Chromosomes fertilization

1. 1

2. 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

3. pollen nucleus
egg cell
nuclei combine cell division (mitosis)
embryo
formed
PLANT
sperms
ovum
nuclei combine cell division (mitosis)
embryo
formed
ANIMAL
3

4. PLANT
This sperm will
fertilize the ovum
0.1 mm
pollen grain
the pollen cell reaches
the egg cell through a
pollen tube
ovary
egg cell
ANIMAL
4
ovule

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.
6

7. 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
7

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

9. Homologous chromosomes
come to lie closely alongside
each other and behave like
a single chromosome
9Meiosis 2

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
14

15. 23
46
46
23
23
23
23
23
23
46
sperm
mother
cell
ovum
mother
cell
sperms produced
by meiosis
fertilization
zygote
ova produced by meiosis
but only one develops to
maturity
15

16. 46 46
46 46
46 46 4646
46 46 46 46
4646
46 46
4646
Cell division continues by
mitosis, so all the cells will
contain 46 chromosomes early embryo
16

17. Genes for any one characteristic
occupy corresponding positions
on homologous chromosomes
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 brown eyes
gene for
blue eyes
gene for
curly hair
gene for
straight
hair
17Genes

18. 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
18

19. In the first stage of meiosis, the illustration (slide 10)
showed one ‘red’ and one ’blue’ chromosome
going to each daughter cell
B
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 B
b
C
c
B = gene for brown eyes
b = gene for blue eyes
C = gene for curly hair
c = gene for straight hair
19Gene combinations

20. It is just as likely that both ‘blue’ chromosomes
will go to one daughter cell and both ‘red’
chromosomes go to the other
B
b
c
C
One gamete will receive the
genes B and c (brown eyes
and straight hair)
The other gamete will receive
genes b and C (blue eyes and
curly hair)
20

21. 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
21

22. 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
22Variation

23. 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
23

24. Bb
BB
bB
bb
b
b
sperm ovum 4 Possible
combinations
BB, Bb and bB have the same effect of producing brown eyes
Only bb gives rise to blue eyes
Although there are 4 possible combinations of genes
B B
24
fertilization

25. Question 1
Which of the following are gametes ?
(a) sperms
(b) dividing cells
(c) ova
(d) nuclei

26. Question 2
In flowering plants, which of the following are
gametes ?
(a) egg cell
(b) ovule
(c) pollen grain
(d) pollen cell

27. 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

28. Question 4
What is the correct sequence of events in meiosis ?
(a) (b) (c) (d) (e) (f)
(a) a, b, d, c, e, f
(b) b, a, d, c, e, f
(c) b, d, a, c, e, f
(d) a, b, d, c, e, f

29. 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

30. Answer
Correct

31. Answer
Incorrect