Download CELL DIVISION

CELL DIVISION
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Multiplication of cells takes place by division of preexisting cells.
Cell multiplication is equally necessary after the birth of
the individual for growth and for replacement of dead
cells.
We also know that Chromosomes within the nuclei of
cells carry genetic information that controls the
development and functioning of various cells and
tissues, and of the body as a whole.
When a cell divides it is essential that entire genetic
information within it be passed on to both the daughter
cells resulting from the division, i.e. must have
chromosomes identical in number to those in the
mother cell.
This type of cell division is called MITOSIS.
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A different kind of cell division called MEIOSIS
occurs during the formation of the gametes.
This consists of 2 successive divisions called
1st and 2nd MEIOTIC DIVISION.
They differ from other cells of the body
because:
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The number of Chromosomes is reduced to half the
normal number, and
The genetic information in the various gametes
produced is not identical.
GENES AND PROTEINS
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Proteins do the work of the cell: growth, maintenance,
response to the environment, reproduction, etc.
Proteins are chains of amino acids. The sequence of amino
acids in each protein is coded in the DNA as a specific
sequence of A, C, G and T bases.
Each gene codes for a different protein.
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Key points:
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All cells within an organism have the same genes.
What makes cells different from each other is that different genes
are turned on and turned off in different cells.
The DNA must be copied and then divided exactly so that
each cell gets an identical copy.
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Mitosis and Meiosis occur in eukaryotes.
Prokaryotes use a different method—”fission”
to divide.
Humans have 46 chromosomes, 23 from each
parent.
Every cell has the same 46 chromosomes
Each species has a characteristic number of
chromosomes: corn has 20, house flies have
10, chimpanzees have 48.
CELL CYCLE
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Some cells divide constantly e.g.: cells in the
embryo, skin cells, gut lining cells, etc. These
cells are called Labile Cells .
Some other cells divide when necessary only to
replace themselves. These cells are called
Stable or Quiescent Cells. E.g. Liver cells
Some other cells almost never divide. These are
called Permanent Cells. Examples include
Neurons, cardiac cells. E.t.c.
Actively dividing cells go through a cycle of
events that results in mitosis.
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Most of the cycle was called “Interphase”
During interphase the cell increases in size, but
the chromosomes are invisible.
The 3 stages of interphase are called
G1,
S, and
G2.
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The S phase (“Synthesis”) is the time when
the DNA is replicated, (when the
chromosome goes from having one
chromatid to having 2 chromatids
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G1 (“Gap”) is the period between mitosis
and S, when each chromosome has 1
chromatid.
Cells spend most of their time in G1: it is the
time when the cell grows and performs its
normal function. (Control of cell division
occurs in G1: a cell that isn’t destined to
divide stays in G1, while a cell that is to
divide enters the S phase).
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G2 is the period between S and mitosis. (The
chromosome has 2 chromatids, and the cell is
getting ready to divide).
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The M stage stands for "mitosis", and is when
nuclear (chromosomes separate) and
cytoplasmic (cytokinesis) division occur.
Control of Cell division (and thus tissue
growth) is very complex. The following terms
are some of the features that are important
in regulation.
 Cdk (cyclin dependent kinase, adds
phosphate to a protein), along with cyclins,
are major control switches for the cell cycle.
 Causing the cell to move from G1 to S or G2
to M.
 MPF (Maturation Promoting Factor) includes
the CdK and cyclins that triggers progression
through the cell cycle.
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p53 is a protein that functions to block the cell
cycle if the DNA is damaged.
If the damage is severe this protein can cause
apoptosis (cell death).
p53 levels are increased in damaged cells. This
allows time to repair DNA by blocking the cell
cycle.
A p53 mutation is the most frequent mutation.
p27 is a protein that binds to cyclin and CdK
blocking entry into S phase.
Breast cancer prognosis is determined by p27
levels.
Reduced levels of p27 predict a poor outcome for
breast cancer patients
Errors can lead to cancer.
 Cancer is a disease where regulation of the cell
cycle goes wrong and normal cell growth and
behavior is lost.
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