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The Cell Cycle
Chapter 12
p. 218-228
Cell Division
 An important part of a
cell’s life cycle is
reproduction
 Cell Division: division
of an existing cell into 2
genetically identical
daughter cells
3 functions:
 1) reproduction,
 2) growth and
development
 3) tissue repair/renewal
Reproduction, Growth & Repair
1) Reproduction
Asexual: produces 2 identical cells (Mitosis)
Sexual: combines DNA from 2 parents to form
new offspring (Meiosis – Ch 13)
2) Growth
Development of organism from 1 cell
3) Repair
Occurs throughout life
i.e. RBC’s
Distribution of Chromosomes
 For most of cell cycle,
DNA is packaged
w/protein in chromatin
(long, thin fiber)
 During Mitosis:
1) Identical copy of
DNA is made
2) DNA condenses into
chromosomes (46)
 Composed of 2
identical sister
chromatids, connected
by centromere
3) Sister chromatids pull apart to opposite sides
of cell
4) Cell splits, forming 2 new identical daughter
cells (46 chromos. = diploid)
During Meiosis, each parent supplies1/2
chromos. needed (23 chromos. = haploid))
Occurs in reproductive organs (ovaries/testes)
Cells must divide twice to cut chromosome number in
half
The Cell Cycle: an overview
 Most of cell’s life
spent in Interphase
(mitosis very brief)
Grow, produce proteins
& organelles
 3 Phases:
1) G1: grows to full size
2) S: replicates DNA
3) G2: prepares for
mitosis
Phases of Mitosis: an overview
 1) Prophase
DNA condenses
Nucleolus disappears
Spindle begins to form
 2) Prometaphase
Nuclear envelope
breaks apart
Spindles attach to
kinetochore
 3) Metaphase
Chromosomes line up
on metaphase plate
 4) Anaphase
Sister chromatids pulled
apart to opposite sides
of cell
 5) Telophase
2 new nuclear
envelopes form
Cytoplasm divides
(cytokinesis), forming
2 new daughter cells
Mitosis in action!!!
The Mitotic Spindle
 Consists of fibers of
microtubules & proteins
 Assembled in
centrosome at either end
(pole) of cell
 Attach to kinetochore of
each sister chromatid
 Align chromosomes along
metaphase plate down
center of cell
Cytokinesis
 “Division of
cytoplasm”
 Cleavage: “pinching
in” of plasma
membrane along
cleavage furrow
Plants develop cell
plate → new cell walls
Mitosis in Plant Cells
Binary Fission
 Prokaryotic cell
division
 DNA replicates, each
copy moves to cell
poles
 Plasma membrane
grows inward & cell
divides
Evolution of Mitosis
Regulation of the Cell Cycle
Chapter 12
p. 228-235
Regulation of Cell Cycle: an overview
Frequency of cell division depends on type
of cell
i.e.: skin (fast), nerve (almost never)
Cell Cycle Control System: directs
events of cell cycle
Built-in molecular “clock” that triggers each
phase
Regulated at internal/external checkpoints
Cell Cycle Checkpoints
 Occur at G1, G2, M
phases
Stop cycle until
requirements for next
phase are met
If not, cell exits cycle &
enters G0 phase (nondividing state)
G1 Checkpoint:
“restriction point” if
clear, cell will usually
complete cycle
Cell Cycle Clock
 Cycle is controlled by
bonding of certain protein
molecules in cell to each
other
 1) Cyclin-dependent kinases
(Cdks)
 2) Cyclin – conc. fluctuates
 Rises until M phase, then
drops off
 Together, make MPF
complex
 Initiates mitosis by
phosphorylating certain
proteins
 Turns off by destroying cyclin
Internal Signals
Occur at anaphase
Will not proceed until all kinetochores are
properly attached to spindles
Anaphase Promoting Complex (APC):
becomes active, cyclin breaks down,
chromatids separate
External Signals
 Most cells depend on
external signals to control
cell division
 1) Growth Factors: proteins
that stimulate other cells to
divide
 i.e.: PDGF: stimulates
platelet growth after injury
(signal-transduction pthwy)
 2) Density-Dependent
Inhibition: cells stop dividing
if become crowded
 3) Anchorage Dependence:
animal cells must be attached
to substratum to divide
Cancer Cells Escape Cycle Controls
 Not affected by growth
factors, density, or
anchorage
 1) Normal cell undergoes
transformation → cancer cell
 2) Cancerous cell reproduces
to form tumor
 3) If remains in place →
benign
 If spreads & impairs organs
→ malignant
 May enter bloodstream, have
abnormal # chromosomes,
affect metabolism
 Spread quickly
(“metastasis”); may require
radiation or chemotherapy