Biology today

Zygotene
• In cells undergoing meiosis, there are two sets of
chromosomes, i.e., the chromosome number is diploid.
There are two similar chromosomes of each type. Such
chromosomes are called homologous chromosomes.
The homologous chromosomes resemble each other in the
position of their centromeres, position of chromomeres,
shape and size.

Asters

Fig.:Zygotene

• During zygotene (Gk. zygon-adjoining), the first essential
phenomenon of meiosis occurs. The two homologous
chromosomes get attached to each other laterally due to the
development of nucleoprotein synaptonemal complex
between them. The process of attachment of the homologous
chromosomes is known as synapsis.
• Depending upon the place of origin of pairing, synapsis
is procentric (starting from centromeres and proceeding
towards ends), proterminal (starting from ends and
proceeding towards centromeres) and intermediate (at
various places in between centromeres and ends).

Synaptonemal/Synaptinemal complex (Moses, 1956)
• Completion of the synaptonemal complex occurs during
zygotene and becomes more conspicuous at pachytene.
During diplotene, the synaptonemal complex disintegrates
and usually disappears.
• In a synaptonemal complex, nucleoprotein core has
a tripartite structure, one central and two lateral
longitudinal elements.
• Central element lies between the two homologous
chromosomes. each lateral element occurs in between two
chromatids of a chromosome.

Chromatid

Recombination

nodule

Lateral element

Central element

Fig.:Recombination nodule and synaptonemal comp .lex

•    On account of synapsis, chromosomes form pairs or

bivalents. The number of bivalents is half the number of

the total chromosomes.

•    Synaptonemal complex stabilises the pairing of

the homologues to facilitate recombination. The

recombination nodule (protein molecules) in the lateral

arms of synaptonemal complex is probably related to

crossing over.

Pachytene

•    Soon after completion of synapsis, the cell enters pachytene

stage, the longest phase of prophase - I.

Synapsis of

homologous

chromosomes

Crossing over

Fig.:Pachytene

•    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

(bivalent/dyad). each bivalent or chromosome pair is made

up of actually four chromatids i.e., two of each chromosome.

The two chromatids belonging to the same chromosome

are called sister chromatids and chromatids belonging

to from different chromosomes are termed as non-sister

chromatids.

Crossing over

•    Process of exchange of genetic material or chromatid

segments between non-sister chromatids of homologous

chromosomes is known as crossing over. It is an enzyme

mediated process.

•    It occurs during pachytene after synapsis between

homologous chromosomes.

•    The probability that crossing-over will occur between two

loci increases with increasing distance between the two loci

(the location of a gene on a chromosome is called a locus)

on the chromosome.

Mechanism of crossing-over

•    The mechanism of crossing over process involves a complex

series of events in which DNa segments are exchanged

between non-sister chromatids. Within the synaptonemal

complex, recombination is thought to be carried out

during pachytene by very large protein assemblies called

recombination nodules.

•    recombination nodules (Zickler et al, 1977) contain multienzyme

complex having endonuclease, exonuclease, unwindase,

r-protein etc.