CELL DIVISION
Cells divide in order to grow, to repair tissues or to create gametes. There are two types of cell
division:
MITOSIS
Mitosis occurs in somatic cells (non-reproductive cells). It is the process in which a parent cell
divides into two daughter cells that have identical genetic information. Before the starts this
process, the DNA and the centriole have been duplicated during the S phase of interphase.
Mitosis has four phases:
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PROPHASE: it is the first phase.
1. The DNA shortens and thickens creating chromosomes.
2. Centrioles move to the poles of the cell where they will
create mitotic spindles.
3. The nucleolus disappears.
4. The cell membrane disappears.
METAPHASE:
1. The chromosomes move in the equatorial plate, or
metaphase plate.
2. The chromosomes attach themselves to the spindles in
the centromeres.
ANAPHASE:
1. The spindles break, breaking the centromeres .Two
chromatids separate into two unconnected strands.
2. The spindles contract, pulling the sister chromatids towards the
opposite poles of the cell.
TELOPHASE: Last phase of mitosis
1. When the chromatids reach their positions the spindle fibers
disappear.
2. New strands of nucleotides attach to the chromatids
forming chromatin.
3. The nuclear membrane appears, forming two daughter
nuclei.
4. The nucleolus appears.
After mitosis another process called cytokinesis takes place. Cytokinesis is
the division of the cytoplasm. There are two types:
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In animals the cytoplasm thins out and eventually separates.
In plants a cell wall is formed that divides the cytoplasm in half.
MITOSIS IN PROKARYOTES
The genetic material in the prokaryotes is
floating in the cytoplasm. It consists on just one
ring of DNA. In order to duplicate , they unzip
the two strands from a specific point : the
replication origin. New strands are added to the
old ones, creating two identical molecules of
DNA.
Then, each of them attaches to separate zones
of the internal cell membrane of the cell and
the cell is divided.
MEIOSIS
Meiosis is the cellular division in reproductive cells. This process ensures that the correct
number of chromosomes is passed onto the next generation. From a diploid cell we obtain two
haploid cells and then four haploid cells called gametes.
The process of meiosis are actually two mitosis, but with different characteristics.
1st MITOSIS
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Prophase:
1. The nuclear membrane disappears, as well as the nucleus (like in simple
mitosis).
2. Centrioles go to the poles of the cell.
3. DNA shortens, forming chromosomes.
4. Instead of appearing alone, they appear in pairs. These homologous
chromosomes attach in pairs by spindle fibers and they cross over*.
Metaphase:
1. As in mitosis, the chromosomes (in pairs) move in the metaphase plate.
2. Pairs of homologous chromosomes are attached .
Anaphase
1. Spindle fibers brake and contract. But instead of separating into chromatids,
the entire chromosome is pulled to the pole of the cell.
Telophase : the same
as mitosis
1. Chromosomes
transform
again into
chromatin.
1st Cytokinesis
Now we have two haploid
cells. No DNA is replicated for
the next mitosis.
2nd MITOSIS: as a simple mitosis. At the end of it new strands attach to the chromatids
forming chromosomes.
2nd Cytokinesis: the same as in a simple mitosis.
RESULT:
4 gametes with n chromosomes are created.
PROCESS THAT ENSURE VARIABILITY:
Independent assortment: position that the chromosomes take . There
are 2 possibilities: mother chromosome up and father chromosome
down or vice versa. As there are 23 chromosomes this provide 223
possibilities.
Crossing over: process that take place during prophase 1. The
homologous chromosomes exchange genetic material
Radom fertilization: from all the sperm cell of the sperm just one will
fertilize the egg.
ORGANELLES SENSITIVE TO MITOSIS
The organelles that are most affected in mitosis are the nucleus
(the cell membrane has to disappear and the genetic material has
to duplicate), the nucleolus (that has to disappear too) and the
centrioles or centrosome, that have to duplicate.
CHROMOSOMES:
Chromosomes are part of the cell that carries hereditary information
in the form of genes.
VOCABULARY:
Diploid cell: cell with 46 chromosomes (2n)
Haploid cell: cell with 23 chromosomes (n)
Chromatid: half of the chromosome (when crhomosomes are separated during anaphase)
Chromatin: genetic information during interphase (when it is stretched)
Centromere: point that joins the two halves (the two chromatids) of a chromosome. It is the
point of attachment to the spindles in anaphase. In this phase, the centromere duplicates and
the chromatid pair separate.
Chiasm: / Synapsis: physical point in the chromosome where crossing over process happens.
Kinetochore : where the chromosomes attaches the spindles.