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THE CELL CYCLE
Chapter 8
A. The Cell Cycle
Events that occur in the life of a cell.
Includes 3 major stages:
 Interphase
 Karyokinesis (mitosis)
 Cytokinesis
1. Interphase
Cell is not dividing.
 G1
Phase - carries out basic functions
& performs specialized activities.
 duration is extremely variable
 contains restriction checkpoint ~
cell “decides” to:
- divide
- enter a quiescent phase (G0)
- die
 G0
Phase - cell maintains specialized
characteristics, but does not divide.
Ex. neurons & muscle cells
S
Phase - cell
replicates
chromosomes &
synthesizes
proteins.
[animal cells replicate
centrioles as well]
 G2
Phase - cell
synthesizes
additional proteins
(ex. tubulin) &
assembles/stores
membrane material.
2. Karyokinesis (mitosis; M phase)
Equal distribution of replicated
genetic material.
 Prophase
 replicated
chromosomes
condense*
 centrosomes separate &
migrate toward opposite
sides of cell
 mitotic spindle forms
(microtubules grow out
from centrosomes)
 nucleolus
disappears
 Prometaphase
 nuclear
membrane
breaks down
 spindle fibers
attach to
centromeres of
chromosomes
 Metaphase
 chromosomes
are lined up
single-file along
equator of
mitotic spindle.
 Anaphase
 centromers
part,
sister
chromatids (now
called
chromosomes)
separate
 chromosomes
move toward
opposite poles
 Telophase
 mitotic
spindle
breaks down
 chromosomes
decondense
 nuclear membranes
reform around two
nuclei
 nucleoli reappear
3. Cytokinesis
Distribution of cytoplasm to daughter
cells.
 begins during anaphase or telophase
 differs in animal & plant cells
Cytokinesis in animal cells:
 cleavage
furrow (slight indentation)
forms around equator of cell.
 actin & myosin microfilaments act like
a drawstring to pinch the cell in two.
 usually an equal division.
Cytokinesis in
animal cells
Cytokinesis in plant cells:
 phragmoplast
(microtubule structure)
forms in cytoplasm & traps vesicles
containing cell wall material.
 vesicles fuse, forming a cell plate
across midline of cell.
 cell plate gives rise to two primary
cell walls.
Does cytokinesis always accompany
karyokinesis?
Karyokinesis in the absence of
cytokinesis results in a syncytium (mass
of multinucleated cells).
Control of the Cell Cycle
Checkpoints - groups of interacting
proteins that ensure cell cycle events
occur in the correct sequence.
Shortening of telomeres - loss of telomere
DNA signals cell to stop dividing.
Some cells produce telomerase (enzyme
that continually adds telomere DNA).
Contact Inhibition - healthy cells stop
dividing when they come in contact with
other cells.
Hormones - stimulate cell division.
Ex. Estrogen stimulates uterine cell division
Growth factors - proteins that
stimulate local cell division.
Ex. Epidermal growth factor (EGF)
stimulates epithelial cell division filling in
new skin underneath a scab
Interaction of kinases & cyclins activate genes that stimulate cell
division.
B. Apoptosis
Programmed cell
death; part of
normal
development.
Steps of Apoptosis:
C. Cancer (loss of cell cycle control)
Condition resulting from excess cell
division or deficient apoptosis.
Characteristics of Cancer Cells:
 can divide uncontrollably & eternally
 are heritable & transplantable
 lack contact inhibition
 readily metastasize
 exhibit angiogenesis
 exhibit genetic mutability
Causes of Cancer:
of oncogenes
Oncogenes are genes that trigger
limited cell division.
 Inactivation of tumor suppressor genes
Tumor suppressor genes prevent a cell
from dividing or promote apoptosis.
 Over-expression
Normal functioning of oncogenes &
tumor suppressor genes may be
affected by environmental factors:
 carcinogens
 radiation
 viruses
 diet
 exercise habits