THE CELL CYCLE
The life cycle of a cell is divided into two major portions that include interphase and a mitotic
phase. Remember that the process of cell division is continuous. It is only divided into stages
for convenience and to help you learn. See Fig 3.35, page 108, which illustrates the cell cycle
as a continuum.
A. Interphase = cell growth and DNA replication. See Fig 3.36a, page110.
1. Not considered part of mitosis
2. Represents the majority of a cell's life and includes:
a. cell growth and
b. duplication of DNA prior to prophase
3. Interphase is divided into 3 parts:
a. G1 = rapid growth and replication of centrioles.
b. S = growth and DNA replication.
c. G2 = growth and final preparations for cell division.
B. MITOTIC PHASE (M) = the mitotic phase (M) is divided into 2 parts that include
mitosis and cytoplasmic division (cytokinesis).
Mitosis = division of nuclear parts includes four parts:
1. Prophase: See Fig 3.36b, page 110.
a. Distinct pairs of chromosomes become apparent (tightly
coiled DNA and histone proteins)
b. Chromosomes pairs are composed of identical sister
chromatids, held together by centromeres.
c. Centriole pairs migrate to opposite ends of cell, and spindle
fibers form between them.
d. Nuclear envelope and nucleolus disintegrate.
2. Metaphase: See Fig 3.36c, page110.
a. Chromosomes line up in an orderly fashion midway between
the centrioles (i.e. along equatorial plate).
b. Centromere holding chromosome together attaches to
spindle fiber between centrioles.
3. Anaphase: See Fig 3.36d, page 110.
a. Centromere holding chromosome pair together splits/
separates.
b. Individual chromosomes migrate in opposite directions on
spindle fibers toward polar centrioles.
c. Cytokinesis begins.
4. Telophase: See Fig 3.36e, page 110.
a. Chromosomes complete migration toward centrioles.
b. Nuclear envelope develops around each set of
chromosomes.
c. Nucleoli develop and reappear.
d. Spindle fibers disintegrate.
e. Cleavage furrow is nearly complete.
5. Cytoplasmic Division (Cytokinesis; forming of 2 daughter cells)
a. Begins during anaphase, when the cell membrane begins to
constrict (pinch) around daughter cells.
b. Is completed at the end of telophase when nuclei and
cytoplasm of two newly formed daughter cells (in interphase)
are completely separated by cleavage furrow.
CONTROL OF CELL DIVISION
A. Significance
1. to form a multi-celled organism from one original cell
2. growth of organism
3. tissue repair
B. Length of the Cell Cycle
1. varies with cell type, location, and temperature
2. Average times are 19-26 hrs.
3. Neurons, skeletal muscle, and red blood cells do not reproduce.
C. Abnormal Cell Division (CANCER)
1. When cell division occurs with no control (goes awry), a tumor, growth, or
neoplasm results.
2. A malignant tumor is a cancerous growth; a non-cancerous tumor is a
benign tumor;
3. Malignant tumors may spread by metastasis to other tissues by direct
invasion, or through the bloodstream or lymphatic system.
4. Oncology is the study of tumors; an oncologist is a physician who treats
patients with tumors.
CELL DEATH
A. Apoptosis = programmed cell death; a fast, orderly contained destruction
that packages cellular remnants into membrane-enclosed pieces that are
then removed.
1. is a normal part of development
2. sculpts organs from tissues that naturally overgrow – important in fetal
development
3. is protective – peeling skin after sunburn to prevent possible cancer
4. is continuous and step-wise, like mitosis:
a. “Death receptor” on doomed cell cell’s membrane receives a signal
to die.
b. Enzymes called caspases are activated and destroy the various cell
components.
c. The dying cell’s shape becomes round as it is cut-off from other
cells.
d. Its cell membrane undulates forming bulges or blebs.
e. The nucleus bursts and mitochondria decompose.
f. The cell shatters (within one hour of caspases release).
g. Inflammation is prevented because fragments are membrane
encapsulated.
B. Necrosis = disordered form of cell death associated with inflammation and
injury.