Molecular mechanism of crossing over, chromosomal evidence of crossing over, genetic factors which affect the frequency of crossing over

Molecular mechanism of crossing over, chromosomal evidence of crossing over, genetic factors which affect the frequency of crossing over:-

Molecular mechanism of crossing over:- The process in which genes are exchanged between the non-sister chromatids of homologous chromosomes is called crossing over.

 

·    This is a process occurring only in the Prophase - I of meiosis.

Mechanism:- It has 5 main steps –

1. Synapsis

2. Duplication

3. Crossing Over

4. Chiasmata Formation

5. Terminalization

1. Synapsis:-

Ø  It occurs in the zygotene sub-stage of prophase - I.

Ø  In this step the homologous chromosomes are arranged near each other at a distance of 1000A°. This process is called Synapsis.

Ø  3 types of coupling occurs in the formation of synapsis.

Ø  Synaptonemal complex:- After formation of synapsis, it forms a fibrous complex in the space of 1000A°. It was discovered by M. J. Moses in the Spermatocyte of Crayfish.

2. Duplication:-

Ø  It occurs in the Pachytene sub-stage of prophase - I.

Ø  In this step, each homologous chromosome splits to form 2 sister chromatids. There is no splitting of the centromere.

3. Crossing Over:-

Ø  It occurs in the Pachytene sub-stage of prophase - I.

Ø  In this step, crossing over takes place between the internal non-sister chromatids. Recombinase enzyme takes part in this process.

Ø  The internal non-sister chromatids are broken into segments. Now these segments are exchanged.

4. Chiasmata Formation:-

Ø  It occurs in the diplotene sub-stage of prophase - I.

Ø  After the exchange of segments between non-sister chromatids, they are joined. As a result, an X-shaped structure appears in the diplotene sub-stage, which is called Chiasma.

Ø  The number of Chiasmata depends on the length of the homologous chromosomes. Usually their number does not exceed 4.

Ø  There are 2 types of Chiasma depending on the position:-

i. Terminal Chiasmata:- Present at the top of the chromosome.

ii. Intercalary Chiasmata:- It is present in the middle of the chromosome.

5. Terminalization:-

Ø  It occurs in the Diakinesis sub-stage of Prophase - I.

Ø  After the process of crossing over is complete, the chromatids move away from each other and the chiasmata get disappeared. This process is called Terminalization.

Chromosomal evidence of crossing over:-

Stern's Experiment:-

> The first cytological evidence in support of genetic crossing over was provided by Curt Stern in 1931 on the basis of his experiments with Drosophila by using cytological markers. 

> He used a Drosophila female fly in which one X- chromosome was broken into two segments and out of these two segments, one behaved as X- chromosome. 

> This chromosome had one recessive mutant allele car (carnation) for eye colour and another dominant allele B (Bar) for eye shape. 

> The other X-chromosome had small portion of Y chromosome attached to its one end. 

> This chromosome had the dominant allele + (wild type allele of car) producing dull red eye colour and a recessive allele + (wild type allele of B) producing normal ovate eye shape. 

> Thus the phenotype of female is barred (since B is dominant to +) with normal eye colour (since car is recessive to +) 

> Both the X-chromosomes in the female had distinct morphology and could be easily identified under microscope. 

> Such females were crossed with male flies having recessive alleles for both genes (car +). 

> As a result of crossing over female flies produce four types of gametes viz., two parental types or non crossover types (car B and + +) and two recombinant types or cross over types (car + and + B). 

> The male flies produce only two types of gametes (car+ and Y), because crossing over does not occur in Drosophila male. 

> A random union of two types of male gametes with four types of female gametes will produce males and females in equal number of four each. 

> Stern cytologically examined the chromosomes of recombinant types i. e. carnation with normal eye shape and barred with normal eye colour. 

> It was found that carnation flies did not have any fragmented X- chromosome but rather had normal X-chromosome. 

> On the other hand barred flies had a fragmented X-chromosome with a segment of Y-chromosome attached to one of the two fragments of X- chromosome. 

> Such chromosome combination in barred is possible only through exchange of segments between non- sister chromatids of homologous chromosomes. 

> This has proved that genetic crossing over was accompanied with an actual exchange of chromosome segments. 

Genetic factors which affect the frequency of crossing over:- The frequency of cross between genes is basically dependent upon the distance between the two, but a number of other genetic, environmental and physiological factors are known to affect the frequency of cross over. The following are some of them: 

1. Sex of the organism:- In certain individuals, cross over may be reduced, suppressed or even totally eliminated altogether. The following table gives a comparison between the male and female individuals of a species with regard to the frequency of cross over. 

2. Age of the organism:- Bridges (1917) in his studies on Drosophila has shown that with the increasing age in the flies, the frequency of cross over particularly of those genes close to centromere would be substantially reduced. 

3. Temperature:- The rate of cross over will be fairly normal at a temperature range between 17 to 29 (Plough 1917). Any alteration in this range, either decrease or increase will correspondingly decrease or increase the frequency of cross over. This is however subject to certain limit. 

4. Decreased Hydration:- Lowering of water content in the surroundings is known to reduce the frequency of chiasma. It may even result in complete suppression of cross over in extreme dehydrated conditions (Sybenga 1972). 

5. Distance between the genes:- This is perhaps the most important factor influencing the frequency of cross over between two gene loci. As has been repeatedly pointed out, proximity between genes reduces the chances of cross over while distance increases the rate of cross over. 

6. Ionizing radiation:- Organisms subject to the effect of ionizing radiations show increased cross over frequency as seen in the case of Drosophila. One of the reports (Sybenga 1972) indicates that even in Drosophila males, crossing over may be induced by radiation. Work on Lilium and Chlamydomonas (Lawrence 1965) have indicated radiation effects on chiasma frequency. 

7. Chemicals:- Certain chemicals which induce chromosomal aberrations are also known to induce or suppress crossing over. Ethyl methane sulphonate is known to induce somatic cross over. Colchicine also prevents cross over by preventing pairing between chromosomes. High dose of selenium will also reduce crossing over frequency. 

8. Chromosomal aberrations:- Inversion in chromosomes alters the gene sequence and thus reduces pairing and crossing over. This is particularly true in inversion heterozygote, when any cross over in the inverted portion would result in duplications and deficiencies in the meiotic products. In addition to the above, some of the cytoplasmic factors are also known to have an influence on the frequency of cross over.