powerpoint meiosis
TRANSCRIPT
MEIOSIS
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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 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
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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 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
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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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
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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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