Biology today

• Crossing over is initiated
by enzyme endonuclease
which induces nicks
(single strand cuts) in each
chromatids.
• Gaps develop in the region
of nicks by the activity of
another enzyme called
exonuclease.
• The free ends on one side
of the cuts are displaced
from their complementary
strands with the aid of
DNa-unwinding proteins
i.e., enzyme unwindase.
• The displaced strands
then base-pair with the
complementary intact strand
of the other homologue, that
is, the cleaved single strand
of homologue 1 base pairs
with the complementary
intact of homologue 2, and
vice versa.
• Gaps left are later filled by DNa polymerase and joined
together by enzyme ligase.

Diplotene
• The nucleoprotein fusion complex of the synapsed
chromosomes dissolves partially. The homologous
chromosomes separate except in the region of crossing
over. It makes chromatid more distinct and tetrad more
clear. The points of attachment between the homologous

chromosomes after the partial dissolution of nucleoprotein

complex are called chiasmata.

•    The number and position of chiasmata varies with the length

of chromosomes and with the species. Chiasma (plural-

chiasmata) formed at the ends of chromosome is called

terminal chiasma and along the length of chromosome is

called interstitial chiasma.

•    Diplotene is extended and metabolically active in animal

cells, especially in oocytes because bulk of gametic growth

occurs during this phase. In human oocytes, diplotene stage

last for many years. Suspended diplotene stage is called as

dictyotene stage.

•    In amphibian oocytes, chromosomes decondense and get

engaged in rapid rNa synthesis. Lampbrush chromosomes

are actually decondensed diplotene chromosomes.

Diakinesis

•    bivalent totally separates and nuclear

Fig.:Diakinesis

membrane disappears.

•    During diakinesis, interstitial

chiasma is displaced at the position

of terminal chiasma. This process is

called terminalisation.