Page 13 - CB1.3 Genetic Varation sciPAD
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CB 1.3 GENETIC VARATION
PRINCIPLES OF GENETIC VARIATION
1. Outline the primary purpose of mitosis.
2. Name two places where mitosis occurs: (a)
(b)
3. (a) Name two types of cells where mitosis does not occur.
(i)
(ii)
(b) What is the general name given to these two types of cells?
4. How many daughter cells are formed by mitosis?
5. How many chromosomes are found in: (a) a normal human body cell?
(b) daughter cells produced by mitosis?
Mitosis
‘Mitosis’ is the process that a cell goes through when it divides to form two new cells. The primary
purpose of mitosis is growth. It is through mitosis that an organism transforms from a single cell (i.e.
a fertilised ovum) to an adult containing trillions of cells. Mitosis also produces new cells when
damaged cells need replacing.
Mitosis occurs in every cell in your body (except for the gametes - sperm and ova). Prior to mitosis,
a copy of every chromosome is created so that after mitosis, each new cell has a complete set of
chromosomes. Therefore, apart from mutations, each new cell will have the same genetic code
as its parent cell. This seems obvious when you consider that damaged skin cells, for example,
must be replaced with cells identical in function to the original cell.
Key stages of mitosis in a cell with only four chromosomes
The DNA has condensed into
chromosomes (two chromatids held
together by a centromere). They are
visible under a light microscope. The
nuclear membrane disappears.
Chromosomes line up randomly at
the cell equator (middle of the cell).
A network of fibres forms across the
cell to control the division.
Chromatids of each chromosome get
separated by the network of fibres
and are moved towards the poles.
The fibre network retracts and a
nuclear membrane forms around
each set of chromosomes. The
chromosomes start to uncoil and
disappear from view.
Two identical daughter cells are
produced, each with a copy of
the original chromosomes.
When a cell is not dividing, the
chromosomes are not visible. They
are being used to make proteins
(protein synthesis). Before cell
division, the DNA replicates and
condenses into chromosomes.
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