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NadiaA

Meiosis
Images’ Source: http://www
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It is when the DNA unravels and replicates
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Stages
Meiosis I
Prophase I
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Chromatin condenses, supercoils so chromosomes shorten and thicken
Chromosomes can be seen under a light microscope
Chromosomes align along their homologous pairs forming bivalents (each chromosome pair
has the same genes at the same loci in a homologous pair)
Non-sister chromatids wrap around each other and attach at points called chiasmata
(singular: chiasma)
Crossing over occurs- Non-sister chromatids swap sections of chromatids with each other
Nucleolus disappears
Nuclear envelope disintegrates
Centrioles move to opposite ends of the cell
A spindle forms which is made of protein microtubules

This stage may last for days, months, years depending on the species involved and the type of
gamete being formed
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Crossing over during prophase I to shuffle alleles
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Genetic re-assortment due to random distribution and segregation of sister chromatids at
meiosis II
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This is called crossing over
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Holding homologous pairs on the spindle equator ensures that when segregation occurs at
anaphase I, one member of each pair will go to each pole

Re-assortment of chromosomes

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Re-assortment occurs due to the random distribution of maternal and paternal
chromosomes on the spindle equator at metaphase I and the subsequent segregation into
the two daughter nuclei at anaphase I
Each gamete acquires a different mixture of maternal and paternal chromosomes e
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cell may get maternal 1 and 2 and paternal 3 and the other cell will get maternal 3 and
paternal 1 and 2
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Due to genetic recombination, crossing over a large number of genetically different gametes
could be produced
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Re-assortment of chromatids
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This is the result of the random distribution on the spindle equator, of the sister chromatids
at metaphase II
Because of crossing over, sister chromatids are no longer genetically identical
How they align at metaphase II determines how they segregate at anaphase II

Fertilisation
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In humans only one ovum (actually a secondary oocyte and has not completed the second
meiotic division) is usually released from an ovary at a time
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Whichever fertilises the ovum, genetic material from the two unrelated individuals is
combined via fusing to produce a zygote
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If mutation occurs in the sperm or egg that are used in fertilisation then the mutated gene
will be present in every cell of the offspring

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