3. Fertilization occurs when the nucleus of a male
reproductive cell combines with the nucleus of a
female reproductive cell
The reproductive cells are called gametes
In animals, the male gamete is the sperm
cell and the female gamete is the ovum
In flowering plants, the male gamete is a cell in the
pollen grain and the female gamete is an egg cell
in the ovule
When the male and female gametes combine,
the resulting cell is called a zygote
2Fertilization
5. As a result of fertilization, the chromosomes from the
male and female cells are combined in the same
nuclear
membrane
Do you see a problem with this?
5
6. The problem is that if the full complement of male and
female chromosomes combine, the zygote and
embryo would have twice as many chromosomes
as its parents
If 46 male chromosomes combined with 46 female
chromosomes, the offspring would have 92 chromosomes
in their cells
And the next generation would have 184 chromosomes,
and so on
In fact, when the gametes are formed, the number of
chromosomes is halved so that the zygote ends up with
the same number of chromosomes as its parents.
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7. The type of cell division which gives rise to gametes
is called
Reduction division in which the chromosome number is
halved from diploid to haploid.
The following slides describe this process.
Only two pairs of chromosomes are shown
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8. The chromosomes appear,
shorten and thicken just as
in mitosis
The ‘blue’ chromosomes
are from the male parent;
the ‘red’ chromosomes
are from the female parent
The two long chromosomes
and the two short
chromosomes are called
homologous pairs
8Meiosis 1
10. The nuclear membrane
disappears
A spindle forms
At this stage it is the
chromosomes (and not
the chromatids) which are
separated
The cell starts to constrict
10Meiosis 3
11. One long chromosome
and one short chromosome
go to either end of the
dividing cell
So the cells now contain
only two, rather than four
chromosomes
By this time the chromatids
have become clear
11Meiosis 4
12. Two new spindles form, at
right angles to the original
spindle
At this stage, it is the
chromatids which
separate and pass to
opposite ends of the
cells
The cell constricts in the plane
at right angles to the first constriction
12Meiosis 5
13. Cell division is completed,
forming four gametes
each with half the number
of chromosomes of the
parent cell
gametes
13Meiosis 6
14. The number of chromosomes in the body cells is called
the diploid number
The number of chromosomes in the gametes is called
the haploid number (half the diploid number)
The diploid number in human cells is 46. The haploid
number in the gametes is 23
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