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Cell Division
Mitosis
During mitosis the cell divides into two
identical daughter cells, each of which has the same number of
chromosomes as the parent cell (see figure below).
When a cell is at rest between divisions, the
chromosomes are packed in the nucleus and are invisible. However,
when a cell is about to divide by mitosis, the chromosomes become
visible. They then become shorter and thicker, split in half along
their length (dividing into two strands of DNA which duplicate
themselves) and then pull apart and move to opposite ends of the
cell. Finally, the cytoplasm (the jelly-like substance inside the
cell) divides and new walls form around the two new cells, with one
set of chromosomes passing into each daughter cell.
The two daughter cells may then also divide by mitosis, producing
more cells which do the same. Mitotic cell division ensures that as
the body grows, every cell receives 46 chromosomes which are an
exact copy of the chromosomes in the original zygote. As growth
proceeds, instructions in the genes ensure that cells differentiate
into tissues, that tissues develop into organs and, eventually,
that individual features, such as eye and hair colour,
appear.
Mitosis does not stop when the body reaches adult size.
Mitosis:- Produces cells in the epidermis which replace
those worn away from the outer surface of the skin.
- Is responsible for producing new red and
white blood cells.
- Is the process by which new cells are formed
to repair damaged tissues.
However, not all cells can be replaced by mitosis. Some cells are
formed by a process called meiosis.
Meiosis
Although humans have 46 chromosomes in every
tissue cell, if sperm and ova had this number, fertilisation would
produce a zygote with 92 chromosomes, and the number of chromosomes
would double with each generation.
However, this does not happen, as gametes are produced by a type of
division called meiosis. Unlike mitosis, meiosis (which only takes
place within the ovaries and testes) produces four daughter cells,
each with half the normal number of chromosomes (see figure
below).
As in mitosis, the chromosomes first become
shorter and thicker and divide in two. However, then the
chromosomes pair so that the one that originally came from the
mother and the other from the father lie side by side.
Next, the chromosomes become very tightly intertwined so that when
they eventually pull apart, each new double chromosome contains
some of the mother's genes and some of the father's genes. Finally,
the two new cells divide again, with only 23 chromosomes entering
each of the four daughter cells.
The interchange of genetic material during this process of meiosis
explains why children do not look exactly like their parents and
why everyone except identical twins has a unique genetic
make-up.
The normal number of chromosomes is restored at fertilisation when
the sperm fuses with the ovum. These 46 chromosomes are actually 23
pairs called homologous pairs, because one member of each pair is
the same shape and size as the other. One pair, the sex
chromosomes, is different in size.
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