Inheritance - Meiosis
Haploid & Diploid Cells
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A diploid cell is a cell that contains two complete sets of chromosomes (2n)
o Nearly all cells in the human body are diploid with 23 pairs (46 individual) of
chromosomes in their nucleus
Haploid cells contain one complete set of chromosomes (n)
o In other words they have half the number of chromosomes compared to
diploid cells
Need for Reduction Division during Meiosis
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During fertilisation the nuclei of gametes fuse together to form the nucleus
of the zygote
Both gametes must contain the correct number of chromosomes in order for the
zygote to be viable. If a zygote has too many or too few chromosomes it may not
survive
For a diploid zygote this means that the gametes must be haploid
o n + n = 2n
o Where n is the haploid number of chromosomes and 2n is the diploid number
of chromosomes
Meiosis produces haploid gametes during sexual reproduction
The reduction in chromosome number during meiosis ensures the gametes formed
are haploid
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Meiosis
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A type of nuclear division that produces haploid cells from a diploid cell. It is used in
the production of gametes in animals and plants.
Meiosis has two divisions, not one as in mitosis. These are called meiosis I and
meiosis II. Each division has the same sequence of stages as in mitosis – prophase,
metaphase, anaphase and telophase (PMAT)
Prophase I
For better understanding, prophase I can further be divided into early, middle and late
prophase:
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Early Prophase
Meiosis begins, as in mitosis, as the chromosomes condense and become visible.
Each chromosome has already been copied, so that each one is made up of two
identical 'sister' chromatids joined at the centromere.
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Middle Prophase
Unlike mitosis, the chromosomes arrange themselves in homologous pairs. The two
chromosomes 1s, for example, line up side by side in a process termed Synapsis.
Each pair of homologous chromosomes is called a bivalent.
Centrosomes move towards opposite poles.
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Late Prophase
The chromosomes in a bivalent are very closely associated. A chromatid of one of
these chromosomes intertwines with a chromatid of the other. Each crossing point is
called a chiasma (plural: chiasmata).
There is almost always at least one, and often several, chiasmata in each pair.
The chiasmata help to hold the chromosomes together in their pair, as they move
through the next stages. Part of a chromatid from one chromosome may break and
rejoin with the chromatid from the other chromosome. This is called crossing over.
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Spindle fibres start forming and they are made from microtubules. These begin to
attach themselves to the centromeres of the homologous pairs of chromosomes.
The nuclear envelope breaks down, and the nucleolus disappears.
Metaphase I
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The spindle fibres, attached to the centromeres, now move the bivalents to the
equator of the cell. The homologous chromosomes in each bivalent remain attached
to each other at the chiasmata.
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Anaphase I
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The spindle fibres begin to pull on the centromeres. This is where the next big
difference from mitosis occurs.
Instead of each chromosome being pulled apart into its two chromatids, it is the two
whole chromosomes in each bivalent that are pulled apart. The centromeres remain
intact, continuing to hold the two chromatids of one chromosome firmly together.
Telophase I
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The chromosomes now arrive at opposite ends of the dividing cell. The spindle fibres
now break down.
In most animals, a nuclear envelope forms around each set, and the nucleolus
generally reforms. This may not happen in plant cells, which may go straight into the
next stage.
At this point reduction division has taken place. Each new nucleus contains one
complete set of chromosomes – one from each homologous pair. The original cell
was diploid, with two sets of chromosomes. The new cells formed will be haploid.
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Cytokinesis
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Usually, the cytoplasm now splits into two, forming two complete cells, each with
the haploid number of chromosomes.
There is no interphase between meiosis I and meiosis II so the DNA is not replicated. The
second division of meiosis is almost identical to the stages of mitosis
Prophase II
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The nuclear envelope breaks down and chromosomes condense
 A spindle forms at a right angle to the old one
Metaphase II
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Chromosomes line up along the equator of the spindle.
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Anaphase II
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Centromeres divide and individual chromatids are pulled to opposite poles
This creates four groups of chromosomes that have half the number of
chromosomes compared to the original parent cell.
Telophase II
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Nuclear membranes form around each group of chromosomes.
Cytokinesis
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Cytoplasm divides as new cell surface membranes are formed creating four haploid
cells
o The cells now only have half the number of chromosomes (previously
chromatids)
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