OCR A2 BIOLOGY MEIOSIS unit 2 2.1.18

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2.1.8 ­ Meiosis
In sexual reproduction, the offspring are genetically different from each other and from the parents.
Each parent produces special reproductive cells, called gametes. Gametes (one from each parent) fuse
together during fertilisation to produce a zygote.
The chromosome number must be halved
When two gametes fuse to make one cell the chromosomes are combined into one nucleus. Therefore the
chromosome number in each gamete has to be haploid (half the original number in the cell). This ensures
that, that after fertilisation, the original chromosome number is restored. Meiosis is the type if nuclear
division where the chromosome number is halved. It involves two separate divisions referred to as meiosis
I and meiosis II. In interphase before meiosis I, the DNA replicates. As a result each chromosome consists
of two identical sister chromatids, joined at the centromere. The cell now contains four, rather than 2
original copies of each chromosome.
Meiosis I
Prophase I
The chromatin condenses and undergoes supercoiling so that chromosomes shorten and thicken.
They can take up stains and be seen under a light microscope .
The chromosomes come together in homologous pairs to form a bivalent. Each member of the pair
has the same genes at the same loci. Each pair consists of one maternal and one paternal
The non sister chromatids wrap around each other and attach at points called chiasmata.
They may swap sections of chromatids with one another in a process called crossing over.
The nucleolus disappears and the nuclear envelope disintegrates.
A spindle forms. It is made of protein microtubules.
This process may last for days, months or even years depending on the species and on the type of
gamete (male or female) being formed.
Metaphase I
Bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres.
The chiasmata are still present.
The bivalents are arranged randomly with each member of a homologous pair facing opposite poles.
This allows the chromosomes to independently segregate when they are pulled apart in anaphase I.
Anaphase I
The homologous chromosomes in each bivalent are pulled by the spindle fibres to the opposite
The centromeres do not divide.
The chiasmata separate and lengths of chromatid that have been crossed over remain with the
chromatid to which they have become newly attached.
Telophase I
In most animal cells two new nuclear envelopes form, one around each set of chromosomes at
each pole, and the cell divides by cytokinesis. There is a brief interphase and the chromosomes
In most plant cells the cell goes straight from anaphase I into meiosis II.

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Meiosis II
This division is in a plane at right angles to meiosis I.
Prophase II
If a nuclear envelope has reformed it breaks down again.
The nucleolus disappears, chromosomes condense, and spindles form.
Metaphase II
The chromosomes arrange themselves at the equator of the spindle. They are attached to the
spindle fibres at the centromeres.
The chromatids of each chromosome are randomly assorted.
Anaphase II
The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. The
chromatids randomly segregate.…read more

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In humans only one ovum is released from an ovary at a time (usually). There are about 300 million
spermatozoa, all genetically different and any one of them can fertilise the ovum, genetic material
from the two unrelated individuals is combined to make the zygote.
DNA mutation may also occur during interphase when DNA replicates. This is not peculiar to meiosis
s it can also occur in mitosis or binary fission. Chromosome mutations may also occur. However,
mutation does increase genetic variation.…read more


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