Question

6) Describe the process of crossing over and synapsis in detail.

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Answer 1

Crossing over

Crossing over is the process in which genetic material is exchanged between non-sister chromatids of homologous chromosomes, resulting in new allelic combinations in the newly formed daughter cells.

Features

a. Crossing over takes place during meiosis, during this stage each pair of a chromosome has four chromatids.

b. Crossing over usually involves the participation of two of the four chromatids of the homologous chromosomes. However, a very rare process of double or multiple crossing over which involves all four also occurs at times.

c. Two recombinant types or crossover types and two non-crossover types or parental types are formed as a result of crossing over.

d. The value of recombinants may vary from 0-50%.

e. The recombination frequency can be calculated from the cross progeny. It is expressed as the percentage ratio of recombinants to the total population.

Mechanism of Crossing Over

1. Crossing over initiates with a double strand break in the DNA molecules.
2. This double-strand break can be instigated by radiation/ carcinogens, or by specific enzymes.
3. The exonucleases, then removes short stretches of nucleotides in the 5’ -> 3’ orientation from the double-strand breaks. This leads to the formation of hanging single-stranded regions that get coated with recombinases that catalyze homologous recombination,
4. During homologous recombination the invading strand behaves like a primer and synthesizes a double-stranded region in DNA using one strand of its non-sister chromatid as a template. Eventually, two strands are being exchanged with the formation of Holliday junction (cross-like structure).
5. Holliday junctions are seen under microscope as ‘chiasma’ towards the end of prophase I, which continue to be visible till the end of anaphase I.

Functions of Crossing Over

1. Crossing over increases the variability of a population
2. It prevents the build-up of deleterious recombination’s of alleles.
3. Traits are passed on to the offspring by this process.

Synapsis

1. Synapsis is the process of the pairing of two homologous chromosomes during prophase I of meiosis.
2. It allows pairing of homologous chromosomes prior to their segregation, and chromosomal crossover.
3. Synapsis begins when proteins that connects different chromosomes to the nuclear membrane starts to migrate. This results in formation of synaptonemal complex, which consists of protein and RNA, and can efficiently hold the homologous chromosomes together.

Function of Synapsis

1. The most important task during meiosis is attachment of chromosomes at metaphasic plate. Synapsis holds the homologous chromosomes together through metaphase I of meiosis I and aligns them on the metaphase plate. Eventually after separation of the chromosomes towards each pole, the genetic information in each gamete is reduced.
2. Exchange of information occurs between homologous chromosomes during crossing over.
3. Synapsis ensures complete distribution of alleles in the new cells.