Crossing Over
Crossing over is a biological process that produces new combination of genes by interchanging the corresponding segments between non-sister chromatids of homologous pair of chromosomes. The term ‘crossing over’ was coined by Morgan (1912). It takes place during pachytene stage of prophase I of meiosis.
Usually crossing over occurs in germinal cells during gametogenesis. It is called meiotic or germinal crossing over. It has universal occurrence and has great significance. Rarely, crossing over occurs in somatic cells during mitosis. It is called somatic or mitotic crossing over.
Mechanism of Crossing Over
Crossing over is a precise process that includes stages like synapsis, tetrad formation, cross over and
terminalization.
(i) Synapsis
Intimate pairing between two homologous chromosomes is initiated during zygotene stage of prophase I of meiosis
I. Homologous chromosomes are aligned side by side resulting in a pair of homologous chromosomes called bivalents. This pairing phenomenon is called synapsis or syndesis. It is of three types,
1. Procentric synapsis : Pairing starts from middle of the chromosome.
2 – Proterminal synapsis: Pairing starts from the telomeres.
3 – Random synapsis: Pairing may start from anywhere. (ii) Tetrad Formation Each homologous chromosome of a bivalent begin to form two identical sister chromatids, which remain held together by a centromere. At this stage each bivalent has four chromatids. This stage is called tetrad stage.
(iii) Cross Over
After tetrad formation, crossing over occurs in pachytene stage. The non-sister chromatids of homologous pair make a contact at one or more points. These points of contact between nonsister chromatids of homologous chromosomes are called Chiasmata (singular-Chiasma). At chiasma, cross-shaped or X-shaped structures are formed, where breaking and rejoining of two chromatids occur. This results in reciprocal exchange of equal and corresponding segments Terminalisation After crossing over, chiasma starts to move towards the terminal end of chromatids. This is known as terminalisation. As a result, complete separation of homologous chromosomes occurs.
