Question

Explain the mechanism of crossing over.

Answer 1

Crossing over is a precise process that includes stages like synapsis, tetrad formation, cross over and terminalization.

1. Synapsis: Intimate pairing between two homologous chromosomes is initiated during the 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:
   a. Procentric synapsis: Pairing starts from the middle of the chromosome.
   b. Proterminal synapsis: Pairing starts from the telomeres.
   c. Random synapsis: Pairing may start from anywhere.

2. Tetrad Formation: Each homologous chromosome of a bivalent begins 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 the tetrad stage.

    

3. Cross Over: After tetrad formation, crossing over occurs in the pachytene stage. The non-sister chromatids of the homologous pairs make a contact at one or more points. These points of contact between non¬sister 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 occurs. This results in a reciprocal exchange of equal and corresponding segments between them. A recent study reveals that synapsis and chiasma formation is facilitated by a highly organized structure of filaments called Synaptonemal Complex (SC). This synaptonemal complex formation is absent in some species of male Drosophila, hence crossing over does not takes place.

    

4. 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.