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CHAPTER 6
Chapter 6
TABLE OF CONTENTS
Section 1 Chromosomes
Section 2 The Cell Cycle
Section 3 Mitosis and Cytokinesis
Chromosomes and Cell Reproduction
Chapter 6
Section 1 Chromosomes
OBJECTIVES
 Identify four examples of cell division in eukaryotes and one example in prokaryotes.
 Differentiate between a gene, a DNA molecule, a chromosome, and a chromatid.
 Differentiate between homologous chromosomes, autosomes, and sex chromosomes.
 Compare haploid and diploid cells.
 Predict how changes in chromosome number or structure can affect development.
Chapter 6
Section 1 Chromosomes
FORMATION OF NEW CELLS BY CELL DIVISION
 Cell division, also called cell reproduction, occurs in humans and other
organisms at different times in their life.
 The formation of gametes involves yet a special type of cell division. Gametes
are an organism’s reproductive cells, such as sperm or egg cells.
 When a cell divides, the DNA is first copied and then distributed.
Chapter 6
Section 1 Chromosomes
FORMATION OF NEW CELLS BY CELL DIVISION,
CONTINUED
Prokaryotic Cell Reproduction
 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.
Chapter 6
Section 1 Chromosomes
FORMATION OF NEW CELLS BY CELL DIVISION,
CONTINUED
Prokaryotic Cell Reproduction
 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.
 Eventually the dividing prokaryote is pinched into two independent cells.
Chapter 6
BINARY FISSION
Section 1 Chromosomes
Chapter 6
Section 1 Chromosomes
FORMATION OF NEW CELLS BY CELL DIVISION,
CONTINUED
Eukaryotic Cell Reproduction
 A gene is a segment of DNA that codes for a protein or RNA molecule.
 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.
Chapter 6
Section 1 Chromosomes
FORMATION OF NEW CELLS BY CELL DIVISION,
CONTINUED
Eukaryotic Cell Reproduction
 The two exact copies of DNA that make up each chromosome are called chromatids.
 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.
Chapter 6
Section 1 Chromosomes
CHROMOSOME STRUCTURE
Chapter 6
Section 1 Chromosomes
COMPARING CELL DIVISION IN PROKARYOTES AND
EUKARYOTES
Chapter 6
Section 1 Chromosomes
HOW CHROMOSOME NUMBER AND STRUCTURE
AFFECT DEVELOPMENT
Sets of Chromosomes
 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.
 The 46 chromosomes in human somatic cells are actually two sets of 23
chromosomes.
Chapter 6
Section 1 Chromosomes
HOW CHROMOSOME NUMBER AND STRUCTURE
AFFECT DEVELOPMENT, CONTINUED
Sets of Chromosomes
 When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid.
 When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid.
 The fusion of two haploid gametes—a process called fertilization—forms a diploid zygote. A zygote is a fertilized
egg cell.
Chapter 6
Section 1 Chromosomes
COMPARING HAPLOID AND DIPLOID CELLS
Chapter 6
CHROMOSOME
NUMBER OF
VARIOUS
ORGANISMS
Section 1 Chromosomes
Chapter 6
Section 1 Chromosomes
HOW CHROMOSOME NUMBER AND STRUCTURE
AFFECT DEVELOPMENT, CONTINUED
Sex Chromosomes
 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.
Chapter 6
Section 1 Chromosomes
HOW CHROMOSOME NUMBER AND STRUCTURE
AFFECT DEVELOPMENT, CONTINUED
Change in Chromosome Number
 Humans who are missing even one of the 46 chromosomes do not survive.
 Humans with more than two copies of a chromosome, a condition called trisomy, will not
develop properly.
 Abnormalities in chromosome number can be detected by analyzing a karyotype, a photo
of the chromosomes in a dividing cell that shows the chromosomes arranged by size.
Chapter 6
KARYOTYPE
Section 1 Chromosomes
Chapter 6
Section 1 Chromosomes
CHROMOSOME NUMBER
Chapter 6
Section 1 Chromosomes
HOW CHROMOSOME NUMBER AND STRUCTURE
AFFECT DEVELOPMENT, CONTINUED
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:
1. deletion mutation
2. duplication mutation
3. inversion mutation
4. translocation mutation
Chapter 6
Section 2 The Cell Cycle
OBJECTIVES
 Identify the major events that characterize each of the five phases of the cell cycle.
 Describe how the cell cycle is controlled in eukaryotic cells.
 Relate the role of the cell cycle to the onset of cancer.
Chapter 6
Section 2 The Cell Cycle
THE LIFE OF A EUKARYOTIC CELL
The Cell Cycle
 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.
Chapter 6
Section 2 The Cell Cycle
THE LIFE OF A EUKARYOTIC CELL, CONTINUED
The Cell Cycle
 The five phases of the cell cycle are:
1. First growth (G1) phase During the G1 phase, a cell grows rapidly
and carries out its routine functions.
2. Synthesis (S) phase A cell’s DNA is copied during this phase.
3. Second growth (G2) phase In the G2 phase, preparations are made
for the nucleus to divide.
4. Mitosis The process during cell division in which the nucleus of a cell is
divided into two nuclei is called mitosis.
5. Cytokinesis The process during cell division in which the cytoplasm
divides is called cytokinesis.
Chapter 6
Section 2 The Cell Cycle
CELL CYCLE G1 PHASE
Chapter 6
Section 2 The Cell Cycle
CELL CYCLE S PHASE
Chapter 6
Section 2 The Cell Cycle
CELL CYCLE G2 PHASE
Chapter 6
Section 2 The Cell Cycle
CELL CYCLE M PHASE
Chapter 6
Section 2 The Cell Cycle
CONTROL OF THE CELL CYCLE
 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).
Chapter 6
Section 2 The Cell Cycle
CONTROL OF THE CELL CYCLE, CONTINUED
 Control occurs at three principal checkpoints:
1. Cell growth (G1) checkpoint This checkpoint makes the decision of whether the cell will divide.
2. DNA synthesis (G2) checkpoint DNA replication is checked at this point by DNA repair enzymes.
3. Mitosis checkpoint This checkpoint triggers the exit from mitosis.
Chapter 6
Section 2 The Cell Cycle
CONTROL OF THE CELL CYCLE, CONTINUED
When Control Is Lost: Cancer
 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.
Chapter 6
Section 3 Mitosis and Cytokinesis
OBJECTIVES
 Describe the structure and function of the spindle during mitosis.
 Summarize the events of the four stages of mitosis.
 Differentiate cytokinesis in animal and plant cells.
Chapter 6
Section 3 Mitosis and Cytokinesis
CHROMATID SEPARATION IN MITOSIS
 During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the dividing cell
with the help of the spindle.
 Spindles are cell structures made up of both centrioles and individual microtubule fibers that are involved in
moving chromosomes during cell division.
Chapter 6
Section 3 Mitosis and Cytokinesis
CHROMATID SEPARATION IN MITOSIS, CONTINUED
Forming the Spindle
 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.
Chapter 6
Section 3 Mitosis and Cytokinesis
CHROMATID SEPARATION IN MITOSIS, CONTINUED
Separation of Chromatids by Attaching Spindle Fibers
 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.
 As soon as the chromatids separate from each other they are called chromosomes.
Chapter 6
Section 3 Mitosis and Cytokinesis
MITOSIS AND CYTOKINESIS
Mitosis
Step 1 Prophase The nuclear envelope dissolves and a spindle forms.
Step 2 Metaphase During metaphase the chromosomes move to the center of the cell and line up
along the equator.
Step 3 Anaphase Centromeres divide during anaphase.
Step 4 Telophase A nuclear envelope forms around the chromosomes at each pole. Mitosis is complete
Chapter 6
STAGES OF MITOSIS
Section 3 Mitosis and Cytokinesis
Chapter 6
MITOSIS
Section 3 Mitosis and Cytokinesis
Chapter 6
Section 3 Mitosis and Cytokinesis
MITOSIS AND CYTOKINESIS, CONTINUED
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.
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE
The illustration below shows the events of the cell cycle. Use the figure below to
answer questions 1–3.
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE, CONTINUED
1.
For about what proportion of the cell cycle is the cell in interphase?
A.
less than 1/2
B.
less than 3/4
C.
more than 3/4
D.
more than 9/10
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE, CONTINUED
1.
For about what proportion of the cell cycle is the cell in interphase?
A.
less than 1/2
B.
less than 3/4
C.
more than 3/4
D.
more than 9/10
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE, CONTINUED
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
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE, CONTINUED
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
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE, CONTINUED
3.
If the phase during which DNA is synthesized did not occur, what effect would it have on the cell cycle?
A.
Mitosis would immediately follow S.
B.
Cytokinesis would immediately follow G2.
C.
G1 would immediately follow mitosis.
D.
G2 would immediately follow G1.
Chapter 6
Standardized Test Prep
MULTIPLE CHOICE, CONTINUED
3.
If the phase during which DNA is synthesized did not occur, what effect would it have on the cell cycle?
A.
Mitosis would immediately follow S.
B.
Cytokinesis would immediately follow G2.
C.
G1 would immediately follow mitosis.
D.
G2 would immediately follow G1.