CROSSING OVER

Crossing over is the exchange of segments between the non-sister chromatids of homologous chromosome. Don’t confuse this term with translocation. Translocation is the movement of segments between non-homologous chromosomes or region of the same chromosome. An example may fix the issue. Philadelphia chromosome; a chromosome formed due to reciprocal translocation between chromosome 9 and 22. A portion of chromosome 9 translocated to chromosome 22 that is between two different chromosomes. This is translocation. O.K.  Fine. 

Types of crossing over

The term crossing over was coined by Morgan. Crossing over can even occur in somatic cells. Somatic crossing over is rare and has no significance. Somatic crossing over is reported in Drosophila by Curt Stern (1935). Now moving into the detail of meiotic crossing over or germinal crossing over; this type of crossing over takes place in germinal cells during gametogenesis. This process is universal in occurrence and has great significance as we discussed in the introductory paragraph.

Mechanism of meiotic crossing over

Crossing over is a crucial process that generates genetic difference within a population.  The prerequisites for crossing over are firstly, 99.7% of DNA replication and 75% of histone synthesis must occur by prophase I. Secondly, each chromosome must attach by its telomeres (ends of the chromosome) to the nuclear envelope through specialized structures called attachment plaques. 

The major steps in meiotic crossing over are 

1) synapsis 

2) duplication of chromosome 

3) crossing over and 

4) terminalisation.

                Synapsis is the intimate pairing between the two homologous chromosomes. It is initiated during the zygotene stages of prophase I of meiosis I. Here chromosomes are aligned side by side and each gene has its counterpart aligned perfectly (gene for gene alignment). The resultant pairs of homologous chromosomes are called bi-valents.

Synapsis is followed by duplication of chromosome (in pachytene). Sister chromatids are held at centromere. At this stage, each bi-valent has four chromatids now called as tetrad.

Crossing over or exchange of segments between the non-sister chromatids of homologous chromosome occurs at the tetrad stage. Homologous chromosome may stay in synapsis for even days during pachytene stage. Now let us have a look into the details of crossing over. 

For easier understanding, crossing over can be divided into three major steps:

1) breakage of chromatid segments 

2) their transposition (movement to the respective site) and 

3) fusion or joining.

Which are the enzymes involved in crossing over?

1) Recombinase is the major enzyme regulating recombination event

2) Endonuclease 

3) Ligase enzyme
Endonuclease is responsible for breakage of 2 non-sister chromatids at corresponding sites. This is followed by the exchange of segments and finally the exchanged segments are joined or the gap is filled by ligase enzyme. 

Crossing over takes place at several points on a tetrad and result in several chiasmata. These are regions were chromosomes are held together.  Larger the chromosome size the more the number of chiasmata. Frequency of crossing over is dependent on the physical distance between genes on the chromosome. The chance of crossing over is more for distantly located genes.

                The final step is terminalisation. After crossing over the non-sister chromatids starts to repel each other. During diplotene, Synaptonemal complexdissolves and desynapsis takes place. During diakinesis, chromosome detaches from the nuclear envelope and the chromatids separates progressively from the centromere towards the chiasmata. Meanwhile chiasma itself moves in a zipper fashion towards the end of tetrad. This movement of chiasma is known as terminalisation. As a result of terminalisation, homologous chromosomes are separated completely.