The Cell Cycle
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Transcript The Cell Cycle
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