What is crossing over ?

Explanation

Crossing Over. Janssens (1909 discovered and defented it. It is the mutual exchange of segments of genetic material between nonsister chromatids of two homologous chromosomes, so as to produce recombinations or new combinations of genes, The nonsister chromatids in which exchange of segments have occurred are called recombinants or cross-overs while the other chromatids in which crossing over has not taken place are known as parental chromatids or non cross-overs. Mechanism of Crossing Over. Chromosomes get replicated in S-phase of interphase. Therefore, leptotene chromosomes are double stranded though the two strands are not visible due to presence of nucleoprotein complex in between the chromatids. (i) Synapsis. Replicated but apparently single homologous chromosomes come to lie side by side with similar gene loci of the two chromosomes exactly opposite. It occurs in the zygotene stage of prophase I. The phenomenon is called synapsis. The synapsed pairs of homologous chromosomes are called bivalents. The small amount of unreplicated chromosome (0.3%), if present, also undergoes replication (Stern and Hotta, 1973). The two homologous chromosomes are held together by a synaptinemal complex. (ii) Crossing Over. It occurs in the pachytene stage, with the help of enzymes (endonuclease, exonuclease R-protein. There is breakage of chromatid segments, exchange of nonsister chromatid segments and later their fusion in new places

(iii) letrad Siage. Synaptinemal complex begins to dissolve except in the region of crossing over. Therefore, chromosomes separate and chromatids become distinct at most of the places. As a bivalent appars have four chromatids now, it is called tetrad stage. It occurs in the diplotene stage of prophase I. The synaptinemal attachment points between the homologous chromosomes are called chiasmata. In later stages the chiasmata tend to shift to the sides. The phenomenon is called terminalisation. Many of them disappear prior to metaphase I.