Download The Cell Cycle

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3
Overview
READI NG WARM-U P
This section introduces the life
cycle of a cell. Students will
learn how cells reproduce and
how mitosis is important.
Finally, students will learn how
cell division differs in plants
and animals.
Objectives
•
•
•
Explain how cells produce more
cells.
Describe the process of mitosis.
Explain how cell division differs in
animals and plants.
Terms to Learn
cell cycle
chromosome
homologous chromosomes
mitosis
cytokinesis
Bellringer
On the board, write the
following:
Biology is the only science in
which multiplication means
the same thing as division.
READI NG STRATEGY
Paired Summarizing Read this section
silently. In pairs, take turns summarizing the material. Stop to discuss ideas
that seem confusing.
Have students write an explanation of this sentence. (When cells
divide, they are multiplying. Some
students may point out that multiplying a number by a fraction is the
same as division.)
The Cell Cycle
In the time that it takes you to read this sentence, your body
will have made millions of new cells! Making new cells
allows you to grow and replace cells that have died.
The environment in your stomach is so acidic that the cells
lining your stomach must be replaced every few days. Other
cells are replaced less often, but your body is constantly
making new cells.
The Life of a Cell
As you grow, you pass through different stages in life. Your
cells also pass through different stages in their life cycle. The
life cycle of a cell is called the cell cycle.
The cell cycle begins when the cell is formed and ends when
the cell divides and forms new cells. Before a cell divides, it
must make a copy of its deoxyribonucleic acid (DNA). DNA is
the hereditary material that controls all cell activities, including
the making of new cells. The DNA of a cell is organized into
structures called chromosomes. Copying chromosomes ensures
that each new cell will be an exact copy of its parent cell. How
does a cell make more cells? It depends on whether the cell is
prokaryotic (with no nucleus) or eukaryotic (with a nucleus).
Making More Prokaryotic Cells
cell cycle the life cycle of a cell
chromosome in a eukaryotic cell,
one of the structures in the nucleus
that are made up of DNA and protein; in a prokaryotic cell, the main
ring of DNA
Prokaryotic cells are less complex than eukaryotic cells are.
Bacteria, which are prokaryotes, have ribosomes and a single,
circular DNA molecule but don’t have membrane-enclosed
organelles. Cell division in bacteria is called binary fission,
which means “splitting into two parts.” Binary fission results
in two cells that each contain one copy of the circle of DNA. A
few of the bacteria in Figure 1 are undergoing binary fission.
v--------------------------g
Making Models Have pairs of
students use string for the cell
membrane and pieces of pipe
cleaners for chromosomes to
demonstrate the basic steps
of mitosis, as described in this
section. l Visual/Interpersonal
Figure 1 Bacteria reproduce
by binary fission.
CHAPTER RESOURCES
Chapter Resource File
CRF
• Lesson Plan
• Directed Reading A b
• Directed Reading B s
Technology
Transparencies
• Bellringer
• The Cell Cycle
Workbooks
Math Skills for Science
• Multiplying Whole Numbers b
• Grasping Graphing g
98
Chapter 4 • The Cell in Action
CONNECTION to
Math ---------------------------------------------------------------b
Cell Multiplication It takes Cell A 1 h to
complete its cell cycle and produce two
cells. The cell cycle of Cell B takes 2 h.
How many more cells would be formed
from Cell A than from Cell B in 6 h?
(After 6 h, Cell A would have formed 64 cells,
and Cell B would have formed 8 cells. Cell A
would have formed 56 cells more than Cell B.)
l Logical/Verbal
Eukaryotic Cells and Their DNA
Eukaryotic cells are more complex than prokaryotic
cells are. The chromosomes of eukaryotic cells contain more DNA than those of prokaryotic cells do.
Different kinds of eukaryotes have different numbers
of chromosomes. More-complex eukaryotes do not
necessarily have more chromosomes than simpler
eukaryotes do. For example, fruit flies have 8 chromosomes, potatoes have 48, and humans have 46.
Figure 2 shows the 46 chromosomes of a human body
cell lined up in pairs. These pairs are made up of similar chromosomes known as homologous chromosomes
(hoh MAHL uh guhs KROH muh SOHMZ).
READING ------------------g
STRATEGY
Prediction Guide Have students guess whether the following statement is true or false
before they read this page:
The number of chromosomes in
eukaryotic cells differs because
some organisms are more complex than others. (false)
✓
Reading Check Do more-complex organisms always
have more chromosomes than simpler organisms do?
(See the Appendix for answers to Reading Checks.)
Making More Eukaryotic Cells
Figure 2 Human body cells have 46
chromosomes, or 23 pairs of chromosomes.
The eukaryotic cell cycle includes three stages. In the first stage,
called interphase, the cell grows and copies its organelles and
chromosomes. After each chromosome is duplicated, the two
copies are called chromatids. Chromatids are held together at a
region called the centromere. The joined chromatids twist and
coil and condense into an X shape, as shown in Figure 3. After
this step, the cell enters the second stage of the cell cycle.
In the second stage, the chromatids separate. The complicated process of chromosome separation is called mitosis.
Mitosis ensures that each new cell receives a copy of each
chromosome. Mitosis is divided into four phases, as shown
on the following pages.
In the third stage, the cell splits into two cells. These cells
are identical to each other and to the original cell.
Have students evaluate their
answer after they read the
page. l Visual
Answer to Reading Check
homologous chromosomes
chromosomes that have the same
sequence of genes and the same
structure
mitosis in eukaryotic cells, a process of cell division that forms two
new nuclei, each of which has the
same number of chromosomes
No, the number of chromosomes is
not always related to the complexity
of organisms.
CONNECTION v
Math ----------------------------------------------------------------------------a
Graphing Cell Division Have
students make a bar graph to
represent the following information about different cell types:
• Cell Type A divides every 3 h
• Cell Type B divides every 6 h
Figure 3 This duplicated
chromosome consists of two
chromatids. The chromatids
are joined at the centromere.
Chromatids
Picking Apart Vocabulary
Brainstorm what words are
similar to the parts of the term
homologous chromosome.
What can you guess about the
meaning of the term’s root
words? Look up the roots of
the words, and explain how
they help describe
the concept.
Centromere
• Cell Type C divides every 8 h
If you start with one cell
of each type, how many A, B,
and C cells will be produced in a
24-hour period? On a graph,
compare the number of cells
produced by each type. (Cell A,
256; Cell B, 16; Cell C, 8) l Visual/
Logical
Answer to Connection
to Language Arts
Is That a Fact!
A fern called Ophioglossum reticulatum
has 1,260 chromosomes per cell, more
than any other organism.
Before sophisticated microscopes were
available, scientists could not see cells
pinching and dividing. Many scientists
believed that cells came into existence
spontaneously—as though crystallizing
out of bodily fluids.
The Greek root of the word homologous means “corresponding in
structure or character.” The roots
for the word chromosome means
“color” and “body.” Homologous
chromosome mean “chromosomes
that have the same structure.”
Section 3 • The Cell Cycle
99
Figure 4
Reteaching -------------------------------------b
Biography of a Cell
Have students write and
illustrate the biography
of a cell. It can be humorous or
serious, but it should include
accurate descriptions of how
materials are transported into
and out of the cell and how cells
reproduce. l Visual/Verbal
Writing
The Cell Cycle
Copying DNA (Interphase)
Mitosis Phase 1 (Prophase)
Before mitosis begins, chromosomes are copied. Each chromosome is then two chromatids.
Mitosis begins. The nuclear membrane dissolves. Chromosomes
condense into rodlike structures.
Mitosis Phase 2 (Metaphase)
The chromosomes line up along
the equator of the cell. Homologous chromosomes pair up.
Quiz ---------------------------------------------------------------------g
1. What is cell division? (It is the
process by which cells reproduce
themselves.)
2. How do prokaryotic cells
make more cells? (binary
fission)
Mitosis and the Cell Cycle
3. How do eukaryotic cells
make more cells? (mitosis
and cytokinesis)
cytokinesis the division of the
Figure 4 shows the cell cycle and the phases of mitosis in
cytoplasm of a cell
an animal cell. Mitosis has four phases that are shown and
described above. This diagram shows only four chromosomes
to make it easy to see what’s happening inside the cell.
Cytokinesis
Alternative
Assessment ---------------------------g
Mitosis and Cancer
Have students research
the role of mitosis in
cancer and write a report or
create a poster or other visual
presentation on what they
learn. Students’ reports should
include information about various cancer treatments, such as
radiation, chemotherapy, and
surgery. l Verbal/Visual
Writing
Cell plate
Figure 5 When a plant cell
divides, a cell plate forms and
the cell splits into two cells.
Answer to Reading Check
During cytokinesis in plant cells,
a cell plate is formed. During
cytokinesis in animal cells, a cell
plate does not form.
In animal cells and other eukaryotes that do not have cell
walls, division of the cytoplasm begins at the cell membrane.
The cell membrane begins to pinch inward to form a groove,
which eventually pinches all the way through the cell, and
two daughter cells form. The division of cytoplasm is called
cytokinesis and is shown at the last step of Figure 4.
Eukaryotic cells that have a cell wall, such as the cells of
plants, algae, and fungi, reproduce differently. In these cells,
a cell plate forms in the middle of the cell. The cell plate contains the materials for the new cell membranes and the new
cell walls that will separate the new cells. After the cell splits
into two, a new cell wall forms where the cell plate was. The
cell plate and a late stage of cytokinesis in a plant cell are
shown in Figure 5.
✓
Reading Check What is the difference between cytokinesis in
an animal cell and cytokinesis in a plant cell?
INCLUSION
Strategies
• Developmentally Delayed • Hearing Impaired
• Learning Disabled
Make a three-column table with these
column headings: Characteristics,
Prokaryotic Cells, and Eukaryotic Cells.
Under Characteristics, use these five row
headings: Small or large?, Complex or simple?, More or less DNA?, Has organelles?,
and Number of stages in cell division.
Have students copy the table and fill
it in. l Logical/Visual
100
Chapter 4 • The Cell in Action
INTERNET v
Sequence Board -------------g
For an internet activity related to this
chapter, have students go to go.hrw.com
and type in the keyword HL5ACTW.
Answers to Section Review
Mitosis Phase 3 (Anaphase)
The chromatids separate and
move to opposite sides of
the cell.
Review
Summary
•
A cell produces more
cells by first copying its
DNA.
cells produce
• Eukaryotic
more cells through the
four phases of mitosis.
produces two
• Mitosis
cells that have the same
•
•
number of chromosomes as the parent cell.
At the end of mitosis,
a cell divides the cytoplasm by cytokinesis.
In plant cells, a cell
plate forms between the
two new cells during
cytokinesis.
Mitosis Phase 4 (Telophase)
Cytokinesis
A nuclear membrane forms
around each set of chromosomes,
and the chromosomes unwind.
Mitosis is complete.
In cells that lack a cell wall, the
cell pinches in two. In cells that
have a cell wall, a cell plate forms
between the two new cells.
Using Key Terms
Critical Thinking
1. In your own words, write a definition for each of the following
terms: cell cycle and cytokinesis.
7. Applying Concepts How does
mitosis ensure that a new cell is
just like its parent cell?
Understanding Key Ideas
2. Eukaryotic cells
a.
b.
c.
d.
6. Predicting Consequences
What would happen if cytokinesis occurred without mitosis?
8. Making Comparisons Compare
the processes that animal cells
and plant cells use to make new
cells. How are the processes
different?
do not divide.
undergo binary fission.
undergo mitosis.
have cell walls.
3. Why is it important for chromosomes to be copied before cell
division?
4. Describe mitosis.
Developed and maintained by the
National Science Teachers Association
Math Skills
5. Cell A takes 6 h to complete
division. Cell B takes 8 h to
complete division. After 24 h,
how many more copies of cell A
would there be than cell B?
For a variety of links related to this
chapter, go to www.scilinks.org
Topic: Cell Cycle
SciLinks code: HSM0235
1. Sample answer: The cell cycle
describes all of the stages a cell
goes through in its life.
Cytokinesis is the last stage of
cell reproduction when a cell’s
cytoplasm is split between the
two new cells.
2. c
3. Chromosomes need to be copied
so that the two new cells have
the same genetic material as the
parent cell.
4. Sample answer: Before mitosis
begins, the chromosomes are
copied. In phase 1, the nuclear
membrane dissolves, and the
chromosomes condense. In
phase 2, the chromosomes line
up along the equator of the cell,
and homologous chromosomes
pair up. In phase 3, the chromatids move to opposite sides of
the cell. In phase 4, a nuclear
membrane forms around the
chromosomes, and the chromosomes unwind.
5. 8
6. If cytokinesis occurred without
mitosis, each cell would only
have half of the parent cell’s
genetic material or less.
7. Mitosis ensures that each new
cell receives a copy of each
chromosome, and hence, an
exact copy of the parent cell’s
genetic material.
8. The processes of animal and
plant cells are different because
plant cells have cell walls.
Cytokinesis is different in plant
cells, but all other stages of mitosis are essentially the same as
they are in animal cells.
CHAPTER RESOURCES
Chapter Resource File
CRF
• Section Quiz g
• Section Review g
• Vocabulary and Section Summary g
• Reinforcement Worksheet b
Workbooks
Science Skills
• Researching on the Web b
• Organizing Your Research g
Section 3 • The Cell Cycle 101