Page 18 - CB1.3 Genetic Varation sciPAD

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18 CB 1.3 GENETIC VARIATION
PRINCIPLES OF GENETIC VARIATION
1. Outline the primary purpose of meiosis. 

2. Name two places where meiosis occurs: (a) 
(b) 
3. How many daughter cells are formed by meiosis? 
4. How many chromosomes are found in: (a) a normal human body cell? 
(b) daughter cells produced by meiosis? 
Cell Division - Meiosis
Meiosis (from the Greek, meaning ‘to make smaller’) is a special type of cell division that produces
male and female ‘gametes’. A gamete is a sex cell (sperm in males and ova in females). A
human body cell contains 46 chromosomes. Gametes only contain half as many chromosomes
(23). During meiosis, genes get shuffled, and this leads to variation within a population. Meiosis is
the reason you look similar, but not identical, to your parents and siblings. The diagram below is
simplified and only shows a cell containing 4 chromosomes.
When the cell is not dividing, the chromosomes are not
visible. They are unwound and are being used to make
proteins (protein synthesis). They are shown here as
discrete chromosomes so you can track the differences
between the parent cell and the daughter cells.
Chromosomes replicate. The nuclear membrane
disappears.
Homologous pairs of chromosomes exchange sections
of DNA (and therefore genes) by a process called
‘crossing over’. They then line up at the cell equator
(the middle of the cell) in random order.
Chromosomes get pulled apart and the parent cell
begins to divide.
The parent cell splits in two.
Chromosomes line up in single file at
the cell equator and get pulled apart.
Four daughter cells (or gametes)
are produced, each with half of the
original number of chromosomes.
1
2
3
4
5
6
7
Crossing Over
This process is very important.
It ensures that there will be
variation among the offspring.
Meiosis only occurs in the testes of
men and the ovaries of women.
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