8 MITOSIS
CHAPTER OUTLINE
Cell Division (p. 156)
8.1
8.2
8.3
8.4
Prokaryotes Have a Simple Cell Cycle (p. 156; Fig. 8.1)
A. Cell division takes place in two stages in prokaryotes: DNA is copied, then the cell splits in a
process called binary fission.
B. Each daughter cell contains one circular copy of the parent cell's DNA and is a functioning
cell.
Eukaryotes Have a Complex Cell Cycle (p. 157)
A. In eukaryotes, cell division, along with replication of segments of DNA called chromosomes,
is more complex.
B. Somatic, or body cells undergo mitosis, while germ cells in reproductive organs undergo
meiosis. The life cycle of a cell is called the cell cycle.
1. The first, or G1 phase, is the primary growth phase and takes up most of the life span of
the cell.
2. The S, or synthesis phase follows, during which DNA is replicated.
3. Next is the G2 phase when the cell readies itself for cell division.
4. These three phases together are known as interphase.
5. Mitosis (M phase) follows next, during which the nucleus and chromosomes of the cell
are divided.
6. During cytokinesis (C phase) the cytoplasm is cleaved, resulting in two daughter cells.
Chromosomes (p. 158; Figs. 8.2, 8.3, 8.4)
A. Discovery of Chromosomes
1. Chromosomes were first observed by Walter Fleming in 1882 while watching the rapidly
dividing cells of salamander larvae.
B. Chromosome Number and Homologous Chromosomes
1. Eukaryotic somatic cells have two copies of each chromosome, known as homologous
chromosomes.
2. When cells have both copies of each type of chromosome, they are called diploid; when
cells have only one of each type, such as after meiosis, they are called haploid.
3. Before cell division, each homologue replicates into two identical copies called sister
chromatids.
4. In duplicated form, eukaryotic chromosomes appear as an “X” still attached in the
middle at a special linkage site called the centromere.
5. Humans have 46, or 23 pairs, of chromosomes; in the duplicated condition, there are 92
chromatids in a human somatic cell.
C. Chromosome Structure
1. Chromosomes are composed of chromatin, a complex of DNA and protein.
D. Chromosome Coiling
1. Eukaryotic DNA is divided into several chromosomes.
2. Eukaryotic DNA is associated with proteins called histones, which help hold the shape of
the large DNA molecules and coil it into tightly compacted chromosomes.
3. A core of eight histones with its coiled DNA forms a nucleosome.
4. Nucleosomes are further coiled up in several ways, until the chromatin becomes highly
condensed and coiled, forming a chromosome.
Cell Division (p. 160; Figs. 8.5, 8.6, 8.7)
A. Interphase
1. During interphase, chromosomes replicate and begin condensation.
B. Mitosis
1. The first stage of mitosis is prophase, during which condensation of chromosomes and
nuclear membrane breakdown occur; centrioles (if present) move toward the poles, and
the mitotic spindle begins to form.
2.
3.
Metaphase occurs second, when chromosomes are aligned at the center of the cell.
Anaphase, the third phase of mitosis, occurs when sister chromatids are separated and
pulled toward the poles.
4. The last phase is telophase, during which the mitotic spindle is disassembled, the nuclear
envelope reforms, and chromosomes decondense.
C. Cytokinesis
1. In animal cells, mitosis is followed immediately by cytokinesis when a cleavage furrow
forms, splitting the cytoplasm between two daughter cells.
2. In plant cells, a cell plate forms and separates the cytoplasm of daughter cells.
D. Cell Death
1. Human cells divide only up to 50 times, after which they are programmed to die.
2. Cancer cells differ in that they can divide rapidly and indefinitely.
8.5
Controlling the Cell Cycle (p. 163; Figs. 8.8, 8.9, 8.10, 8.11)
A. The complex cell cycle of eukaryotes is controlled by feedback at three checkpoints.
1. Cell growth is assessed at the G1 checkpoint.
2. DNA replication is assessed at the G2 checkpoint.
3. Mitosis is assessed at the M checkpoint.
Cancer and the Cell Cycle (p. 166)
8.6
What Is Cancer? (p. 166; Fig. 8.12)
A. Cancer is unrestrained cell proliferation caused by damage to genes regulating cell division.
B. A tumor, or cluster of undifferentiated cells, may result.
C. The cancerous growth may metastasize, forming new tumors at distant sites.
8.7
Cancer and Control of the Cell Cycle (p. 166: Fig. 8.13)
A. The p53 gene plays a key role in the G1 checkpoint of cell division.
B. Mutations disabling key elements of the G1 checkpoint are associated with many cancers.
KEY TERMS
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binary fission (p. 156) Prokaryotes carry out this simple process of splitting into two cells.
chromosome (p. 157) A eukaryotic chromosome is a single DNA molecule with associated proteins.
mitosis (p. 157) The type of cell division used for growth and repair, mitosis occurs in somatic cells.
complex cell cycle (p. 157) This consists of the G1 phase, S phase, G2 phase, mitosis, and cytokinesis.
G1 phase (p. 157)
G2 phase (p. 157)
prophase (p. 160)
metaphase (p. 161)
anaphase (p. 161)
telophase (p. 161)
somatic cells (p. 155)
meiosis (p. 157)
germ cells (p. 155)
cytokinesis (p. 159) This occurs with a cleavage furrow in animal cells and a cell plate in plant cells.
homologous chromosomes (p. 158) Two copies of the same chromosome, each coming from a
different parent.
sister chromatids (p. 158)
centromere (p. 158)
diploid (p. 158)
karyotype (p. 1562)
histones (p. 159) Proteins with positive charges around which the DNA molecule coils.
chromatin (p. 159)
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nucleosome (p. 159)
G0 phase (p. 164)
growth factor (p. 164)
cancer (p. 166) Simply put, cancer is a growth disorder of cells.
tumor (p. 166)
metastases (p. 166) The spread of cancer cells through the body.
P53 protein (p. 166)
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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3.
The Mechanics of Mitosis. Give each student in the class a length of string to represent a nuclear
membrane and two (or more) pairs of pipe cleaners. Two individual pipe cleaners represent two
chromosomes in normal condition; the second set of two can each be twisted together with one of the
first pair to represent chromosomes in duplicated condition. Using a suitable diagram on an overhead
projector or chalkboard, have students manipulate the “chromosomes” through the various stages of
mitosis. Something about actually going through the motions of manipulating cell parts in this way
helps students remember the stages of mitosis.
Cancer and Its Causes. Discuss with students what types of things cause cancer, including the
mutagens in cigarette smoke, inadequate nutrition or fats in the diet, mutagenic chemicals, and
ultraviolet radiation. Relate mutations in DNA to losing control over the cell cycle and having more
rapid than normal rates of cell division, producing numerous immature cells. End your discussion with
ideas on how students could cut their chances of developing cancer, based especially on behavior
modification.
How Cancer Chemotherapy Works. Discuss with your students how cancer chemotherapy works and
tie it in with the cell cycle and mitosis. Many different forms of chemotherapy are available, each
tailored to work on specific types of cancer in the body. Chemotherapy operates by specifically killing
cells when they are dividing. Since cancer cells divide much more rapidly than normal body cells,
chemotherapy drugs should kill the cancer out before they kill too many of the body's normal cells.
Cells of the body that are killed by chemotherapy are those that the body replaces frequently, such as
the lining of the small intestine or the lining of the mouth and throat. Thus, chemotherapy patients
often suffer intestinal disturbances plus dry or sore mouths and throats, among other side effects of the
treatment.
CRITICAL THINKING QUESTIONS
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2.
What are the possible consequences of an inaccurate process of mitosis during growth in a
multicellular organism?
Devise an evolutionary explanation for why eukaryotes, such as humans, have two copies of each
chromosome.