I. The key roles of cell division
A. Reproduction, Growth, and Repair
B. Distributes identical sets of chromosomes to
daughter cells.
1. Genome = all the
genes in a cell; an
organism’s
genetic material
C. Terms you should know
1. Somatic cells =
2. Gametes =
3. Gene =
4. Chromatin =
5. Chromosome =
6. Sister Chromatids =
7. Centromere =
8. Mitosis =
9. Cytokinesis =
10. Meiosis =
11. Parent cell =
12. Daughter cells =
13. Centrosome =
14. Mitotic spindle =
15. Kinetochore =
16. Centriole =
II. The Mitotic Cell Cycle
A. An overview
1. Mitosis or the “Mitotic (M) Phase” includes mitosis and
cytokinesis -- this is the shortest part of the cell cycle.
2. Interphase -- 90% of the cycle. Divided into 3 subphases:
a. G1 phase (“first gap”) -- Cell grows (makes proteins and
organelles)
b. S phase (synthesis) -- chromosomes are copied (DNA
replication)
c. G2 phase (“second gap”) -- Cell grows (makes proteins
and organelles)
B. Mitosis : the phases
1. Prophase, Prometaphase, Metaphase, Anaphase, Telophase
C. Mitosis: a closer look…refer to book for details
G1, S, G2
Prophase
Prometaphase
D. The mitotic spindle distributes chromosomes to daughter cells
1. Made of microtubules and associated proteins (while
assembling, microtubules of cytoskeleton disassemble
providing material)
a. assembly takes place in the centrosome which also
contains the centrioles.
3. At the end of prophase some spindle microtubules attach
to kinetochores while others interact with microtubules
from opposite pole.
a. protein and specific sections of chromosomal DNA at the
centromere of a duplicated chromosome
4. During metaphase, the Metaphase plate is formed
a. plane midway between poles where chromosomes are
aligned.
5. During anaphase kinetochore microtubules shorten by
depolymerizing (hydrolysis)…see picture on next slide,
while nonkinetochore microtubules elongate the cell.
E. Cytokinesis: A closer look
1. In animal cell, it occurs by a process known as cleavage
a. cleavage furrow begins as a shallow groove in the cell
surface near the old metaphase plate.
b. on cytoplasmic side, there is a contractile ring of actin
microfilaments associated with myosin proteins
b. contraction of this ring is like pulling a drawstring until
parent cell is pinched in two.
Read about the
experiment that
was done to show
that the
shortening of the
microtubule
occurs at the
kinetochore, not
that pole. Pg. 235
2. Plant cells are different due to the cell wall
a. No cleavage furrow…produce a cell plate instead.
b. Vesicles, made in the golgi apparatus and containing cell
wall material, move along microtubules to middle of cell
where they grow together . This produces the cell plate.
c. The cell plate enlarges until the vesicle walls fuse with
the plasma membrane along the perimeter of the cell.
d. Two daughter cells result, each with own plasma
membrane and cell wall material between them.
E. Evolution of Mitosis
1. Given that prokaryotes preceded eukaryotes on Earth, it
is hypothesized that mitosis evolved from binary fission
“division in half”
a. bacteria reproduces this way
Bacterial chromosome is attached to
plasma membrane
Copies remain attached
III. Regulation of the cell cycle
A. Sequential events of the cell cycle are directed by a
distinct “cell cycle control system”
1. proceeds on its own, driven by a built-in clock
2. HOWEVER, it is regulated at certain checkpoints by
internal and external controls.
3. In an animal cell, there are built in stop signals that stop
the cell cycle at certain checkpoints until they are
overidden by “go-ahead” signals.
4. Signals at checkpoints come from cellular surveillance
(is a process completed or not?) or from outside the cell
5. Three major checkpoints – found in G1, G2, and M
phases.
6. There is a G0 phase
B. Checkpoints – a closer look
1. G1 – “go ahead signal”
a. if it gets signal – cell continues to S phase
b. if no signal – cell goes into nondividing state (Go)
2. G2 – go ahead signal initiates M phase
3. M phase – all chromosomes lined up at the
metaphase plate initiates separation of sister
chromatids
C. Process of giving signals is Signal Transduction
1. Cell receiving signal has specific receptor protein in is
plasma membrane
a. the binding of a messenger (molecule) to the
receptor protein triggers a chain reaction involving
other proteins, which relay the message to a
molecule that performs a specific activity inside
the cell.
D. Type of molecules that make up the cell cycle control system
1. Cyclins and Cyclin-Dependent Kinases
a. Cyclin-Dependent Kinases (Cdks) – proteins that
activate or inactivate other proteins by phosphorylating
them.
b. They are “cyclin-dependent” because they are only active
when attached to a cyclin. They are present in the cell at a
constant concentration and usually in the inactive form.
c. Cyclin – a protein that cyclically fluctuates its concentration
within the cell.
2. MPF – “maturation promoting factor”
a. an example of cyclin-cdk complex
b. triggers the cell’s passage past the G2 checkpoint into
the M phase.
c. Cyclins associated with Cdk molecules accumulate during
interphase
d. MPF initiates mitosis by phosphorylating nuclear envelope,
fragmenting it.
e. MPF initiates a sequence of events leading to the
breakdown of its own cyclin.
E. Internal Signals
1. Anaphase does not begin until all the chromosomes are
at the metaphase plate. How does the cell know when
this happens?
a. Kinetochores that are not attached to spindle
microtubules give off a signal that inhibits the
anaphase-promoting complex (APC) to become
active
b. Once the APC is active it breaks down proteins
holding sister chromatids together.
F. External signals
1. Presence of growth factor – protein that stimulates cells to divide
a. each cell type responds specifically to a certain growth factor or
combination of different growth factors
2. Density-dependent inhibition – crowded cells stop dividing
a. when a cell population reaches a certain density, the amount of
required growth factors and nutrients available to each cell
becomes
insufficient to allow continued cell growth
3. Anchorage Dependence – to divide, cells must be attached to a
substrate
a. most cells in your body are attached to an extracellular matrix.
[Cancer cells do not exhibit density dependent inhibition nor
anchorage dependence]
Anchorage Dependence
This is similar to cutting
yourself