Phases of prophase 1
This is very prolonged phase. The chromosomes behave as homologous pair during
meiosis. It begins after interphase. Chromosomes have already been duplicated in
interphase so each chromosomes have two chromatids.
Stages of meiosis are:
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis
Leptotene:
This is the first stage of prophase I and the shortest phase of prophase I.
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It the phase of condensation of replicated chromosomes.
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The chromosomes condense and become compact and visible hence making it
possible to distinguish between sister chromatids.
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Size of nucleus increases.
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The chromosomes appear like strings with beads and the beads are known as
chromomeres.
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Each of the sister chromatids gets attached to the nuclear envelopes.
Zygotene:
-also known as zygonema.
This is the phase where homologous chromosomes associate closely to forms pairs of
chromosomes, a mechanism known as synapsis. The pairs of chromosomes have four
chromatids (tetrads).
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The synaptic association forms up and down the chromosome creating several
points of contact known as synaptonemal complex which has a zipper-like
structure created by coils of the chromatids.
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Synapsis is facilitated by the synaptonemal complex by holding the aligned
chromosomes together.
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After synapsis of the homologous pairs, they form four chromatids which are
known as tetrads or bivalents (two pairs).
Pachytene
This is the phase where the cross over of genetic materials takes place
between non-sister chromatids i.e pairs of homologous chromosomes.
This forms chiasmata.
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Crossing over is achieved by synapsis by the attachment created
by the chromatid in a homologous chromosome.
The synapses complete the crossing over of genetic information,
creating a variation in the genetic materials due to the exchange
between the mother and father genetic elements.
Separation of sister chromatids occurs and the homologous
chromosomes remain attached, hence a thick complex is formed
known as the synaptonemal complex.
The synaptonemal complex allows the formation of chiasma which
function by allowing the crossing over of alleles within small
regions of the chromosomes.
Diplotene
This is the stage of synaptonemal complex disappearance while the
homologous pairs remain attached at the chiasmata.
 The disintegration of the synaptonemal complex occurs between
the two chromosomal arms causing repulsion of the arms.
 This causes the chromosomes to move apart from each other while
still being held by the chiasmata.
 The chromosomes then start to uncoil slowly and the chiasmata
can be visualized microscopically and they can be seen moving
close to the ends of the chromatids. This process is known as
terminalization.
Diakinesis- The fifth and final phase of prophase I. It sets up the cell for
metaphase.
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It takes place after the chromatids have condensed and the sister chromatids are
bivalent of tetrad as visualized under a microscope.
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The chiasmata finally arrive at the end of the chromatid arms of the
chromosomes finalizing terminalization.
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the Chromosomes become more condensed but remain connected by the
chiasmata and can not move further toward the poles.
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At this stage the nucleolus and the nuclear envelope dissolves allowing the
centrioles (the centrosome forming microtubules) that form the mitotic spindle,
to migrate freely along with the remaining spindles formed during mitosis.