,
chromosomes in his or her cells.
photographs of the
Humans who are missing even one
chromosomes are cut out,
arranged in pairs from largest
of the 46 chromosomes do not surto smallest, and numbered.
vive. Humans with more than two
copies of a chromosome, a condition
called trisomy (TRY soh mee), will
not develop properly. Abnormalities
in chromosome number can be
detected by analyzing a karyotype
(KAR ee uh tiep), a photo of the chromosomes in a dividing cell that
shows the chromosomes arranged by
size. Figure 5 shows a typical karyotype. A portion of a karyotype from
an individual with an extra copy
of chromosome 21 is also shown in
Figure 5. This condition is called
People with Down syndrome have
Down syndrome, or trisomy 21.
three copies of chromosome 21 in
Short stature, a round face with
their karyotype.
122
IPC Benchmark Fact
pp. 122–123
Student Edition
TAKS Obj 2 Bio 4B
TAKS Obj 2 Bio 6A
TAKS Obj 2 Bio 6C, 6D
TEKS Bio 4A, 4B, 6A, 6C, 6D, 6E, 6F
Teacher Edition
TAKS Obj 2 Bio 6B
TAKS Obj 2 Bio 6C, 6D
TAKS Obj 5 IPC 5B
TEKS Bio 3D, 3F, 6C, 6D
122
Tell students the colored dye that makes chromosomes more visible is the result of the reflection of
light. Indeed it is the bouncing back of waves to our
eyes that makes any object visible. Although there is
a tendency to think of reflection as images we can
see in a mirror, any object that we can observe with
our eyes, whether or not that image is aided with a
scientific instrument, is caused by light that bounces
back or is reflected from an object. In this instance,
the dye increases the amount of reflected light thus
making the chromosomes more visible. TAKS 5
IPC 5B (grade 11 only)
Chapter 6 • Chromosomes and Cell Reproduction
Career
GENERAL
Cytogenetic Technologist Cytogenetic technologists aid physicians in diagnosis of genetic
disorders by preparing specimens, such as karyotypes. Have students use library references or
on-line resources to research the training
requirements and duties of a cytogenetic technologist. (Technologist may prepare cells, prepare
slides in the correct mitotic stage, stain slides, and
prepare accurate karyotypes from photographic
prints or computer images.) Bio 3D
upper eyelids that cover the inner corners of the eyes, and varying
degrees of mental retardation are characteristics of people with
Down syndrome.
In mothers younger than 30, Down syndrome occurs in about 1
in 1,500 births. In mothers 37 years old, the incidence doubles to 1
in 290 births. In mothers over 45, the risk is as high as 1 in 46
births. Older mothers are more likely to have a baby with Down
syndrome because all the eggs a female will ever produce are present in her ovaries when she is born, unlike males who produce new
sperm throughout adult life. As a female ages, her eggs can accumulate an increasing amount of damage. Because of this risk, a
pregnant woman over the age of 35 may be advised to undergo prenatal testing that includes fetal karyotyping.
What events can cause an individual to have an extra copy of a
chromosome? When sperm and egg cells form, each chromosome
and its homologue separate, an event called disjunction (dihs
JUHNK shuhn). If one or more chromosomes fail to separate properly—an event called nondisjunction—one new gamete ends up
receiving both chromosomes and the other gamete receives none.
Trisomy occurs when the gamete with both chromosomes fuses
with a normal gamete during fertilization, resulting in offspring
with three copies of that chromosome instead of two. In Down syndrome, nondisjunction involves chromosome 21.
Teaching Tip
TAKS 2 Bio 6C; Bio 6D (grade 10 only)
www.scilinks.org
Topic: Genetic Disorders
Research in Texas
Keyword: HXX4008
O
A Faulty Chromosome
Karyotypes of people with DiGeorge syndrome show that
they have one normal and one
faulty 22nd chromosome. The
faulty chromosome is missing a
small region that contains 25
genes. To understand how this
chromosomal deletion results in
Finding the Crucial Gene
Using a technology called chromosome engineering, the Baylor
TAKS 2
Bio 6C;
Bio 6D
(grade
10 only)
Teacher’s Notes
Cut note card pieces prior to
the lab.
TAKS 2
DiGeorge syndrome, researchers
at Baylor College of Medicine
in Houston have been studying
the disorder in mice.
First, the researchers found
that they could produce similar
heart defects in mice by deleting
a part of mouse chromosome 16.
When these mice were bred with
mice that had a duplication of the
same part of chromosome 16,
their offspring had no heart
defects. Because the deleted
part contained only 15 genes, the
search for the cause of DiGeorge
syndrome was narrowed from 25
genes to 15 genes.
Modeling
Chromosomal
Mutations
Skills Acquired
Modeling, sequencing,
interpreting
On the Trail of a Chromosomal Deletion
ne of every 4,000 babies is
born with a genetic disorder
called DiGeorge syndrome. This
disorder causes serious heart
defects that must be surgically
corrected within a few days after
birth. Children born with DiGeorge
syndrome can also have blood
ailments, facial abnormalities, a
deficient immune system, and
other problems.
GENERAL
Everyone Needs an X Tell students that most nondisjunctions
involving autosomes are lethal,
whereas most involving sex chromosomes are not. However, a YO
combination in humans is lethal,
leading biologists to suspect that at
least one X chromosome is necessary for development and survival.
Answers to Analysis
researchers eventually zeroed in
on a gene called Tbx1. They
showed that deleting Tbx1 on
one chromosome 16 in mice
causes the heart defects of
DiGeorge syndrome. Tbx1 is also
required for the development of
other embryonic structures
besides the heart. Thus, the
results of research on DiGeorge
syndrome may provide clues
about the genetic causes of
other birth defects.
Answers will vary based on the
type of mutation: deletion: cell
would be missing a gene, which
could prove fatal; duplication:
cell would have an extra gene,
which could prove fatal or result
in malfunctioning of the cell;
inversion: cell may not be able to
use gene because it is located in a
different area on the chromosome, which could prove fatal;
translocation: cell may not be
able to use the gene because it is
located on a different chromosome, which could prove fatal.
123
Cultural
Awareness
Cultural Attitudes Toward Genetic Defects
From the Middle Ages through the nineteenth
century, most Europeans believed that genetic
defects reflected inner corruption. However,
many other cultures, including the Celtic people of Europe and Native Americans, thought
individuals with such defects had a special
insight and a closer connection with nature.
These unique members of the community were
given responsibility as tribal leaders or healers.
Many physically disabled leaders made careful
astronomical observations of the sun and
moon that helped them advise their communities on the optimum times for planting, harvesting, or migrating to another area.
Bio 3F, TAKS 2 Bio 6C
Chapter 6 • Chromosomes and Cell Reproduction
123
Change in Chromosome Structure
Changes in an organism’s chromosome structure are called mutations.
Breakage of a chromosome can lead to four types of mutations. In a
deletion mutation, a piece of a chromosome breaks off completely. After
cell division, the new cell will lack a certain set of genes. In many cases
this proves fatal to the zygote. In a duplication mutation, a chromosome
fragment attaches to its homologous chromosome, which will then
carry two copies of a certain set of genes. A third type of mutation is an
inversion mutation, in which the chromosome piece reattaches to the
original chromosome but in a reverse orientation. If the piece reattaches
to a nonhomologous chromosome, a translocation mutation results.
Close
Reteaching
Ask students to differentiate each of
the following terms by defining and
sketching them: chromosomes, chromatids, DNA, and genes. Students
should label their sketches. (chromosomes: coiled DNA and associated
proteins; chromatids: identical copies
of a given chromosome; DNA: chain
of nucleic acids containing genetic
code; genes: sequences of DNA
that code for specific proteins
or RNA.)
English Language
LS Visual
Modeling Chromosomal Mutations
You can use paper and a pencil to model the ways
in which chromosome structure can change.
Procedure
1. Write the numbers 1–8 on
note-card pieces (one number per piece). Tape the
pieces together in numerical
order to model a chromosome with eight genes.
GENERAL
True or False:
1. At cell division, each chromosome consists of two chromatids
attached at the centromere.
(True)
2. The normal diploid number for
humans is 23. (False)
3. A person with the sex chromosomes XX would be female.
(True)
GENERAL
Have each student prepare a complete human karyotype based on
Figure 5. Each chromosome should
be drawn and then cut out. Allow
students to choose the sex of their
karyotype and determine whether
there are mutations, deletions, or
extra chromosomes. Have students
exchange their diagrams with a
partner. Each student must now
match homologues and determine
the sex of the individual, as well as
identify any possible abnormalities.
LS Interpersonal TAKS 2 Bio 6C, Bio 6D
(grade 10 only); Bio 6F
pp. 124–125
Student Edition
TAKS Obj 2 Bio 4B
TAKS Obj 2 Bio 6A, 6D
TAKS Obj 2 Bio 6C
TEKS Bio 4A, 4B, 6A, 6C, 6E, 6F
Teacher Edition
TAKS Obj 2 Bio 4B, 6A, 6B, 6C, 6D
TEKS Bio 4A, 4B, 6A, 6B, 6C, 6D,
6E, 6F
124
3
4
5
Original chromosome
14 note-card pieces, pencils, tape
Learners
Alternative
Assessment
2
Materials
TAKS 2 Bio 6A; Bio 4A
Quiz
1
TAKS 2 Bio 6D
(grade 10 only)
6C
2. Use the “chromosome” you
made to model the four alterations in chromosome structure discussed on this page
and illustrated at right. For
example, remove the number
3 and reconnect the remaining chromosome pieces to
represent a deletion.
1
3. Reconstruct the original
chromosome before
modeling a duplication,
an inversion, and a translocation. Use the extra
note-card pieces to make
the additional numbers you
need.
Describe how a cell might
be affected by each mutation
if the cell were to receive a
chromosome with that
mutation.
4
5
Deletion
1
2
1
2
3
4
5
Duplication
1
Analysis
2
2
4
3
5
Inversion
1
2
9
3
4
5
Translocation
Section 1 Review
Summarize how prokaryotic cells divide by
4A 4B 6E
binary fission.
Critical Thinking Evaluating Conclusions
Do you agree or disagree that homologous chro6A 6E
mosomes are found in gametes. Explain.
Identify the point in a eukaryotic cell cycle
TAKS Test Prep How does the karyotype of a
person with Down syndrome differ from a normal
4B 6F
karyotype?
A It lacks a chromosome.
B It has two sex chromosomes.
C It has a damaged chromosome.
D It has an extra copy of a chromosome.
at which DNA coils up to form
6A 6E
chromosomes.
Summarize the difference between a haploid
6E
cell and a diploid cell.
124
Answers to Section Review
1. DNA is first copied. Then the cell divides into
equal halves by adding new cell membrane
between the two DNA copies. The growing cell
membrane pushes inward, and the cell constricts to form two new, identical cells. A new
cell wall forms around each new membrane.
TAKS 2 Bio 4B
2. Chromosomes become visible in a eukaryotic
cell when the cell prepares to divide (during
prophase). TAKS 2 Bio 6A
3. A haploid cell (n) contains one set of chromosomes. A diploid cell (2n) contains two sets of
chromosomes. Bio 6E
Chapter 6 • Chromosomes and Cell Reproduction
4. Students should disagree. Homologous chromosomes are pairs of similar chromosomes. Because
gametes are haploid (n) they contain only one set
of chromosomes. Thus, homologous chromosomes are not normally found in gametes.
TAKS 2 Bio 6A
5.
A. Incorrect. Lacking a
chromosome is usually a fatal abnormality.
B. Incorrect. All normal individuals have two
sex chromosomes. C. Incorrect. Damaged chromosomes can lead to death or abnormalities,
but do not cause Down syndrome. D. Correct.
People with Down syndrome have an extra
copy of chromosome 21. TAKS 2 Bio 4B
The Cell Cycle
Section 2
Section 2
Focus
The Life of a Eukaryotic Cell
Objectives
Cell division in eukaryotic cells is more complex than cell division
in bacteria because it involves dividing both the cytoplasm and the
chromosomes inside the nucleus. Many internal organelles must be
correctly rearranged before the eukaryotic cell can properly divide
and form two fully functioning cells.
The Cell Cycle
The life of a eukaryotic cell is traditionally shown as a cycle, as
illustrated in Figure 6. The cell cycle is a repeating sequence of cellular growth and division during the life of an organism. A cell
spends 90 percent of its time in the first three phases of the cycle,
which are collectively called interphase. A cell will enter the last
two phases of the cell cycle only if it is about to divide. The five
phases of the cell cycle are summarized below:
● Describe how the cell
cycle is controlled in TAKS 2
eukaryotic cells.
4B 6E
● Relate the role of the
cell cycle to the onset
4B 6C
of cancer.
3. Second growth (G2) phase. In the G2 phase,
preparations are made for the nucleus to
divide. Hollow protein fibers called microtubules are assembled. The microtubules are
used to move the chromosomes during mitosis.
4. Mitosis. The process during cell division in
which the nucleus of a cell is divided into two
nuclei is called mitosis (mie TOH sihs). Each
nucleus ends up with the same number and
kinds of chromosomes as the original cell.
5. Cytokinesis. The process during cell division
in which the cytoplasm divides is called
cytokinesis (SIET oh kih nee sihs).
Mitosis and cytokinesis produce new cells
that are identical to the original cells and allow
organisms to grow, replace damaged tissues,
and, in some organisms, reproduce asexually.
TAKS 2
Key Terms
cell cycle
interphase
mitosis
cytokinesis
cancer
1. First growth (G1) phase. During the G1 phase, a cell grows
rapidly and carries out its routine functions. For most organisms, this phase occupies the major portion of the cell’s life. Cells
that are not dividing remain in the G1 phase. Some somatic cells,
such as most muscle and nerve cells, never divide. Therefore, if
these cells die, the body cannot replace them.
2. Synthesis (S) phase. A cell’s DNA is copied during this phase.
At the end of this phase, each chromosome consists of two chromatids attached at the centromere.
Overview
● Identify the major
events that characterize
each of the five phases of
the cell cycle.
4B 6E TAKS 2
Bellringer
Figure 6 The eukaryotic
cell cycle. The cell cycle
consists of phases of growth,
DNA replication, preparation
for cell division, and division of
the nucleus and cytoplasm.
INTE
RP
HA
SE
S
• Lesson Plan GENERAL
• Directed Reading
• Active Reading GENERAL
Ask students to finish the following
sentence: A typical eukaryotic cell
spends 90 percent of its time in
________. Write the following
choices on the board: mitosis,
anaphase, interphase, and
cytokinesis. (interphase) Bio 4A
Motivate
(DNA synthesis)
G1
Demonstration
(Cell growth)
G2
(Growth and
preparation for
mitosis)
Cytokinesis
Mitosis
125
Chapter Resource File
Before beginning this section
review with your students the
objectives listed in the Student
Edition. In this section, students
will examine the three phases that
take place in the life cycle of a cell,
which are collectively known as the
cell cycle. They will learn that cells
spend most of their time in interphase, during which cells grow and
DNA is replicated. The cell enters
mitosis as it prepares to divide,
then divides into two cells during
cytokinesis. Students will learn that
cancer may result if the controls for
the cell cycle break down.
Planner CD-ROM
• Reading Organizers
• Reading Strategies
• Portfolio Project
Genetics Project GENERAL
Display either a model or a photograph of a human brain. Point out
that once the brain is fully formed,
most of the nerve cells do not
divide, again. These cells remain in
the G1 phase of the cell cycle. Then
display either a model or a photograph of a human bone. Point out
that red blood cells are produced
from cells in the marrow of long
bones. An average red blood cell
lives for about 120 days. Each second, about 2 million red blood cells
are produced by cell division in the
bone marrow. Cells in the marrow,
unlike those in the brain, continue
going through the cell cycle as long
as a person lives. TAKS 2 Bio 4B
Transparencies
TT Bellringer
Chapter 6 • Chromosomes and Cell Reproduction
125
Control of the Cell Cycle
Teach
Teaching Tip
GENERAL
Interphase Have students draw
a Graphic Organizer like the one
below to summarize the events that
occur during the phases of interphase. Have them use arrows
between the phases, pointing to
the right, to indicate the sequence
of events. TAKS 2 Bio 4B; Bio 3E
Teaching Tip
If a cell spends 90 percent of its time in interphase, how do cells
“know” when to divide? How is the cycle controlled? Just as traffic
lights control the flow of traffic, cells have a system that controls
the phases of the cell cycle. Cells have a set of “red light–green light”
switches that are regulated by feedback information from the cell.
The cell cycle has key checkpoints (inspection points) at which
feedback signals from the cell can trigger the next phase of the cell
cycle (green light). Other feedback signals can delay the next phase
to allow for completion of the current phase (yellow or red light).
The cell cycle in eukaryotes is controlled by many proteins. Control occurs at three principal checkpoints, as shown in Figure 7.
Reviewing Information
Learn the stages of interphase by reviewing the
steps numbered 1–5 on the
previous page. You can see
in Figures 6 and 7 that the
cell cycle is a repeating
series of three steps
followed by mitosis
and cytokinesis.
1. Cell growth (G1) checkpoint. This checkpoint makes the decision of whether the cell will divide. If conditions are favorable
for division and the cell is healthy and large enough, certain
proteins will stimulate the cell to begin the synthesis (S) phase.
During the S phase, the cell will copy its DNA. If conditions are
not favorable, cells can typically stop the cell cycle at this checkpoint. The cell cycle will also stop at this checkpoint if the cell
needs to pass into a resting period. Certain cells, such as some
nerve and muscle cells, remain in this resting period permanently and never divide.
GENERAL
Busy Cells Remind students that
during interphase, a cell is not only
growing, it is also producing the
proteins and carrying out the
functions that are characteristic of
that type of cell. TAKS 2 Bio 4B
Group Activity
Writing
Cell Cycle Phases
Have students read Section 2,
then write all the activities and
events described during the cell
cycle on index cards (one event per
card). Draw a large version of
Figure 6 at the front of the room.
Then collect the students’ cards.
Allow one student at a time to
come to the front of the room, read
a card out loud, and then tape the
card to the proper phase of the cell
cycle. LS Verbal TAKS 2 Bio 4B
2. DNA synthesis (G2) checkpoint. DNA replication is checked at
this point by DNA repair enzymes. If this checkpoint is passed,
proteins help to trigger mitosis. The cell begins the many molecular processes that are needed to proceed into mitosis.
Figure 7 Control of the
cell cycle. The cell cycle in
eukaryotes is controlled at
three inspection points, or
checkpoints. Many proteins
are involved in the control of
the cell cycle.
G1
checkpoint
3. Mitosis checkpoint. This checkpoint triggers the exit from
mitosis. It signals the beginning of the G1 phase, the major
growth period of the cell cycle.
When Control Is Lost: Cancer
INTE
RP
HA
S
Certain genes contain the information necessary to make the proteins that regulate cell
growth and division. If one of these genes is
mutated, the protein may not function, and
regulation of cell growth and division can be
disrupted. Cancer, the uncontrolled growth
of cells, may result. Cancer is essentially a
disorder of cell division. Cancer cells do not
respond normally to the body’s control
mechanisms.
Some mutations cause cancer by overproducing growth-promoting molecules,
thus speeding up the cell cycle. Others
cause cancer by inactivating the control
proteins that normally act to slow or stop
the cell cycle.
E
S
G1
G2
Cytokinesis
Mitosis
G2
checkpoint
Mitosis
checkpoint
126
Graphic Organizer
Use this graphic organizer with
Teaching Tip: Interphase on
this page.
G1
pp. 126–127
Student Edition
TAKS Obj 2 Bio 4B
TAKS Obj 2 Bio 6C
TEKS Bio 4A, 4B, 6C, 6E, 6F
Teacher Edition
TAKS Obj 2 Bio 4B, 6C
TEKS Bio 3E, 4B, 6C
126
Cell
growth
Chapter 6 • Chromosomes and Cell Reproduction
Interphase
S
DNA
synthesis
G2
Growth and
preparation for cell division
Exploring Further
TAKS 2
Cancer
Although all cancers are not curable, great
progress has been made in cancer research over
the last 30 years. We now know that cancer
results from damage to a small set of genes that,
in normal cells, limits the ability of cells to divide.
What causes this damage? Certain environmental
factors appear to be associated with cancer. For
example, the incidence of cancer per thousand
people is not uniform throughout the United
States. Rather, it is higher in cities and in the
Mississippi delta, suggesting that pollution and
pesticide runoff may contribute to cancer. When
pollutants, radiation, and other environmental factors associated with cancer are analyzed, a clear
pattern emerges. Most cancer-causing agents are
powerful mutagens—that is, they readily damage
DNA. The conclusion that cancer is caused by
mutation of a cell’s DNA is now supported by a
very large body of evidence.
How many mutations are required to produce
cancer? Research in the last several years indicates
that mutation of only a few genes can transform
normal cells into cancerous ones. All of these
cancer-causing genes are involved with regulating
how fast cells grow and divide. How is cell division
regulated? As a crude analogy, imagine a car
parked on the side of a road. To get it going, you
must step on the accelerator and release the brake.
Stepping on the Accelerator
A cell divides when it receives a signal to do so. A
“divide” signal is usually in the form of a chemical
substance released by another cell. The substance is bound by a protein on the surface of
the receiving cell. This binding activates a second
protein inside the cell—relaying the signal from
the outside of the cell to the inside. Here, a family
of proteins then relay the signal inward to the
nucleus. One protein molecule passes the signal to
the next like a baton in a
relay race. The genes for
these signal-carrying
proteins are called oncogenes (onco is Greek for
“mass” or “tumor.”). If
oncogenes are changed
by mutation to become
more active, cancer can
result. Like stepping on
Melanoma cells
the accelerator of a car,
an increase in the activity
of these proteins amplifies the “divide” signal.
This causes the cell to divide more often.
Understanding Cancer
Releasing the Brakes
At the nucleus, the divide signal overrides a
set of genes that act as “brakes.” These braking
genes—called tumor supressor genes—prevent cell
division from occurring too often. In cancer, these
tumor suppressor genes are damaged. Like removing pressure from the brakes of a car increases a
car’s speed, decreasing the activity of tumor suppressors speeds up cell division.
Cells have three kinds of tumor suppressors, all of
which must be disabled before cancer can occur.
First, cells have proteins that inhibit DNA replication
for limited periods. In cancer cells they are
permanently inactivated. Second, cells have errorcorrecting proteins that detect damage to genes.
In most cancers this error-detection has been
disabled. Third, cancer cells rebuild the tips of
their chromosomes. A little is lost from the ends
of chromosomes at
each replication, limiting the number of
www.scilinks.org
times a normal cell
Topic: Cancer Cells
can divide. Adding
Keyword: HX4030
the deleted material
back to the tips
removes this limit to
a cell’s life span.
Section 2 Review
Differentiate between the G1, G2, and S phases
of the eukaryotic cell cycle.
4B 6E
Relate what occurs at each of the three principal
checkpoints in the cell cycle.
4B 6E
Critical Thinking Evaluating Information
TAKS 2 Bio 4B
Reteaching
Assign students to groups. Have
each group visually interpret a
stage in the cell cycle. Have each
group display their graphics in the
appropriate space in a circle graph
that represents the cell cycle.
TAKS Test Prep In the cell cyle of typical
cancer cells, mutations have caused
4B 6C
A slower growth.
C uncontrolled growth.
B a failure in mitosis.
D a halt in cell division.
Quiz
GENERAL
1. The three phases of the cell cycle
are interphase, mitosis, and
________. (cytokinesis)
TAKS 2 Bio 4B
2. When normal control of the cell
3. Chromosomes are formed right before a cell
divides in mitosis. During most of interphase,
the DNA exists as chromatin, which is more
elongated and harder to see with a microscope.
TAKS 2 Bio 4B
2. Cell growth checkpoint (G1): determines if cell
is ready to undergo division; DNA synthesis
checkpoint (G2): DNA repair enzymes check
progression; mitosis checkpoint: triggers exit
from mitosis at the beginning of the G1 cycle.
Close
LS Visual Co-op Learning
Answers to Section Review
1. G1 phase: cell grows rapidly and carries out
routine functions; G2 phase: mitochondria and
other organelles replicate, microtubules are
assembled; S phase: DNA is copied
Teaching Strategies
• Tell students that most anticancer drugs interfere with
the cell cycle of cancer cells.
Unfortunately, the drugs also
interfere with healthy cells,
which is why the patient
suffers side effects from the
medications.
Discussion
• Where in the cell cycle could
scientists target anticancer
drugs? (Answers will vary but
may mention proteins involved
in the three checkpoints, DNA
replication, or cytokinesis.)
• What type of environmental
factors have been associated
with the onset of cancer?
(Answers may include diet,
UV radiation, hormones, and
environmental pollution.)
Why are individual chromosomes more difficult
to see during interphase than during mitosis?
4B 6E
127
TAKS 2
Bio 4B,
6C
4.
cycle fails, ________ may
develop. (cancer) TAKS 2 Bio 4B
TAKS 2 Bio 4B
Alternative
Assessment
A. Incorrect. Cancer is associated with accelerated cell growth. B. Incorrect.
Cancer does alter the cell cycle, but not by
halting mitosis. C. Correct. Uncontrolled cell
growth is the defining characteristic of cancer.
D. Incorrect. Cancer cells continue to divide.
Have students write and illustrate a
description of the cell cycle in the
style of a children’s book called “A
Cell’s Life.” Be sure students include
each phase, including G1, S, G2,
mitosis, and cytokinesis. LS Verbal
TAKS 2 Bio 4B, 6C
Chapter 6 • Chromosomes and Cell Reproduction
127
Section 3
Section 3
Mitosis and Cytokinesis
Focus
Chromatid Separation in Mitosis
Objectives
Overview
Before beginning this section
review with your students the
objectives listed in the Student
Edition. In this section, students
will examine the processes of mitosis and cytokinesis in more detail.
During mitosis, spindle fibers pull
the chromatids to opposite ends of
the cell, ensuring that each new cell
receives the proper assortment of
chromosomes. During cytokinesis,
the cytoplasm is divided by a new
cell membrane or cell wall.
Every second about 2 million new red blood cells are produced in
● Describe the structure and
your body by cell divisions occurring in the bone marrow. These cells
function of the spindle
during mitosis.
4B 6E TAKS 2 have received the signal to divide. The cells continue past the G2 phase
and enter into the last two phases of the cell cycle—mitosis and
cytokinesis. During mitosis the nucleus divides to form two nuclei,
each containing a complete set of the cell’s chromosomes. During
cytokinesis the cytoplasm is divided between the two resulting cells.
● Differentiate cytokinesis in
During mitosis, the chromatids on each chromosome are physi4B 6E
animal and plant cells.
TAKS 2 cally moved to opposite sides of the dividing cell with the help of
the spindle, shown in Figure 8. Spindles are cell structures made up
Key Terms
of both centrioles and individual microtubule fibers that are
spindle
involved in moving chromosomes during cell division.
● Summarize the events
of the four stages
of mitosis.
4B 6E TAKS 2
Forming the Spindle
Animal cells usually have one pair of centrioles, with the centrioles at
right angles to each other. During the G2 phase of the cell cycle, the
centriole pair is replicated so that the cell has two pairs of centrioles
as it enters the mitotic phase. When a cell enters the mitotic phase,
the centriole pairs start to separate, moving toward opposite poles of
the cell. As the centrioles move apart, the spindle begins to form.
Centrioles and spindle fibers are both made of hollow tubes of protein called microtubules. Each spindle fiber is made of an individual
microtubule. Each centriole, however, is made of nine triplets of
Bellringer
Draw a football field on the board
with the players lined up at midfield and the goal posts at each
end. Have students write a short
paragraph describing how the players, the midfield line, and the goal
posts compare to the structures of
a cell involved in mitosis and cell
division. (The players represent the
chromosomes, the midfield line represents the equator of the cell, and the
goal posts represent the centrioles at
the poles of the cell.)
Figure 8 The spindle
The spindle, made up of centrioles
and spindle fibers, helps move
chromosomes apart during mitosis.
Microtubule
triplets
TAKS 2 Bio 4B; Bio 3E, 6E
Centromere
Cell
Chromatids
Motivate
Activity
GENERAL
Mitosis Set up several microscopes with prepared slides of cells
at various stages of mitosis, but
presented out of order. Have students examine the slides and try to
place each slide into the proper
order of events. If microscopes
and slides are not available, the
various stages can be drawn on the
board, unlabeled
English Language
Learners
and out of order.
TAKS 2 Bio 4B; Bio 6E
pp. 128–129
Student Edition
TAKS Obj 2 Bio 4B
TEKS Bio 4B, 6E
Teacher’s Edition
TAKS Obj 2 Bio 4B
TEKS Bio 3E, 3F, 4B, 6E
128
Spindle
fibers
Centrioles
Each centriole is composed of
nine triplets of microtubules
arranged in a circle.
128
Chapter Resource File
• Lesson Plan GENERAL
• Directed Reading
• Active Reading GENERAL
• Data Sheet for Math Lab
• Data Sheet for Quick Lab
Chapter 6 • Chromosomes and Cell Reproduction
Transparencies
TT Bellringer
TT Stages of Mitosis
GENERAL
Planner CD-ROM
GENERAL
• Reading Organizers
• Reading Strategies
• Supplemental Reading
The Lives of a Cell
microtubules arranged in a circle. Unlike animal cells, plant cells do
not have centrioles, but they form a spindle that is almost identical
to that of an animal cell.
Teach
18
493
Some of the microtubules in the spindle interact with each other.
Others attach to a protein structure found on each side of the centromere. The two sets of microtubules extend out toward opposite poles
of the cell. Once the microtubules attach to the centromeres and
poles, the two chromatids in each chromosome can be separated.
The chromatids are moved to each pole of the cell in a manner
similar to bringing in a fish with a fishing rod and reel. When the
microtubule “fishing line” is “reeled in,” the chromatids are
dragged to opposite poles. The reeling in occurs because the ends
of the spindle fibers are broken down bit by bit at each of the poles.
As the fibers become shorter, the chromatids they are pulling move
closer and closer to the poles.
As soon as the chromatids separate from each other they are
called chromosomes. When the chromosomes finally arrive, each
pole has one complete set of chromosomes.
x 2+ 6x
-7-0
-7-0
Calculating the
Number of Cells
Resulting from
Mitosis TAKS 2 Bio 4B; Bio 6E
Teacher’s Notes
Review numbers in scientific
notation before starting the lab.
Review how to cancel units and
set up equal unit proportions.
For example, 60 seconds/
1 minute is an equal proportion.
Analysis Answers
0
2
5
<
x 2+ 6x
0
2
5
Skills Acquired
Calculating, predicting
8
493
76
<
Separation of Chromatids
by Attaching Spindle Fibers
1. 9.0 1010 cells 90,000,000,000 cells
2. 2.16 1012 cells
3. Factors that might increase or
decrease the rate of mitosis
include mutated genes, diet,
and exposure to ultraviolet
light and tobacco products.
Calculating the Number of Cells
Resulting from Mitosis 4B 6E TAKS 2
Background
MATH TAKS Obj 9 8.3B; Obj 10 8.14A
Scientists investigating cancer might need to know the
number of cells produced in a certain amount of time. In
the human body the rate of mitosis is about 25 million
(2.5 107) cells produced every second! You can calculate
the number of cells produced by mitosis in a given amount
of time.
1. Calculate the number of cells produced by mitosis in the given time. For example, to find
the number of cells produced in 3 minutes, determine how many seconds there are in 3 minutes (since
the rate is given in seconds).
60 seconds
3 minutes 180 seconds
1 minute
2. Multiply the rate of mitosis by the time (in seconds) asked for in the problem
(180 seconds).
2.5 107 cells
180 seconds 4.5 109 cells (4,500,000,000 cells)
second
Analysis
1. Calculate the number of
cells that would be produced
in 1 hour.
2. Calculate the number of
cells that would be produced
in 1 day.
3. Critical Thinking Predicting Patterns Identify factors
that might increase or decrease
the rate of mitosis.
129
did you know?
Fly Chromosomes The fruit fly Drosophila
melanogaster has been used to study chromosomal mutations since 1933. The salivary glands
of these flies, as in many insects, are composed
of cells that do not divide during the larval
stage. However, the chromosomes continue to
replicate, producing many copies. The copies of
each chromosome are closely aligned, resulting
in thick chromosomes that are easy to study
with a microscope. Bio 3F
Chapter 6 • Chromosomes and Cell Reproduction
129
Mitosis and Cytokinesis
Although mitosis is a continuous process, biologists traditionally
divide it into four stages, as shown in Figure 9.
Teach, continued
continued
SKILL
Mitosis
BUILDER
Vocabulary To help students master vocabulary, relate the word
mitosis to its Greek origin, mitos,
meaning “thread.” Ask students to
explain why mitosis is associated
with thread. (The hereditary material
consists of long, threadlike molecules.)
Using the Figure
Lead students through
each stage shown in
Figure 9, focusing on the
behavior of chromosomes. Point
out that the various stages are not
of equal duration. Using movie film
as an analogy, help students avoid
the misconception that mitosis
“jumps” from stage to stage.
Although a movie consists of individual frames of film, the images
on the film appear to change continuously. Explain that mitosis
progresses in a similar fashion.
IO
B
graphic
Step
Metaphase During metaphase the chromosomes move to the
center of the cell and line up along the equator. Spindle fibers
link the chromatids of each chromosome to opposite poles.
Step
Anaphase Centromeres divide during anaphase. The two
chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten.
Step
Telophase A nuclear envelope forms around the chromosomes at each pole. Chromosomes, now at opposite poles,
Stages of Mitosis
The chromosome copies in the nucleus of a dividing cell are separated into two nuclei.
1
2
Prophase
• Chromosomes
become visible
• Nuclear envelope
dissolves
• Spindle forms
Chromosome
(already copied)
The chromosomes
replicate during
interphase.
INTER
PH
AS
E
GENERAL
Have students hypothesize about
how a cell’s ATP use changes during mitosis. (The events of mitosis
require a lot of additional energy,
which is supplied by ATP.)
TAKS 2 Bio 4B; Bio 6E
pp. 130–131
Student Edition
TAKS Obj 2 Bio 4B
Teacher Edition
TAKS Obj 2 Bio 4B
TEKS Bio/IPC 3C
TEKS Bio 3E, 3F, 4A, 4B, 6E
Centrioles
S
Prefixes Tell students that the prefixes of the different stages of mitosis describe the order of events or
the events themselves. Pro- means
“earlier than;” meta- means “later
than” or “after;” ana- means “up”
or “back” and describes the movement of the chromosomes “up”
toward the poles; and
telo- means
English Language
Learners
“end.”
Spindle fibers
G1
Cy
to
ki
n
es
i
s
Magnification: 567ⴛ
G2
Mitosis
130
MEDICINE
CONNECTION
When cultured, cancer cells can divide indefinitely if they are given a continual supply of
nutrients. One cell line has been continuously
cultured since 1951. The cells in this cell line
are called HeLa cells because they were originally from a tumor removed from a woman
named Henrietta Lacks. The patient suffered
from uterine cervical carcinoma. Cells from
this cell line are still used by researchers
around the world, especially in research
on viruses. Bio/IPC 3C; Bio 3F
Chapter 6 • Chromosomes and Cell Reproduction
Metaphase
• Chromosomes
line up along
equator
Nucleus
Teaching Tip
130
Prophase Chromosomes coil up and become visible during
prophase. The nuclear envelope dissolves and a spindle forms.
Figure
Figure 96-9
IO
B
graphic
TAKS 2 Bio 4B; Bio 6E
Teaching Tip
Step
uncoil and the spindle dissolves.
The spindle fibers break down and
disappear. Mitosis is complete.
Teaching Tip
Belt of protein
threads
Cytokinesis
As mitosis ends, cytokinesis begins. During cytokinesis, the cytoplasm of the cell
is divided in half, and the cell membrane
grows to enclose each cell, forming two
separate cells as a result. The end result
of mitosis and cytokinesis is two genetically identical cells where only one cell
existed before.
During cytokinesis in animal cells and
other cells that lack cell walls, the cell is
pinched in half by a belt of protein
threads, as shown in Figure 10.
3
Figure 10 Cytokinesis
in animal cells. The cell
membrane is pinched in
half by a belt of protein
threads.
LS Visual TAKS 2 Bio 4B; Bio 6E
Using the Figure
4
Anaphase
• Centromeres divide
• Chromatids (now
called chromosomes)
move toward opposite
poles
GENERAL
Stages of Mitosis Have students
draw pictures of each stage of
mitosis on four note cards. On one
side, have students write the name
of the stage. On the opposite side,
have them draw an accurate sketch
of the stage. They can then use
these as flash cards to help each
other learn the stages of mitosis.
Two genetically
identical cells
Tell students that the belt of
protein threads that are labeled in
Figure 10 is a ring of the proteins
actin and myosin, the same two
proteins that interact to contract
muscle cells. The furrowing of the
cell membrane occurs perpendicular to the long axis of the spindle.
Cytokinesis usually begins in
anaphase but is not completed until
after the two nuclei have formed.
Telophase
• Nuclear envelope
forms at each pole
• Chromosomes uncoil
• Spindle dissolves
• Cytokinesis begins
TAKS 2 Bio 4B; Bio 4A, 6E
Group Activity
GENERAL
Performing Mitosis and
Cytokinesis Have students work in
groups of three or four students.
Ask each group to design a “performance” using the entire class as
the cast, as well as props, that
would demonstrate the process of
mitosis and cytokinesis. Ask groups
to present their designs to the class,
and have the class decide which
“show” is best. If time and materials permit, allow students to stage
their mitosis and cytokinesis show.
LS Interpersonal
Co-op Learning
TAKS 2 Bio 4B; Bio 3E, 6E
131
MISCONCEPTION
ALERT
Mitosis vs. Cytokinesis Students often think
that mitosis is the same as cell division. Be
sure they understand that mitosis refers
strictly to the division of the chromosomes,
whereas cytokinesis refers to the division of
the cytoplasm. Remind students that cell
division is just one of the four events that
make up the cell cycle. TAKS 2 Bio 4B; Bio 6E
Trends in Cell Biology
Turning Off Cancer Currently, researchers at
the National Cancer Institute are working to
find the proteins that “turn on” or “turn off”
gene ECT2, an oncogene that appears to be a
critical regulator of cytokinesis. Eventually,
researchers may be able to “turn off” the
mechanism that causes cancer cells to divide
uncontrollably. Bio/IPC 3C
Chapter 6 • Chromosomes and Cell Reproduction
131
In plant cells and other cells that have rigid cell walls, the
cytoplasm is divided in a different way. In plant cells, vesicles
formed by the Golgi apparatus fuse at the midline of the
dividing cell and form a cell plate. A cell plate is a membranebound cell wall that forms across the middle of
the plant cell. A new cell wall then forms on both
Cell wall
sides of the cell plate, as shown in Figure 11. When
Nucleus
complete, the cell plate separates the plant cell
into two new plant cells.
In both animal and plant cells, offspring cells are
about equal in size. Each offspring cell receives an
identical copy of the original cell’s chromosomes.
Each offspring cell also receives about one-half of
Forming
the original cell’s cytoplasm and organelles.
Teach, continued
continued
Observing Mitosis
and Cytokinesis TAKS 2
Bio 4B;
Bio 6E
Skills Acquired
Comparing,
inferring
Answers to Analysis
cell plate
1. Prophase: chromosomes are
visible as dark threads;
metaphase: chromatids line up
along the equator; anaphase:
chromosomes appear to pull
toward opposite poles;
telophase: chromosomes are
at opposite poles
2. Answers will vary but should
indicate more cells in interphase than in the other stages.
3. Cells spend the majority of
their time in interphase.
Observing Mitosis and Cytokinesis
TAKS 2
You can identify the stages of mitosis and the process
of cytokinesis by observing slides of tissues undergoing
mitosis using a compound microscope.
Materials
compound microscope, prepared slide of mitosis,
paper, pencil
Analysis
Procedure
1. View a prepared slide of cells
undergoing mitosis under low
power of a compound microscope.
2. Move the slide until you find a
section where different stages
of mitosis are visible.
Close
Reteaching
2
Provide students with yarn to TAKS
Bio 4B
represent chromosomes, nuclear
envelopes, and cell membranes; tie
tabs to represent centromeres; and
string to represent spindle fibers.
Have them recreate on their desktops a cell undergoing
mitosis.
English Language
LS Kinesthetic
Figure 11
Cytokinesis in
plant cells. A
cell wall forms
in the center of
the dividing cell.
Learners
Quiz
GENERAL
1. In mitosis, the chromatids move
toward opposite poles during
________. (anaphase) TAKS 2 Bio 4B
2. During mitosis, the chromosomes line up along the equator
during ________. (metaphase)
TAKS 2 Bio 4B
3. Switch to high power. Use
the photos in Figure 9 to help
you locate and identify cells
in interphase and in each
stage of mitosis.
4. On a separate piece of paper,
sketch an example of each
stage. Label each sketch with
the following terms where
appropriate: chromosomes,
cell membrane, cytoplasm,
nucleus, spindle, and cell wall.
5. Switch to low power, and
estimate how many cells are
clearly in interphase and how
many cells are in one of the
stages of mitosis.
Student Edition
TAKS Obj 2 Bio 4B
TAKS Obj 2 Bio 6C
TEKS Bio 4B, 6C, 6E
Teacher Edition
TAKS Obj 2 Bio 4B, 6C
TEKS Bio 4B, 6C
132
2. Compare the number of
cells in interphase with the
number of cells in one of the
stages of mitosis.
3. Critical Thinking Inferring Relationships What
does your answer to item 2
indicate about the relative
length of interphase?
Section 3 Review
Describe the function of the microtubules
Compare how cytokinesis occurs in plant cells
during anaphase.
with how it occurs in animal cells.
4B 6E
Describe the events that occur during each of
the four stages of mitosis.
4B 6E
4B 6E
TAKS Test Prep Mitosis could not proceed if a
mutation interrupted the assembly of
4B 6C
A the cell wall.
C the cell membrane.
B spindle fibers.
D the nuclear envelope.
132
Answers to Section Review
1. The microtubules, attached to the centromeres,
shorten and pull the chromatids to opposite
poles, similar to a fishing line reeling in a fish.
TAKS 2 Bio 4B
pp. 132–133
1. Describe the activity of
chromosomes in each stage
of mitosis.
2. Prophase: chromosomes become visible,
nuclear envelope dissolves, spindle forms;
metaphase: chromosomes line up at the equator, spindle fibers attach to each chromatid;
anaphase: centromeres divide, chromatids
move to opposite poles due to shortening spindle fibers; telophase: nuclear envelope forms
around the chromatids at each pole, chromosomes uncoil, spindle fibers break down and
disappear TAKS 2 Bio 4B
Chapter 6 • Chromosomes and Cell Reproduction
3. In plant cells, vesicles formed by the Golgi
apparatus fuse at the equator and form the cell
plate. A new cell wall forms on both sides of
the cell plate. In animal cells, the cell is pinched
in half by a belt of protein threads. TAKS 2 Bio 4B
4.
A. Incorrect. The cell wall does
not play a key role in mitosis. B. Correct.
Without the spindle fibers, chromosomes cannot segregate properly into two complete
nuclei. C. Incorrect. The cell membrane does
not play a key role in mitosis. D. Incorrect. The
nuclear envelope breaks down before mitosis
and reforms after the completion of mitosis.
TAKS 2 Bio 4B, 6C
Study
CHAPTER HIGHLIGHTS
ZONE
Key Concepts
Alternative
Assessment
Key Terms
Section 1
1 Chromosomes
●
Cell division allows organisms to reproduce asexually, grow,
replace worn-out or damaged tissues, and form gametes.
●
Bacteria reproduce by binary fission.
●
Before cell division, DNA coils tightly around proteins and
forms chromosomes. At cell division, each chromosome
consists of two chromatids attached at the centromere.
●
Each organism has a characteristic number of chromosomes.
●
Human somatic cells are diploid, with 23 pairs of homologous chromosomes. Human gametes are haploid, with 23
chromosomes.
●
Sex chromosomes carry information that determines an
organism’s sex.
●
Changes in chromosome number or structure can cause
abnormal development. Karyotypes are used to examine an
individual’s chromosomes.
gamete (118)
binary fission (119)
gene (119)
chromosome (119)
chromatid (119)
centromere (119)
homologous chromosome (120)
diploid (121)
haploid (121)
zygote (121)
autosome (122)
sex chromosome (122)
karyotype (122)
●
The life of a eukaryotic cell—the cell cycle—includes
interphase, mitosis, and cytokinesis.
●
Interphase consists of 3 phases: growth, DNA synthesis (replication), and preparation for cell division. A cell about to divide
enters the mitosis and cytokinesis phases of the cell cycle.
●
The cell cycle is carefully controlled; failure of cellular
control can result in cancer.
Set up a lab practical with various
stations. Include microscopes with
slides at various stages of mitosis,
pictures of chromosomes, and
karyotypes for interpretation.
Chapter Resource File
• Science Skills Worksheet GENERAL
• Critical Thinking Worksheet
• Test Prep Pretest GENERAL
• Chapter Test GENERAL
IPC Benchmark
Review
Section 2
2 The Cell Cycle
GENERAL
To prepare students for the TAKS, have
students review Changes in Matter:
Physical Changes and Chemical
Changes TAKS Obj 4 IPC 8A on p. 1052
of the IPC Refresher in the Texas
Assessment Appendix of this book.
cell cycle (125)
interphase (125)
mitosis (125)
cytokinesis (125)
cancer (126)
Section 3
3 Mitosis and Cytokinesis
●
During mitosis, spindle fibers drag the chromatids to opposite
poles of the cell. A nuclear envelope forms. Each resulting
nucleus contains a set of the original cell’s chromosomes.
●
Cytokinesis in animal cells occurs when a belt of protein
threads pinches the cell membrane in half. Cytokinesis in
plant cells occurs when vesicles from the Golgi apparatus
fuse to form a cell plate.
spindle (128)
Unit 4—Cell Reproduction Use Topics 1–4
BIOLOGY
in this unit to review the key concepts and
terms in this chapter.
133
Answer to Concept Map
The following is one possible answer
to Performance Zone item 15.
Cell cycle
includes
interphase
mitosis
cytokinesis
includes
divides
divides
synthesis phase
chromosomes
cytoplasm
first growth phase
second growth phase
Chapter 6 • Chromosomes and Cell Reproduction
133
Performance
ZONE
CHAPTER 6
ANSWERS
Using Key Terms
1. b Bio 6E
2. a TAKS 2 Bio 4B; Bio 6E
3. c Bio 4A, 6E
4. d TAKS 2 Bio 4B; Bio 6E
5. a. Gametes, or reproductive cells,
are haploid, or n, and contain
one set of chromosomes.
When gametes unite, they
form a zygote, which is
diploid, or 2n, and contains
two sets of chromosomes.
b. Of the 23 pairs of chromosomes, 22 pairs are called
autosomes. The individual’s
sex is determined by the 23rd
pair, the sex chromosomes. A
karyotype is a photograph of
chromosomes arranged by size.
c. A person’s sex is determined
by their sex chromosomes. An
individual with two X chromosomes (XX) is female; an
individual with XY chromosomes is male.
Understanding Key Ideas
6. b TAKS 2 Bio 4B
7. d TAKS 2 Bio 6C
8. c TAKS 2 Bio 4B
9. a Bio 6E
10. b TAKS 2 Bio 4B; Bio 6E
11. a TAKS 2 Bio 4B; Bio 6E
12. Normal cells can become cancer
cells if a genetic mutation impairs
a cell’s ability to regulate cell
growth and division. The
mutation leads to cancer, or
uncontrolled cell growth.
TAKS 2 Bio 6C
CHAPTER REVIEW
8. When the cell cycle is not controlled,
Using Key Terms
1. Prokaryotes reproduce asexually by
a. disjunction.
c. cytokinesis.
b. binary fission.
d. mitosis.
2. The stage of the cell cycle in which a cell’s
DNA is copied is called the _______ phase.
4B 6E
a. S
c. G2
b. G1
d. mitosis
each chromosome.
d. structures that move chromosomes
during mitosis.
4. The process in which the cytoplasm of a
cell is divided is called
4B 6E
a. disjunction.
c. binary fission.
b. interphase.
d. cytokinesis.
134
mosomes present in the original cell.
b. receives most of the chromosomes from
the original cell.
c. donates a chromosome to the original
cell.
d. receives exactly half the chromosomes
from the original cell.
10. During the metaphase stage of mitosis,
4B 6E
a. the cell membrane folds inward.
b. chromosomes line up at the cell’s equator.
c. spindle fibers shorten, pulling chromo-
somes to the poles of the cell.
d. chromosomes are at opposite ends of
5. For each set of terms, write one or more
sentences summarizing information
learned in this chapter.
a. haploid, diploid, gametes, zygote
b. autosome, sex chromosome, karyotype
c. XX chromosomes, XY chromosomes
the cell.
11. How does cell division differ between
animal and plant cells?
4B 6E
a. Plant cells do not have centrioles.
b. Animal cells form a cell plate.
c. Plant cells are always haploid.
d. Animal cells do not have centrioles.
Understanding Key Ideas
6. In humans, females have _______ sex
chromosomes.
a. XY
b. XX
12. Summarize how normal cells can become
4B
cancer cells.
c. YY
d. XO
1
2
c. inversion
d. duplication
3
4
fragment before the cytoplasm divides.
(Hint: See Chapter 3, Section 2.)
4B 6E
2
1
2
3
14.
How did researchers narrow
the search for genes involved in DiGeorge
syndrome to 15 genes?
6C
15.
Concept Mapping Make a concept
map that shows the events in the cell cycle.
Try to include the following words in your
map: cell cycle, interphase, synthesis phase,
chromosomes, cytokinesis, mitosis, second
growth phase, and first growth phase.
3E
5
Original chromosome
1
6C
13. List five organelles that must divide or
7. The diagram below represents a(n) _______
mutation.
6C
a. deletion
b. translocation
4B
9. As a result of mitosis, each resulting cell
6E
a. receives an exact copy of all of the chro-
3. Chromatids are
4A 6E
a. dense patches within the nucleus.
b. prokaryotic chromosomes.
c. two exact copies of DNA that make up
4
5
? mutation
134
13. Answers will vary, but should include
organelles such as lysosomes, the Golgi apparatus, the endoplasmic reticulum, ribosomes,
and vacuoles. TAKS 2 Bio 4B; Bio 6E
14. When the mice had two copies of those genes,
DiGeorge syndrome did not appear.
TAKS 2 Bio 6C
pp. 134–135
Review and Assess
TAKS Obj 1 Bio/IPC 2B, 2D
TAKS Obj 2 Bio 3D, 4B, 6C, 6E
TEKS Bio 3D, 4B, 6C, 6E
TEKS Bio/IPC 2B, 2D
_______ may result.
a. Down syndrome
b. binary fission
c. cancer
d. a spindle
6E
15. The answer to the concept map is found at
the bottom of the Study Zone page. Bio 3E
Chapter 6 • Chromosomes and Cell Reproduction
Assignment Guide
Section
1
2
3
Questions
1, 3, 5 ,6, 14, 17, 19
2, 7, 8, 12, 15, 17, 18, 20, 21, 22
4, 9, 10, 11, 13
Critical Thinking
Alternative Assessment
16. Predicting Outcomes Predict the result
20. Finding and Communicating Information
if cytokinesis were omitted from the cell
cycle?
4B 6E
17. Inferring Relationships Explain the
relationships between mitosis in
eukaryotic cells and binary fission
in prokaryotes.
4A 4B 6E
18. Evaluating Conclusions Damage to the
brain or the spinal cord is usually permanent. Use your knowledge of the cell cycle
to explain why damaged cells in the brain
or spinal cord are not replaced.
4B 6E
19. Evaluating Viewpoints A newspaper article
describes the concern that more infants
with Down syndrome will be born in the
United States as more women delay having
children. Do you agree or disagree with
this concern? Explain.
4B 6C
Critical Thinking
Scientists have determined that telomeres
(the tips of chromosomes) are shaved down
slightly every time a cell divides. When the
telomeres reach a certain length, the cell
may lose its ability to divide. Find out what
scientists have recently uncovered about
telomeres and their association with cell
division and cancer. Prepare a brief written
report to share with your class.
2B 2D 4B 6E
21. Career Connection Oncologist Research
the field of oncology, and write a report
that includes a job description, training
required, kinds of employers, growth
prospects, and starting salary.
2B 3D
22. Interactive Tutor Unit 4 Cell Reproduction
Write a report summarizing how different
cancer-fighting drugs kill cancer cells by
interrupting the cell cycle. How is cell division related to cancer?
2B 4B 6E
16. Without cytokinesis, the cell
would not divide into two
daughter cells.
TAKS 2 Bio 4B; Bio 6E
17. Both result in new cells with the
same number of chromosomes as
in the original cell.
TAKS 2 Bio 4B; Bio 4A, 6E
18. Most nerve cells are permanently
resting in interphase G1. Because
they do not undergo mitosis,
most damaged nerve cells are
not replaced.
TAKS 2 Bio 4B; Bio 6E
19. Answers will vary, but should
include statistics about how risk
factors change with increasing
maternal age. TAKS 2 Bio 4B, 6C
Alternative Assessment
TAKS Test Prep
The illustration below shows the events of
the cell cycle. Use the illustration and your
knowledge of science to answer questions 1–3.
I NT E
RP
HA
SE
S
G1
1. For about what proportion of the cell cycle
is the cell in interphase?
4B 6E
A less than 1/2
C more than 3/4
B less than 3/4
D more than 9/10
2. List the phases of the cell cycle starting
immediately after cell division.
F G1, S, G2, mitosis, cytokinesis
G cytokinesis, G1, S, G2, mitosis
H S, G2, mitosis, cytokinesis, G1
J G1, cytokinesis, mitosis, G2, S
4B 6E
TAKS 1 Bio/IPC 2B, 2D;
TAKS 2 Bio 4B; Bio 6E
3. If the phase during which DNA is synthe-
G2
Cytokinesis
Mitosis
sized did not occur, what effect would it
have on the cell cycle?
4B 6E
A Mitosis would immediately follow S.
B Cytokinesis would immediately follow G2.
C G1 would immediately follow mitosis.
D G2 would immediately follow G1.
Test
When several questions refer to the same graph,
table, drawing, or passage, answer the questions
you are sure of first.
135
Standardized Test Prep
1. A. Incorrect. Cells spend the majority of time
in interphase. B. Incorrect. Cells spend the
majority of time in interphase. C. Correct.
Cells spend most of their time in interphase.
D. Incorrect. While cells spend most of their
time in interphase, the processes of mitosis and
cytokinesis take longer than 1/10 of the total
cell cycle. TAKS 2 Bio 4B; Bio 6E
2. F. Correct. The cell cycle can be divided into
recognized phases that complete the continuous process. G. Incorrect. While the phases are
presented in proper order, cytokinesis does not
immediately follow cell division. H. Incorrect.
20. Answers will vary. Cells from
large tumors often have unusually
short telomeres. Telomerase, an
enzyme that catalyzes the lengthening of telomeres, stabilizes
telomere length, especially in cancer cells. Researchers are focusing
on telomerase as a target for cancer diagnosis and chemotherapy.
While the phases are presented in proper order,
S does not immediately follow cell division.
J. Incorrect. The phases here are written backwards of their proper order.
TAKS 2 Bio 4B; Bio 6E
3. A. Incorrect. The S phase would not exist,
because DNA synthesis takes place during the
S phase. B. Incorrect. Cytokinesis would continue to follow mitosis. C. Incorrect. G1 would
continue to immediately follow cytokinesis.
D. Correct. G2 normally follows S, the phase
during which DNA synthesis takes place.
21. Oncologists treat and manage cancer patients by using radiation
therapy or drugs. Training requires
an M.D. or D.O. degree, passing a
licensing exam, an internship, and
a residency. Employers include
solo, partnership, or group practices, as well as universities and
government agencies. Growth
prospects are excellent. Starting
salary varies by region.
TAKS 1 Bio/IPC 2B; Bio 3D
22. Answers will vary. Examples
include vincristine and taxol,
which prevent the mitosis spindle
microtubules from functioning.
Cancer occurs when cell division
does not respond to the normal
signals that regulate the cell cycle.
TAKS 1 Bio/IPC 2B; TAKS 2 Bio 4B; 6E
TAKS 2 Bio 4B; Bio 6E
Chapter 6 • Chromosomes and Cell Reproduction
135