In the meiotic cross over there are four steps. These are:
The homologous chromosomes present in the germinal cells come closer and pair longitudinally. This process called pairing or synapses takes place during the zygotene of prophase 1.
This is very important from the point of view of exchange of segments. The pairing is very specific and allelic loci in each homologue only undergo pairing. The paired chromosomes are called bivalents.
Synapses, which begins during zygotene continues up to pachytene, when chromosomes maintain physical contact at one or more points called chiasmata.
Mutual attraction brought about by allelic gene loci is believed to be responsible for synapses.
The force of attraction is called the synaptic force. In order to account for the synaptic force Darlington has proposed a theory. The theory is known as the precocity theory.
This theory proposed by Darlington C.D. (1937) assumes, that the chromosomes, to have an independent behavior during division, must possess two chromatids. In other words they must replicate as it happens dung mitosis.
However, during early prophase the chromosomes do not replicate and they have only one chromatid which represents an unbalanced or unsaturated state. As a result, the chromosomes seek their partners and pair.
During pachytene, when the chromosomes start exhibiting the double chromatids, their pairing need is satisfied and the chromosomes move apart. Darlington opines that synapses are due to the precocious movement of the chromosomes.
Recent studies at the molecular level of the synaptic phenomenon have indeed supported the precocious movement theory of Darlington.
The studies of Hotta, I to and Stern (1966) have indicated that, some amount of DNA (about 0.3%) is actually synthesised during zygotene and pachytene in the pollen mother cells of Lilium.
The bulk of the DNA however is synthesized during premeiotic interphase. This shortage of DNA (0.3%) according to Stem in responsible for pairing the studies of Bogadnov et al (1968) is Glyclus domesticalis have also revealed a similar synthesis of DNA during meiotic division.
The appearance of a complex anastamaose of filamentous structures during the process of pairing has been reported in a number of organisms. This structure called synaptonemal complex is thought to function as a promoter of synapses.
(b) Duplication of chromosomes:
The bivalents undergo duplication during pachytene. Each chromosome now consists of two chromatids (but the Centromere will not divide); thus there will be four chromatids (tetrad).
(c) Crossing over:
This takes place at the tetrad or the four strand stage. At the time of cross over the two opposing non sister chromatids (chromatids belonging to two different chromosomes), have a break at identical points.
This is brought about due to the action of the enzyme endonuclease. The two chromatids exchange an identical length of the genome. After exchange the segments fuse with the chromatids due to the action of the enzyme called ligase (Stem and Hotta, 1969).
There is some synthesis of DNA during this stage, as it is necessary to repair the broken points of the chromosome. The crossing over is physically demonstrable in the form of chiasma.
After the exchange of segments, the two chromosomes start moving away from each other as the synaptic force lapses. The separation begins from the centromere and moves towards the ends of the chromosomes.
It is this uncoupling of chiasma that is called terminalization. During diakinesis the homologous chromosomes get separated except at their ends.