.Crossing over occurs in
(1) Prophase II
(2) Zygotene of prophase I
(3) Metaphase II
(4) Pachytene of prophase I
Answer ▽ ✅Verified
Answer (4) Crossing over occurs in pachytene of prophase I
👉MEIOSIS
Meiotic events can be grouped under the following phases:
Meiosis I Meiosis II
Prophase I Prophase II
Metaphase I Metaphase II
Anaphase I Anaphase II
Telophase I Telophase II
👉Prophase I:
Prophase of the first meiotic division is typically longer and
more complex when compared to prophase of mitosis. It has been further
subdivided into the following five phases based on chromosomal
behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
During leptotene stage the chromosomes become gradually visible
under the light microscope. The compaction of chromosomes continues
throughout leptotene. This is followed by the second stage of prophase
I called zygotene. During this stage chromosomes start pairing together
and this process of association is called synapsis. Such paired
chromosomes are called homologous chromosomes. Electron
micrographs of this stage indicate that chromosome synapsis is
accompanied by the formation of complex structure called
synaptonemal complex. The complex formed by a pair of synapsed
homologous chromosomes is called a bivalent or a tetrad. However,
these are more clearly visible at the next stage. The first two stages of
prophase I are relatively short-lived compared to the next stage that is
pachytene. During this stage, the four chromatids of each bivalent
chromosomes becomes distinct and clearly appears as tetrads. This stage
is characterised by the appearance of recombination nodules, the sites
at which crossing over occurs between non-sister chromatids of the
homologous chromosomes. Crossing over is the exchange of genetic
material between two homologous chromosomes. Crossing over is also
an enzyme-mediated process and the enzyme involved is called
recombinase. Crossing over leads to recombination of genetic material
on the two chromosomes. Recombination between homologous
chromosomes is completed by the end of pachytene, leaving the
chromosomes linked at the sites of crossing over.
The beginning of diplotene is recognised by the dissolution of the
synaptonemal complex and the tendency of the recombined
homologous chromosomes of the bivalents to separate from each other
except at the sites of crossovers. These X-shaped structures, are called
chiasmata. In oocytes of some vertebrates, diplotene can last for
months or years.
The final stage of meiotic prophase I is diakinesis. This is marked by
terminalisation of chiasmata. During this phase the chromosomes are
fully condensed and the meiotic spindle is assembled to prepare the
homologous chromosomes for separation. By the end of diakinesis, the
nucleolus disappears and the nuclear envelope also breaks down.
Diakinesis represents transition to metaphase.
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