Download The Cell Cycle

All Animal Industry – Ag Notes
The Cell Cycle
The life cycle of a cell consists of growth and division. There are several stages to the cell cycle:
G1 phase
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first gap is usually longest period of cell cycle
however, in some embryonic cells that are rapidly dividing, G1 might only last a few
minutes
Some cells, like nerve cells never leave G1 and this is sometimes called a G0 state
G1 prepares the cell to undergo the next stage of synthesis
S phase
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all chromosomes are duplicated
DNA is replicated
new proteins synthesized to assemble with new DNA to form new chromosomes
replication begins at many sites along the length of a chromosome
time to complete S phase varies between different life stages and between species
G2 phase –
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Cell prepares itself for mitosis by synthesizing needed components
Some cells remain in interphase their whole lives because they do not divide. Two such
examples are nerve cells and adult muscle cells. The long "G1" phase is sometimes called G0
phase.
Why do cells duplicate??
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To regulate growth
As cells get larger they have more trouble moving nutrients and wastes through the
membrane
CANCER is caused by the cells lost ability to control growth
Control of the Cell Cycle
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result of cell cycle is cell proliferation
critical in embryological development
critical in repairing tissue damage
critical in immune response
uncontrolled proliferation is called cancer - more later in course
Mitosis
Mitosis
Interphase
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cells are not dividing
chromosomes are decondensed (called chromatin) and their information is available to
the cell for synthesizing products
cells spend most of their time in this intermediate non-mitotic state
during interphase (in S phase), all the cell's DNA is duplicated – resulting in 4 copies
of each gene instead of the normal 2 in a diploid cell
Prophase – Chromosomes appear
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chromatin begins to coil and condense to form chromosomes
each chromosome appears to have two strands (each containing a single molecule of
DNA)
each strand is called a chromatid
each chromatid is attached to its sister chromatid at the centromere
at this stage, the number of chromosomes (containing a pair of chromatids) is considered
to be equal to the number of centromeres
the two chromatids are the result of DNA replication that takes place just before mitosis
starts.
the nuclear envelope disappears
the nucleolus disappears
in cytoplasm, the spindle apparatus forms
eventually the spindle guides the separation of sister chromatids into the two daughter
cells
Metaphase – Chromosomes line up near cell center
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spindle grows and forms attachments to the chromosomes at the centromeres
chromosomes move to an equatorial plate (metaphase plate) which is formed along the
midline of the cell between the poles
chromosomes are at their most condensed state now
metaphase chromosomes can be stained and will show distinctive banding patterns
Anaphase – Sister Chromosomes separate
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centromeres divide to create two chromosomes instead of a pair of attached chromatids
spindle fibers shorten and the sister chromosomes are drawn to the opposite poles of
the cell
poles of the spindle apparatus are pushed apart as the cell elongates
anaphase results in the exact division of chromosome, distributing one complete diploid
complement of genetic information to each daughter cell
Telophase – Chromosomes condense and form new nucleus
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nuclear envelopes reassemble and surround each set of daughter chromosomes
nucleoli reappear inside the newly formed nuclei
in animal cell, a furrow appears around the cell that eventually pinches the cell into
two new cells
in plants, a cell plate forms between the two daughter nuclei as the cell wall divides the
cell
chromosomes decondense in the daughter cells to become chromatin and the cells are
once again in Interphase
Meiosis
In meiosis, the process is quite similar to mitosis. However, another cell division takes place in
which there is no extra DNA replication step. Instead of having a pair of genes (as in a diploid
cell), there is only one copy of each gene (a haploid cell). This one copy of genetic information
produces gametes of either sperm or eggs. Thus, only one copy of a gene is passed on to each
gamete. It is not until the sperm and egg join that there will be two halves of genetic information.
This process is the basis for all of Mendel's laws.