Biology 211 Intro Molecular and Cell Biology
Lecture 21and 22 Mitosis and Cell Cycle
Reading: Campbell et al., Chap. 12, skip pp. 213-220, but do look at Figs. 12.8,
12.9 and 12-13.
Outline:
1. Cell cycle
2. Chromosome structure
3. Mitosis
4. Regulation of the cell cycle and cancer
Lecture:
The ability of organisms to reproduce is one of the distinguishing properties of
life. Cells reproduce in order to propagate the species (unicellular organism), to
grow larger and to replace dead or damaged cells.
1. Cell cycle: The life of a cell from from its origin until it divides into two cells.
Figure 12.4
MITOTIC PHASE: Cell division, 10% of cell cycle
Mitosis--Division of the nucleus.
Cytokinesis--Division of the cytoplasm
INTERPHASE: Time between cell divisions, 90% of cell cycle
G1 phase--First gap, cell grows and produces more proteins
S phase--DNA is synthesized
G2 phase--Second gap, cell continues to grow
2. During the cell division phase, the chromosomes are evenly distributed
between two cells (mitosis) and the cell contents are evenly distributed between
daughter cells (cytokinesis).
Changes in chromosome condensation during the cell cycle.
A. In eukaryotic cells, the DNA is associated with proteins (histones) to form
chromatin.
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B. Prior to cell division, DNA synthesis occurs and the chromatin condenses to
form chromosomes. The number of chromosomes is a unique property of each
species; humans have 46 chromosomes.
Parts of a chromosome:

Sister chromatids: Two halves of a duplicated chromosome.

Centromere: Constriction on chromosome where the duplicated
chromosomes are attached to each other.

Kinetochore: Proteins at the centromere that the chromosome uses to
attach to the mitotic apparatus.
Mitotic spindle:

Structure that assembles to separate the chromosomes during mitosis.

Made of microtubules: Long filament formed from tubulin protein
subunits.

Poles are organized by centrosomes.
3. Mitosis--Series of events in the nucleus of cells that divides the genetic
material evenly between daughter cells.
Distinct subphases: Figure 12.5
A. prophase--Chromosomes condense and become visible. Mitotic spindle
(microtubule structure) begins to form.
B. metaphase--Nuclear envelope breaks down. Chromosomes attach to the
spindle and line up in the middle of the cell (=metaphase plate).
C. anaphase--Centromeres divide and daughter chromosomes migrate to
opposite poles. Chromosome movement is due to depolymerization of
microtubules and motor proteins carrying chromosome toward ends of
microtubules.
D. telophase--Daughter nuclei reform, chromosomes become more dispersed,
and cytokinesis (cytoplasmic division) occurs.
Cytokinesis


Animal cells: Cleavage furrow forms between daughter cells.
Plant cells: Cell plate forms between daughter cells.
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4. Regulation of the cell cycle and cancer

Frequency of cell division varies with the type of cell.
Some cells do not divide at all in adults.

Cell division is tightly controlled at "checkpoints".
For example, a critical checkpoint is at the boundary between G1 and S. Many
cells "exit" the cell cycle switching to a non-dividing state called G0. Cells that
are in G0 do not divide unless environmental signals are received such as growth
factors.

Cancer cells have escaped from cell cycle controls.
Normal cells
Cell division controlled
Too many cells--growth stops
Cancer cells
Cell division out of control
Too many cells--growth continues;
growth not density inhibited
Cells escape from their normal location
and spread throughout the
body=metastasis
Often abnormal numbers of
chromosomes
Mutations in several genes for proteins
involved in cell growth
DNA damage not repaired
Cells in the correct location
Normal number of chromosomes
Normal proteins controlling cell growth
Repair of DNA damage
Progression of a normal cell to a cancer cell:
1. Single cell is exposed to radiation or chemicals.
2. Damage to DNA is not repaired or cell has a defect in the enzymes of DNA
repair.
3. Damage alters some genes for proteins controlling cell growth (these are
often called oncogenes). Usually 2-5 genes are altered.
4. Cells beginning to divide when they should be resting. Too many cells are
formed. A tumor is a mass of abnormal cells in normal tissues.
5. If tumor remains at the original site and does not spread, it is a benign tumor
and usually can be removed by surgery.
6. If cells leave the site of the original tumor, it becomes metastatic, spreading
to other parts of the body. Cancers that spread are more likely to be malignant
(may lead to death).
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