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Transcript of MEIOSIS The great gene shuffling machine Copyright Dr. Michael Knee Source: //hcs.osu.edu/hcs300/ ©...
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MEIOSISThe great gene shuffling machine
Copyright Dr. Michael Knee Source: http://hcs.osu.edu/hcs300/ © 2007 Paul Billiet ODWS
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Meiosis performs two functions
It halves the number of chromosomes to make haploid sets
It shuffles the genes to produce new combinations (recombinations)
© 2007 Paul Billiet ODWS
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Meiosis and sexual reproduction Meiosis is needed to produce sex cells
(gametes) with unpaired sets of chromosomes (haploid)
Sex cells are used in fertilisation At fertilisation two sets of genes come
together to form a hybrid with a set of paired chromosomes (diploid)
The hybrid, whilst similar to the parents, is unique
© 2007 Paul Billiet ODWS
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Haploid and Diploid
Karyotypes of somatic cells show paired sets of chromosomes
The origin of the pairs are the maternal and paternal chromosomes of the egg and the sperm
The number of types of chromosomes of a species is constant = n
So the diploid (paired set) = 2n
© 2007 Paul Billiet ODWS
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The sexual reproduction life cycle
MeiosisFertilisation
Haploid (n)
Diploid (2n)
© 2007 Paul Billiet ODWS
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Meiosis a two step process
Meiosis 1 is the reduction division Meiosis 2 resembles mitosis
© 2007 Paul Billiet ODWS
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Meiosis 1: Early Prophase 1
Chromosomes condense
Homologous pairs linked by chiasmata (chiasma sing.)
© 2007 Paul Billiet ODWS
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Meiosis 1: Late Prophase 1
Spindle fibres form and spread out between the centrioles
© 2007 Paul Billiet ODWS
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Meiosis 1: Metaphase 1
The pairs of chromosomes line up on the equator
The orientation of the maternal and the paternal chromosomes is random
© 2007 Paul Billiet ODWS
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Meiosis 1: Anaphase 1
Maternal and paternal chromosomes segregate (pulled separate on the spindle)
They move to opposite poles
© 2007 Paul Billiet ODWS
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Meiosis 1: Metaphase 1 revisited The pairs of
chromosomes could orientate in different ways
© 2007 Paul Billiet ODWS
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Meiosis 1: Anaphase 1 revisited Resulting in different
combinations of chromosomes
This means there are 2n combinations
In humans this means 223 or over 8 million combinations
© 2007 Paul Billiet ODWS
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Meiosis 2: Prophase 2
Now the cells are haploid
The chromosomes do not decondense at the end of meiosis 1
Each chromosome has still two chromatids
© 2007 Paul Billiet ODWS
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Meiosis 2: Prophase 2
Spindles form again
© 2007 Paul Billiet ODWS
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Meiosis 2: Metaphase 2
The chromosomes line up on the spindle equator independently
© 2007 Paul Billiet ODWS
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Meiosis 2: Anaphase 2
The sister chromatids separate on the spindle
Each cell will receive a copy of each chromosome type (i.e. it will receive n chromatids all different)
The genes on the different chromosomes are recombined (shuffled)
© 2007 Paul Billiet ODWS
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Meiosis 2: Telophase 2
Four haploid sex cells are produced
© 2007 Paul Billiet ODWS
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Crossing over During prophase 1 not only do
the homologous pairs link They exchange genetic material The genes on each chromosome
are not identical they may be alleles
Alleles are different versions of a gene
e.g. Ear shape gene has two alleles the ear lobe allele and the no ear lobe allele
© 2007 Paul Billiet ODWS
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Meiosis: Anaphase 1
So when the pairs are separated, the alleles of the genes on the same chromosome are recombined (reshuffled)
Genes on the same chromosome are called linked genes
© 2007 Paul Billiet ODWS
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Meiosis: Prophase 2
Each cell is haploid (n) The sister chromatids
are no longer identical copies
© 2007 Paul Billiet ODWS
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Meiosis: Anaphase 2
At anaphase 2 the chromatids segregate (separate) randomly
Even greater variation is achieved in the sex cells
© 2007 Paul Billiet ODWS
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Meiosis: Telophase 2
Thus an infinite variety of sex cells is possible
Combined with random mating between males and females an infinite variety of individuals is conceived at fertilisation
© 2007 Paul Billiet ODWS