MOLECULAR MECHANISM OF
CHROMOSOMAL PAIRING
Homologous chromosomes start pairing at one or more
points along the lengths of chromosomes during this stage. Pairing extends
along the rest of the chromosome length in a zipper like fashion. Although
heredity, temperature, nutrition, etc.are known to affect pairing, the
mechanism of pairing is not well understood. Pairing or synapsis is a very
precise process in which only homologous parts o chromosomes come together.
Non-homologous parts are looped out and thus remain unpaired.
A pair of
homologous chromosomes is called bivalent
.At any one point only two chromosomes can get paired. This is evident when
three or four homologous chromosomes are held together to form multivalent, but
even then at any given point only two chromosomes are paired. Under the
electron microscope the synaptonemal complex appears multilayered. There is a
central axis which is flanked on both the sides by an axial element.
There is a
central space in between the central and axial elements. Chromatin is dispersed
towards the outer faces of the axial element. DNase destroys the chromatin but
not the axial or central elements. These are destroyed by trypsin, indicating
thereby that both the central and axial elements are proteinaceous in nature.
The central axis is composed of a substance which is synthesized during
pairing. Synaptonemal complexes not formed if the homologous are experimentally
prevented from pairing. The ends of chromosomes are attached to the inner
surface of the nuclear membrane at this stage. A part of the DNA (about 0.3%in
lilium) that remains unreplicated during interphase is replicated during this
stage of meiosis.
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