Meiosis summary

Meiosis summary

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5.5 Cellular division for gametes (meiotic division) for reproduction
Organisms which reproduce sexually produce offspring which are genetically dissimilar from their parents and each other,
and the process of meiosis is required for this. Unlike mitosis, meiosis is used only by special reproductive cells, called
gametes. One gamete from each parent fuses together to produce a zygote at fertilisation. When two gametes fuse to
make one cell, the chromosomes from each parent cell are combined into one daughter nucleus. Therefore, the number
of chromosomes in each gamete needs to be haploid (one half the number of the original cell). This is as opposed to a
diploid cell which contains the full number of chromosomes.
paternal spindle homologous chromosomes
chiasma chiasmata
homologue fibres pulled to opposite poles
Metaphase I Anaphase I
maternal nuclear envelope
homologue Centromeres attach to the spindle One of each pair of homologous
breaks down
fibres, and the bivalents arrange chromosomes is pulled by spindle
Prophase I fibres to opposite poles
themselves randomly on the
Chromosomes shorten and fatten
equator of the cell with each of a
(supercoil) and come together in
pair of homologous chromosomes
homologous pairs to form a bivalent. cells divide by nuclear
facing the opposite pole
The chromatids wrap around one cytokinesis envelopes form
another and attach at points called
chiasmata where they may break and
swap small sections of DNA in a
process called crossing over. The
nuclear envelope breaks down
Telophase I
Prophase II Nuclear envelopes reform around
The nuclear envelopes break down the chromosomes at each pole and
again, chromosomes supercoil and the cell divides by cytokinesis
spindle fibres form
Anaphase II
chromosomes lying on
The centromeres divide and the
the equator of the cell
Metaphase II chromatid chromatids are pulled to
The chromosomes arrange moving opposite poles by the spindle
themselves on the equator of each towards the fibres, and the chromatids
pole randomly segregate
cell, and are attached by their
centromeres again to the spindle.
Again, chromatids are randomly
arranged along the equator
Telophase II
The tetrad of four cells is formed. Each cell
has a haploid number of chromosomes,
Meiosis is complete, and the cell rests haploid
and crossing over during prophase I has
until the next meiotic division cells
introduced genetic variety giving cells of
different composition

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Genetic variation
Meiosis and germline fertilisation increase genetic variation in a number of ways:
through the process of crossing over during prophase I (this occurs where alleles can be shuffled between
chromosomes where they cross over at points called chiasmata)
through genetic reassortment due to the random distribution and subsequent segregation of the chromosomes
during meiosis I (and the chromatids during meiosis II)
through random mutation
fertilisation itself introduces random variation: only one sperm contributes its DNA, and yet there are about 300
million sperm…read more


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