explain in detail the phases that occur during meosis I
Answers
Answer:
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Explanation:
Four haploid daughter cells are produced at the end, unlike two diploid daughter cells in mitosis. Meiosis 1 separates the pair of homologous chromosomes and reduces the diploid cell to haploid. It is divided into several stages that include, prophase, metaphase, anaphase and telophase.
Meiosis I:
It consists of four stages:
(a) Prophase I:
Prophase I is a long and
elaborate process and is further
divided into five sub-stages.
(i) Leptotene-
- Condensation and coiling of
Chromatin fibres begins during
Leptotene and Chromosomes are
formed, which are visible under
the light microscope.
Bouquet formation( telomeres of
the chromosomes get polarized
towards the centrioles)
(ii) Zygotene-
- Each diploid cell contains a pair
of Homologous chromosomes.
One of them is the Paternal
chromosome while the other one
is Maternal chromosome.
- Pairing of Homologous
chromosomes takes place during
the Zygotene , they come to lie
side by side. This phenomenon is
called Synapsis.
- The complex formed as a
result of Synapsis is called
Bivalent (Pair of chromosomes)
.or
- The process of Synapsis is
accompanied by the formation of
Synptonemal complex. It is a
protein structure which forms
between the homologous pair of
chromosomes and stabilizes
them during Synapsis and
Crossing over (recombination).
(iii) Pachytene-
- The chromosomes continue to
become short and thick. All the
sister chromatids are clearly
visible at this stage and the
synapsed pair is called Tetrad (
Equivalent to four
chromosomes).
- The two chromatids from the
same Chromosome are called
‘Sister- Chromatids’ and the two
chromatids from the other
Chromosome are called ‘Non-
sister chromatids’ .
- During Pachytene, Crossing
Over takes place between the
non-sister chromatids, which
means that there is exchange of
Genetic material between two
non-sister chromatids. It takes
place by breaking up and reunion
of segments of Chromatids.
- During Pachytene, there is
mutual exchange of
corresponding segments between
the non sister chromatids,
resulting in a ‘Recombination of
Genetic material’.
- The Recombination is mediated
by an enzyme called
Recombinase. The site where
crossing over occurs forms a
‘Recombination Nodule’.
(iv) Diplotene-
- The Synptonemal complex
dissolves during Diplotene and
the homologous chromosomes
start separating from each
other.
- However, they remain attached
at the region of crossing over.
This point of attachment of the
non –sister chromatids of the
homologous chromosomes is
called Chiasma.
It appears as a X-shaped
structure.
- The Diplotene stage may last
for months to years. E.g.
Oocytes of some vertebrates
.This is called ‘Dictyotene’.
(v) Dialknesis-
- The separation of non-sister
chromatids continues and the
Chiasma itself starts moving
towards the tip of the tetrad in
a Zipper fashion.