1. fertilization occurs when the nucleus of a male reproductive cell combines with the nucleus of a...
TRANSCRIPT
1
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 thepollen grain and the female gamete is an egg cellin the ovule
When the male and female gametes combine, the resulting cell is called a zygote
2Fertilization
pollen nucleus
egg cell
nuclei combine cell division (mitosis)embryo formed
PLANT
sperms
ovumnuclei combine cell division (mitosis) embryo
formed
ANIMAL
3
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
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 andfemale chromosomes combine, the zygote and embryo would have twice as many chromosomes as its parents
If 46 male chromosomes combined with 46 femalechromosomes, the offspring would have 92 chromosomesin 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 gametesis 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
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The chromosomes appear,shorten and thicken just asin mitosis
The ‘blue’ chromosomesare from the male parent; the ‘red’ chromosomesare from the female parent
The two long chromosomes and the two short chromosomes are called homologous pairs
8Meiosis 1
Homologous chromosomes come to lie closely alongsideeach other and behave like a single chromosome
9Meiosis 2
The nuclear membrane disappears
A spindle forms
At this stage it is thechromosomes (and notthe chromatids) which areseparated
The cell starts to constrict
10Meiosis 3
One long chromosomeand one short chromosomego to either end of the dividing cell
So the cells now containonly two, rather than fourchromosomes
By this time the chromatidshave become clear
11Meiosis 4
Two new spindles form, at right angles to the originalspindle
At this stage, it is the chromatids which separate and pass to opposite ends of thecells
The cell constricts in the planeat right angles to the first constriction
12Meiosis 5
Cell division is completed,forming four gameteseach with half the numberof chromosomes of the parent cell
gametes
13Meiosis 6
The number of chromosomes in the body cells is called the diploid number
The number of chromosomes in the gametes is calledthe haploid number (half the diploid number)
The diploid number in human cells is 46. The haploidnumber in the gametes is 23
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23
46
4623
23
23
23
23
23
46
sperm mothercell
ovummothercell
sperms produced by meiosis
fertilizationzygote
ova produced by meiosis but only one develops tomaturity
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46 46
46 46
46 46 4646
46 46 46 46
4646
46 46
4646
Cell division continues by mitosis, so all the cells willcontain 46 chromosomes early embryo
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Genes for any one characteristicoccupy corresponding positions on homologous chromosomes
But they do not necessarily controlthe characteristic in the same way
For example, one of the gene pairresponsible for eye colour mightdetermine brown eyes and its partner determine blue eyes*
gene for brown eyesgene forblue eyes
gene for curly hair
gene forstraight hair
17Genes
Usually only one of a gene pair will be expressed in anindividual
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 tothe gene for blue eyes. The gene for blue eyes is notexpressed in this individual
The gene for blue eyes is said to be recessive to thegene for brown eyes
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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
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 eyesC = gene for curly hairc = gene for straight hair
19Gene combinations
It is just as likely that both ‘blue’ chromosomeswill go to one daughter cell and both ‘red’ chromosomes go to the other
B
b
c
C
One gamete will receive thegenes B and c (brown eyes and straight hair)
The other gamete will receive genes b and C (blue eyes andcurly 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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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
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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Bb
BB
bB
bb
bb
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
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fertilization
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 aregametes ?
(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) (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
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