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Introduction
Meiosis - involves two successive divisions
of a diploid (2N) eukaryotic cell of a sexually
reproducing organism that result in four
haploid (N) progeny cells, each with half of the
genetic material of the original cell. Through
the mechanisms by which paternal and
maternal chromosomes segregate, and the
process of crossing-over, genetic variation is
produced in the haploid cells.
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Premeiotic Interphase• Outside the nucleus of
animal cells are two
centrosomes, each containing
a pair of centrioles. The two
centrosomes are produced by
the duplication of a single
centrosome during premeiotic
interphase. The centrosomes
serve as microtubule
organizing centers (MTOCs).
Microtubules extend radially
from centrosomes, forming an
aster.
• Plant cells do not have
centrosomes. Different kinds
of microtubule organizing
centers serve as sites of
spindle formation.
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Prophase I
• At the start of prophase I, the
chromosomes have already
duplicated. During prophase I,
they coil and become shorter and
thicker and visible under the light
microscope.
• Crossing-over is the process
that can give rise to genetic
recombination. At this point, each
homologous chromosome pair is
visible as a bivalent (tetrad), a
tight grouping of two
chromosomes, each consisting of
two sister chromatids.
• The nucleolus disappears
during prophase I.
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Prophase I
Prophase I has so many processes happening that it is usually separated
into five stages. They are listed, in order, below with their explanations.
• Leptonema
During this stage, the chromosomes begin to condense and become visible.
Researchers also believe that homologous pair searching begins also at
this stage.
• Zygonema
The chromosomes continue to become denser. The homologous pairs have
also found each other and begin to initially align with one another, referred
to as 'rough pairing'. Lateral elements also form between the two
homologous pairs, forming a synaptonemal complex.
• Pachynema
Coiling and shortening continues as the chromosomes become more
condense. A synapsis forms between the pairs, forming a tetrad.
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• Diplonema
The sister chromatids begin to separate slightly, revealing
points of the chiasma. This is where genetic exchange occurs
between two non-sister chromatids, a process known as
crossing over.
• Diakinesis
The chromosomes continue to pull apart, but non-sister
chromatids are still loosely associated via the chiasma. The
chiasma begin to move toward the ends of the tetrad as
separation continues. This process is known as terminalization.
Also during diakinesis, the nuclear envelope breaks down and
the spindle fibers begin to interact with the tetrad.
Where chromatids overlap is called a CHIASMATA, and it
allows for CROSSING OVER of genetic information between
chromosomes
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Metaphase I• The centrioles are at opposite
poles of the cell.
• The pairs of homologous
chromosomes (the bivalents), now
as tightly coiled and condensed
as they will be in meiosis, become
arranged on a plane equidistant
from the poles called the
metaphase plate.
• Spindle fibers from one pole of
the cell attach to one chromosome
of each pair (seen as sister
chromatids), and spindle fibers
from the opposite pole attach to
the homologous chromosome
(again, seen as sister chromatids).
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Anaphase I• Anaphase I begins when the
two chromosomes of each
bivalent (tetrad) separate and
start moving toward opposite
poles of the cell as a result of
the action of the spindle.
• Notice that in anaphase I the
sister chromatids remain
attached at their centromeres
and move together toward the
poles. A key difference between
mitosis and meiosis is that
sister chromatids remain joined
after metaphase in meiosis I,
whereas in mitosis they
separate.
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Telophase I
• The homologous chromosome
pairs complete their migration to
the two poles as a result of the
action of the spindle. Now a
haploid set of chromosomes is at
each pole, with each chromosome
still having two chromatids.
• A nuclear envelope reforms
around each chromosome set, the
spindle disappears, and
cytokinesis follows.
• Many cells that undergo rapid
meiosis do not decondense the
chromosomes at the end of
telophase I. Other cells do exhibit
chromosome decondensation at
this time; the chromosomes
recondense in prophase II.
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The homologous chromosome pairs reach the poles of
the cell, nuclear envelopes form around them, and
cytokinesis follows to produce two cells.
