The Cell Cycle
October 12, 2005
Cell Division
Functions in
Reproduction,
Growth, and
Repair
Important Terms (part one)
Genome = all of a cell’s DNA
Chromosome = individual DNA molecule
Chromatin = chromosomal DNA and
its protein scaffold
Sister chromatids = the two identical
copies of a duplicated chromosome
Centromere = the point of attachment for two sister chromatids
Somatic cells = (most cells) have two copies of each chromosome
(diploid)
Gametes = (reproductive cells; sperm and eggs) have one copy of each
chromosome (haploid)
More Important Terms
Cell cycle = the ordered sequences of
events involving cell division
Interphase = the main part of the cell
cycle:
G1 phase - cell prepares for
DNA synthesis, usually grows
in size
S phase - DNA is duplicated
G2 phase - cell checks fidelity of DNA replication, prepares for
nuclear division
Mitosis (M phase) = nuclear division
Mitosis subphases - prophase, prometaphase, metaphase, anaphase,
telophase
Cytokinesis = division of cytoplasm
Mitosis (1)
Mitosis (2)
The Mitotic Spindle
The mitotic spindle
is composed of
microtubules, and
directs the alignment
of chromosomes on
the metaphase plate.
During anaphase, the
microtubules attached
to each centromere to
guide the movement
of chromosomes
toward spindle poles.
Polar Depolymerization
of Microtubules and
Movement of NonKinetochore
Microtubules
Cytoplasmic Division
Microfilaments
contract to divide
the cytoplasm. Plant
cells are less
flexible, because of
the cell wall, and
deposit cell wall
material between the
two daughter cells.
Mitosis (differential staining)
Mitosis in Growing Tissue
Binary
Fission in
Bacteria
Evolution
of Mitosis
Cell Cycle Control
A growth signal leads to
passage through the G0
restriction point into G1.
The cell then executes a
programmed series of
steps that culminate in
mitosis and cytokinesis.
The cell’s place in the
cell cycle is signaled by
the presence or absence
of cell cycle kinases.
MPF is an example of kinase
control of the cell cycle. It is
like a license that allows the
cell to begin M phase of the
cell cycle.
MPF is a two-subunit enzyme.
One subunit is Cdk (present
during the whole cell cycle).
The other subunit is a cyclin
protein that builds up as M
phase approaches.
When the cyclin concentration
is high enough, cyclin binds to
Cdk, forming the MPF
enzyme. MPF activity signals
the start of mitosis.
Cancer
Normal cells are transformed to
tumor cells when cell division
is not controlled.
Cancer cells are growth-factor
independent.
Cancer cells have escaped
density-dependent inhibition.
Most cancerous cells are
destroyed by the immune
system, but some cells progress
from benign to malignant, and
possibly to metastatic.
Cell Culture and Growth Factors
Density-dependent
Inhibition
Density-dependent inhibition
is a property of normal cells.
Cancer cells lack this inhibition.
Metastatic Cancer
Metastatic cancer cells
spread throughout the
body.
Summary
Regulation of the cell cycle is largely through the action of kinases,
often associated with regulatory subunits called cyclins.
The cell cycle is controlled at major
checkpoints that will block:
1. During G1 prior to the entry to S stage
2. During S and G2 prior to the entry to
M stage
3. During the M stage prior to the end of
mitosis
DNA damage or incomplete replication is a major reason the cell
cycle is blocked. Failure to correct the problem usually leads to cell
death. If cells do not die, the uncontrolled cell division may lead to
tumors.