Chromosome - a linear DNA molecule
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Homologous chromosomes - chromosomes that have the same kind of genes in the same order
o 1 copy from father, 1 copy from mother
o humans have 46 chromosomes with 23 homologous pairs
o also know as sister chromosomes
Chromatid - after a chromosome has replicated in S phase, there are two chromatids. Each chromatid
possess an identical sequence of DNA.
Centromere - special region where spindle fibers attach
Grand Scheme
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Mitosis
o
o
o
Meiosis
o
o
o
o
2N diploid cell
replicates
divides - two diploid daughter cells
2N diploid cell
replicates
division - two daughter cells, each 2N
division (without replication)- four daughter cells, each N (haploid)
Mitosis
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Cell Cycle - phases
o Interphase
 G0 - Gap 0 phase, a resting stage, low levels of MPF (Maturation Promoting Factor), 2N
 G1 - Gap 1 phase, cell enters mitosis, low levels MPF, 2N
 S - DNA synthesis occurs, low levels of MPF, 4N
 G2 - Gap 2 phase, time before metaphase, MPF is low at the start and increases at the end
of G2, 4N
o M - M phase
 Prophase - chromosomes condense, nuclear membrane disappears, MPF is increasing
 Metaphase - chromosomes line up on spindle, MPF is at its peak level
 Anaphase - chromosomes begin to pull apart, MPF declines to initiate anaphase
 Telophase - spindles disappear, MPF is low, nuclear membranes begin to reappear
 Cytokinesis - cell division, MPF low
Sequence of Events
o GO resting cell is triggered to enter cell cycle
o progresses through G1, S, G2
o enters M phase
 short prophase
 metaphase
 nuclear membrane or envelope brakes-down
 chromosomes attach to spindles via the centromeres
 spindles are microtubules made of tubulin
 homologous chromosomes align along side of each other
 MPF activity is very high
 anaphase
 chromatids pull apart
 MPF activity decreases
 telophase
 microtubules disappear
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 nuclear envelope reforms
cytokinesis
 cell division
Meiosis
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Sequence of Events
o Interphase - I
 G0 to G2 are the same as meiosis except this is in a primary spermatocyte or primary
oocyte
 important to remember that S phase has occurred and gamete is 4N
o Prophase - I
 very long in meiosis, in spermatocyte might be days while in the oocyte it can be years or
decades
 leptotene
 chromosome condense
 zygotene
 homologous chromosomes pair
 note that this is NOT sister chromatids
 called bivalents or tetrad in the literature
 pachytene
 pairing is complete
 crossing over occurs between homologous chromosomes NOT sister chromatids
 diplotene
 oocytes stop here before puberty (dictyate arrest)
 homologous chromosomes pull apart but remain attached at crossover points
(chiasma)
 note that chiasma are not the centromeres
 RNA synthesis can occur up to this point
 diakinesis
 RNA synthesis stops
 the 4 chromatids of each pair of homologous chromosomes become visible
 homologous chromosomes remain linked at chiasma
 chromatids are linked at centromere
o Metaphase - I
 nuclear membrane breaks down
 spindles attach to centromeres
 homologous chromosomes are initially attached at chiasma
 homologous chromosomes go to opposite poles and then pair across from one another in
the center of the metaphase plate
o Anaphase - I
 homologous chromosomes pull apart
 chiasma separate
 homologous chromosomes move to opposite poles
o Telophase - I
 microtubules disappear and nuclear envelopes begin to reappear
o Cytokinesis - I
 division to form secondary spermatocyte or secondary oocyte and polar body - I
o Interphase - II
 short
 No DNA synthesis
o Prophase - II
 brief
o Metaphase - II (same as mitosis)
 oocyte stops here until ovulated and resumes only if fertilized
o
o
o
Anaphase - II (same as mitosis)
Telophase - II (same as mitosis)
Cytokinesis - II (same as mitosis)
 results is haploid daughter cells
 remember for oocyte this stage cell never exists because meiosis is only
completed after penetration of the oocyte by a sperm
Mitosis and Meiosis
What is Mitosis?
Mitosis produces two daughter cells that are identical to the parent cell. If the parent cell is haploid (N), then the
daughter cells will be haploid. If the parent cell is diploid, the daughter cells will also be diploid.
N→N
2N → 2N
This type of cell division allows multicellular organisms to grow and repair damaged tissue.
Click here to go to the chapter on Mitosis.
Summary of the Phases of Mitosis
The drawings below show chromosome movement and alignment in a cell from a species of animal that has a
diploid number of 8. As you view the drawings, keep in mind that humans have a diploid number of 46.
Interphase (G1 and G2)
Chromosomes are not easily visible because they are
Prophase
The chromosomes begin to coil.
The spindle apparatus begins to form as centrosomes
Prometaphase
The nuclear membrane disintegrates.
Kinetochores form on the chromosomes.
Kinetochore microtubules attach to the chromosomes
Metaphase
The chromosomes become aligned on a plane.
Anaphase
The chromatids separate (The number of chromosom
Telophase
The nuclear membrane reappears.
The chromosomes uncoil.
The spindle apparatus breaks down.
The cell divides into two.
G1 Interphase
The chromosomes have one chromatid.
G2 Interphase
The chromosomes are replicated. Each one has two s
chromatids.
What is Meiosis?
Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell.
2N → N
Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid.
Meiosis involves two divisions producing a total of four daughter cells.
Click here to go to the chapter on meiosis.
Summary of the Phases of Meiosis
A cell undergoing meiosis will divide two times; the first division is meiosis 1 and the second is meiosis 2. The
phases have the same names as those of mitosis. A number indicates the division number (1st or 2nd):
meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1
meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2
In the first meiotic division, the number of cells is doubled but the number of chromosomes is not. This results in 1/2
as many chromosomes per cell.
The second meiotic division is like mitosis; the number of chromosomes does not get reduced.
Prophase I
Homologous chromosomes become
paired.
Crossing-over occurs between
homologous chromosomes.
Crossing over
Metaphase I
Homologous pairs become aligned in
the center of the cell.
The random alignment pattern is
called independent assortment. For
example, a cell with 2N = 6
chromosomes could have any of the
alignment patterns shown at the left..
Anaphase I
Homologous chromosomes separate.
Telophase I
This stage is absent in some species
Interkinesis
Interkinesis is similar
to interphase except DNA synthesis
does not occur.
Prophase II
Metaphase II
Anaphase II
Telophase II
Daughter Cells
Mitosis
1. It occurs in the vegetative or somatic cells.
2. After this division two daughter cells are produced.
3. Daughter cells process the same chromosome number as that of parent cell.
4. It is of short duration.
5. Daughter cells are similar to parent cell genetically.
6. There is no crossing over in mitosis.
7. With the splitting of centomere, chromatids are pulled apart towards the respective poles. Each
chromatid behaves as an independent chromosome.
8. This is an equational division.
9. Homologous chromosomes are not arranged in pairs in the equatorial plate.
10. Genetic variation does not occur between daughter cells.
11. DNA synthesis is completed in the interphase.
12. In metaphase, the centromeres are lined up on the equatorial plane and the arms extend into the
eytoplasm.
Meiosis
1. It occurs in the reproductive cells.
2. Four daughter cells are produced after melotic division.
3. Chromosome number of daughter cells is reduced to half.
4. It is of long duration. The complete process is divided in to two divisions.
5. Daughter cells have genetic differences from the parent cell.
6. Crossing over takes place between the nonsister chromatids.
7. Whole chromosome moves apart in anaphase I of meiosis because there is no splitting of centromere.
8. In meiosis, the first division is reductional and the second one is equational.
9. Homologous chromosomes are arranged in pairs in the equatorial plate.
10. Exchange between maternal and paternal chromosomal segments does not render the daughter cell
identical.
11. DNA synthesis is not completed in the interphase.
12. In metaphaseI, the centromeres of the homologous chromosomes lie towards the two opposite poles
and their arms extend towards the equator.
Regulation of the Cell Cycle
The cell cycle is controlled by a cyclically operating set of reaction sequences that both trigger and coordinate key
events in the cell cycle
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The cell-cycle control system is driven by a built-in clock that can be adjusted by external stimuli (chemical
messages)
Checkpoint - a critical control point in the cell cycle where stop and go-ahead signals can regulate the cell
cycle
o Animal cells have built-in stop signals that halt the cell cycles and checkpoints until overridden by
go-ahead signals.
o Three Major checkpoints are found in the G1, G2, and M phases of the cell cycle
The G1 checkpoint - the Restriction Point
o The G1 checkpoint ensures that the cell is large enough to divide, and that enough nutrients are
available to support the resulting daughter cells.
o If a cell receives a go-ahead signal at the G1 checkpoint, it will usually continue with the cell cycle
o If the cell does not receive the go-ahead signal, it will exit the cell cycle and switch to a nondividing state called G0
o Actually, most cells in the human body are in the G0 phase
The G2 checkpoint ensures that DNA replication in S phase has been completed successfully.
The metaphase checkpoint ensures that all of the chromosomes are attached to the mitotic spindle by a
kinetochore.
Cyclins and Cyclin-Dependent Kinases - The Cell-Cycle Clock
Rhythmic fluctuations in the abundance and activity of cell-cycle control molecules pace the events of the cell cycle.
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Kinase - a protein which activates or deactivates another protein by phosphorylating them.
Kinases give the go-ahead signals at the G1 and G2 checkpoints
The kinases that drive these checkpoints must themselves be activated
o The activating molecule is a cyclin, a protein that derives its name from its cyclically fluctuating
concentration in the cell
o Because of this requirement, these kinases are called cyclin-dependent kinases, or Cdk's
MPF - Maturation Promoting Factor (M-phase promoting factor)
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Cyclins accumulate during the G1, S, and G2 phases of the cell cycle
By the G2 checkpoint (the red bar in the figure), enough cyclin is available to form MPF complexes
(aggregations of Cdk and cyclin) which initiate mitosis
o MPF apparently functions by phosphorylating key proteins in the mitotic sequence
Later in mitosis, MPF switches itself off by initiating a process which leads to the destruction of cyclin
o Cdk, the non-cyclin part of MPF, persists in the cell as an inactive form until it associates with
new cyclin molecules synthesized during interphase of the next round of the cell cycle
PDGF - Platelet-Derived Growth Factors - An Example of an External Signal for Cell Division
PDGF is required for the division of fibroblasts which are essential in wound healing
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When injury occurs, platelets (blood cells important in blood clotting) release PDGF
Fibroblasts are a connective tissue cells which possess PDGF receptors on their plasma membranes
The binding of PDGF activates a signal-transduction pathway that leads to a proliferation of fibroblasts
and a healing of the wound
Density Dependent Inhibition
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Cells grown in culture will rapidly divide until a single layer of cells is spread over the area of the petri
dish, after which they will stop dividing
If cells are removed, those bordering the open space will begin dividing again and continue to do so until
the gap is filled - this is known as contact inhibition
Apparently, 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
Anchorage Dependence
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For most animal cells to divide, they must be attached to a substratum, such as the extracellular matrix of
a tissue or the inside of the culture jar
Anchorage is signaled to the cell-cycle control system via pathways involving membrane proteins and the
cytoskeleton
Cells Which No Longer Respond to Cell-Cycle Controls - Cancer Cells
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Cancer cells do not respond normally to the body's control mechanism.
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o They divide excessively and invade other tissues
o If left unchecked, they can kill the organism
Cancer cells do not exhibit contact inhibition
o If cultured, they continue to grow on top of each other when the total area of the petri dish has
been covered
o They may produce required external growth factor (or override factors) themselves or possess
abnormal signal transduction sequences which falsely convey growth signals thereby bypassing
normal growth checks
Cancer cells exhibit irregular growth sequences
o If growth of cancer cells does cease, it does so at random points of the cell cycle
o Cancer cells can go on dividing indefinitely if they are given a continual supply of nutrients
 Normal mammalian cells growing in culture only divide 20-50 times before they stop
dividing