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Fertilization
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
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pollen nucleus
nuclei combine
PLANT
cell division (mitosis)
egg cell
sperms
ovum
embryo
formed
ANIMAL
nuclei combine
cell division (mitosis)
embryo
formed
0.1 mm
pollen grain
ovary
ovule
egg cell
This sperm will
fertilize the ovum
PLANT
the pollen cell reaches
the egg cell through a
pollen tube
ANIMAL
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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?
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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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The type of cell division which gives rise to gametes
is called
At cell division, before the chromatids separate, the
chromosomes are shared equally between the two
daughter cells
The following slides describe this process.
Only two pairs of chromosomes are shown
Meiosis 1
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
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Meiosis 2
Homologous chromosomes
come to lie closely alongside
each other and behave like
a single chromosome
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Meiosis 3
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
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Meiosis 4
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
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Meiosis 5
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
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Meiosis 6
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Cell division is completed,
forming four gametes
each with half the number
of chromosomes of the
parent cell
gametes
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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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23
46
sperms produced
by meiosis
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sperm
mother
cell
23
23
fertilization
zygote
ovum
mother
cell
23
23
46
46
ova produced by meiosis
but only one develops to
maturity
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Cell division continues by
mitosis, so all the cells will
contain 46 chromosomes
early embryo
46
46
46
46
46
46
46
46
46
46
46
46
46
46
46 46
46
46
Genes
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gene for brown eyes
Genes for any one characteristic
occupy corresponding positions
on homologous chromosomes
gene for
blue eyes
gene for
curly hair
But they do not necessarily control
the characteristic in the same way
For example, one of the gene pair
responsible for eye colour might
determine brown eyes and its
partner determine blue eyes*
gene for
straight
hair
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Usually only one of a gene pair will be expressed in an
individual
A person inheriting the gene for brown eyes and the gene
for blue eyes will have brown eyes
The gene for brown eyes is said to be dominant to
the gene for blue eyes. The gene for blue eyes is not
expressed in this individual
The gene for blue eyes is said to be recessive to the
gene for brown eyes
Gene combinations
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In the first stage of meiosis, the illustration (slide 10)
showed one ‘red’ and one ’blue’ chromosome
going to each daughter cell
One gamete will receive the
gene combination for brown
eyes and curly hair. The other
will receive the genes for blue
eyes and straight hair
C
B
B
b
c = gene for straight hair
C = gene for curly hair
b = gene for blue eyes
B = gene for brown eyes
c
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It is just as likely that both ‘blue’ chromosomes
will go to one daughter cell and both ‘red’
chromosomes go to the other
One gamete will receive the
genes B and c (brown eyes
and straight hair)
c
B
The other gamete will receive
genes b and C (blue eyes and
curly hair)
b
C
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So, there could be 4 types of gamete with different
combinations of the genes
BC
brown eyes, curly hair
bc
blue eyes, straight hair
Bc
brown eyes, straight hair
bC
blue eyes, curly hair
Variation
Meiosis not only halves the number of
chromosomes but can also rearrange
the genes
This is one cause of the variations that occur
in members of the same species
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Rearrangement of genes can also take
place at fertilization
A sperm may carry a gene for brown eyes (B)
or a gene for blue eyes (b)
An ovum may carry a gene for brown eyes (B)
or a gene for blue eyes (b)
At fertilization, four possible combinations
can occur
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sperm
ovum
B
B
4 Possible
combinations
BB
Bb
fertilization
bB
b
b
bb
Although there are 4 possible combinations of genes
BB, Bb and bB have the same effect of producing brown eyes
Only bb gives rise to blue eyes
Question 1
Which of the following are gametes ?
(a) sperms
(b) dividing cells
(c) ova
(d) nuclei
Question 2
In flowering plants, which of the following are
gametes ?
(a) egg cell
(b) ovule
(c) pollen grain
(d) pollen cell
Question 3
Which of the following occur in both mitosis
and meiosis ?
(a) chromatids separate
(b) homologous chromosomes separate
(c) nuclear membrane disappears
(d) four cells are formed
Question 4
What is the correct sequence of events in meiosis ?
(a)
(b)
(c)
(d)
(e)
(a) a, b, d, c, e, f
(c) b, d, a, c, e, f
(b) b, a, d, c, e, f
(d) a, b, d, c, e, f
(f)
Question 5
Which of the following represent variation
within a species ?
(a) black cats and tabby cats
(b) collie dogs and dachshunds
(c) goldfinch and greenfinch
(d) shire horses and race horses
Answer
Correct
Answer
Incorrect