Chromosomes

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Chromosomes and Fertilization
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Fertilization
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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
PLANT
nuclei combine
cell division (mitosis)
embryo formed
egg cell
sperms
ANIMAL
nuclei combine
cell division (mitosis)
embryo formed
ovum
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0.1 mm
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pollen grain
ovary
ovule
egg cell
This sperm will fertilize the ovum
the pollen cell reaches the egg cell through a
pollen tube
ANIMAL
PLANT
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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
Meiosis
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
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Meiosis 1
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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
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Homologous chromosomes come to lie closely
alongside each other and behave like a single
chromosome
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Meiosis 3
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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
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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
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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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sperms produced by meiosis
sperm mother cell
fertilization
zygote
ovum mother cell
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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
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Genes
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gene for blue eyes
gene for brown eyes
Genes for any one characteristic occupy
corresponding positions on homologous chromosomes
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
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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
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)
The other gamete will receive genes b and C
(blue eyes and curly hair)
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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
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Variation
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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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4 Possible combinations
sperm
ovum
BB
Bb
fertilization
bB
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
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Question 1
Which of the following are gametes ?
(a) sperms
(b) dividing cells
(c) ova
(d) nuclei
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Question 2
In flowering plants, which of the following
are gametes ?
(a) egg cell
(b) ovule
(c) pollen grain
(d) pollen cell
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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
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Question 4
What is the correct sequence of events in meiosis
?
(a)
(b)
(c)
(d)
(e)
(f)
(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
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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
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Answer
Correct
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Answer
Incorrect