How Cells Divide
Chapter 6
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Outline
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Simple Cell Cycle
Complex Cell Cycle
Chromosomes
Mitosis
 Cancer
Meiosis I
Meiosis II
Evolutionary Consequences of Sex
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Simple Cell Cycle
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Cell division in bacteria takes place in two
stages (Simple Cell Cycle).
 DNA is copied
 Cell Splits (Binary Fission)
- Forms two daughter cells
Heredity information in bacteria encoded in
single circle of DNA.
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Complex Cell Cycle
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Eukaryotic DNA is contained in linear
chromosomes.
 Long DNA molecules packaged with
proteins.
Mitosis - Mechanism of cell division occurring
in non-reproductive (somatic) cells.
Meiosis - Mechanism of cell division
occurring in reproductive (germ) cells.
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Complex Cell Cycle
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G1 phase - Primary growth phase
S phase - DNA replicates
G2 phase - Microtubule synthesis
M phase - Chromosomes pull apart
C phase - Cytokinesis
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Complex Cell Cycle
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Chromosomes
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Human cells each have 23 nearly identical
pairs of chromosomes, for a total of 46.
 Each chromosome contains thousands of
genes that play important roles in body
development and function.
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Chromosome Structure
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DNA helix is wrapped around proteins with
positive charges (histones) and negative
charges counteracting each other.
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Chromosome Structure
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Homologues - Nearly identical copies of the
same chromosomes.
Diploid Cells - Two copies of chromosomes.
 Before cell division, each homologue
replicates, resulting in two identical copies
(sister chromatids).
- Remain attached at centromere.
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Cell Division
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Interphase - Chromosomes replicate and
begin to wind up tightly.
Mitosis
 Prophase - Nuclear envelope breaks
down, DNA further condenses, and spindle
fibers form.
 Metaphase - Chromosomes align at center
of cell and kinetochore fibers attach to
centromeres.
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Mitosis
Anaphase - Centromeres replicate and
sister chromatids separate and move
towards opposite poles.
 Telophase - Nuclear envelope reappears
and chromosomes decondense.

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Cytokinesis
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Cytokinesis - Division of cytoplasm.
 Animal cells - Cell pinches in two with
contracting belt of microtubules, forming a
cleavage furrow.
 Plant cells - Membrane assembled at right
angle to mitotic spindle. Cell plate grows
outward until it reaches the interior surface
of plasma membrane.
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Controlling Cell Cycle
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At critical points, further cell progress
depends on a central set of switches
regulated by cell feedback.
 G1 – Cell growth assessed
 G2 DNA replication assessed
 M mitosis assessed
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Cancer
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Cancer - Unrestrained cell growth and
division.
 Tumor - Cluster of cells.
- Benign - Encapsulated and noninvasive.
- Malignant - Not encapsulated, invasive,
and shed cells.
 Metastases - Process of cells
shedding from a malignant tumor and
spreading to distant parts of the body.
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Cell Cycle Control
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Gene p53 plays a key role in G1 checkpoint
of cell division.
 Gene’s product monitors integrity of DNA,
checking for successful replication.
- If protein detects damaged DNA, it halts
cell division and stimulates repair
enzymes.
 Nonfunctional p53 genes allow cancer
cells to repeatedly divide.
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Curing Cancer
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Preventing cancer from starting by focusing
on decision-making process to divide.
 Receiving the signal to divide
 Relay switch
 Amplifying the signal
 Releasing the brake
 Checking that everything is ready
 Stepping on the gas
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Curing Cancer
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Preventing the Spread of Cancer
 Tumor growth
- Angiogenesis inhibition
 Metastasis
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Molecular Cancer Therapies
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Meiosis
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Gamete formation must involve some
mechanism to halve the number of
chromosomes.
 Two sets of chromosomes are present in
somatic cells of adults (diploid), but only
one set is present in gametes (haploid).
- Sexual Reproduction involves the
alteration of meiosis and fertilization.
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Alteration of Generations
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Stages of Meiosis
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Meiosis I
 Prophase I - Chromosomes pair up and
exchange segments (Crossing Over).
 Metaphase I - Chromosomes align at cell’s
center (Independent Assortment).
 Anaphase I - Homologous pairs pulled
apart.
 Telophase I - Individual chromosomes
gather at each of the poles.
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Crossing Over
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Meiosis II
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Mitotic division involving products of Meiosis I.
 Prophase II - Nuclear envelope breaks
down and spindle fibers form.
 Metaphase II - Spindle fibers bind to both
sides of centromeres.
 Anaphase II - Spindle fibers contract,
moving sister chromatids to opposite poles.
 Telophase II - Nuclear envelope reforms
around four sets of daughter cells.
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Evolutionary Consequences of Sex
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Genetic Diversity
 Independent Assortment
 Crossing Over
 Random Fertilization
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Review
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Simple Cell Cycle
Complex Cell Cycle
Chromosomes
Mitosis
 Cancer
Meiosis I
Meiosis II
Evolutionary Consequences of Sex
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