Crossing Over and its mechanism

February 22, 2019

The phenomenon of separation of linked genes is termed as crossing over.
Linked genes separate during inheritance and produce new or non-parental combinations in case of incomplete linkage.
The parental combinations are more than the non-parental combinations.
It is also a physical process of exchange of genetic materials or segments between non-sister chromatids of two homologous chromosomes during meiosis.
It results in production of new or non-parental combinations.
It occurs during prophase I of first meiotic division.
Generally, the two non-sister chromatids cross each other at one or more points.
Such points of crossing over are known as chiasma.
There is a break at the point of crossing over which is followed by a re-union of corresponding segments of chromatids.

Mechanism of crossing over

Mechanism of crossing over can be described under following headings:

1.Synapsis

The paternal and maternal chromosome of a homologous pair come close together during the prophase I of meiosis cell division.
They pair at the zygotene sub-stage which is called synapsis.
During synapsis, the homologous chromosomes lie side by side and coil around each other to form a bivalent.

2.Duplication of chromosomes

Synapsis is followed by duplication of chromosomes.
Here, each of the homologous chromosome in a bivalent splits longitudinally into two sister chromatids during diplotene stage.
Thus the bivalent now consists of four chromatids which is called tetrad.

3.Crossing over

During pachytene stage, when the paired chromosomes start separating, the chromatids remain attach at one or more points and thus establish one or more exchange per bivalent.
This points of attachment are known as chiasma.
At each chiasma, two non-sister chromatids of the bivalent break at the corresponding points and then rejoin with the exchange of segments forming new chromatids.

4.Terminalization

The non-sister chromatids start repelling each other during diakinesis sub stage after completion of crossing over.
The chromatids separate from the centromere.
The chiasma shifts towards the terminal ends.
The movement of chiasma towards ends is known as terminalization.
Gradually the chromatids get condensed and thus shortened and the homologous chromosomes become separated.

Kinds of crossing over

The number of chiasma usually depends upon the length of chromosome.
Crossing over is of three types depending upon the number of chiasma. They are:

1.Single crossing over

In single crossing over there is only one chiasma in the chromosome i.e. the chromatids of homologous chromosome contact and break only at one point along their entire length.

2.Double crossing over

In double crossing over chiasma are formed at two points i.e. chromatids break and rejoin at two points.
It means two chiasmata are formed along the entire length of the chromosome.

3.Multiple crossing over

When crossing over occurs at more than two places in the same chromosome pair and more than two chiasmata are formed, the crossing over is known as multiple crossing over.

Frequency of crossing over

The frequency of crossing over depends upon the distance of genes between which crossing over occurs.
The frequency of crossing over between any two linked genes increases with the distance between them.
Similarly, the closer the two genes are linked, the lesser the chance for a chiasma occurring between them.

Factors influencing Crossing Over

a) Sex

In male Drosophila, the crossing over is completely suppressed.
However, in the male of mammals, there is a tendency of reduction of crossing over.

b)Temperature

Plough demonstrated that when female Drosophila are subjected to high or low temperature, the percentage of the crossing over is increased.
Age
Bridges demonstrated in Drosophila that as the female becomes older the crossing over tends to increase.

c)X-radiations: The irradiations by x-rays and by radium increase the crossing over.

d)Nutritional effect

Levine fed young Drosophila on a high calcium diet and found a decrease in crossing over.
Neel found that the larval starvations at certain age increases crossing over.

e)Distance between two genes

The distance between genes is the main factor in crossing over.
If two genes are distance apart, they have greater chances of crossing over than those which are closely set together.

f)Mutant genes: Certain mutant genes increase the frequency of somatic crossing over.

g)Near centromere and tips of chromosomes, crossing over is less frequent.

Significance of Crossing Over

It affords a proof for the linear arrangement of genes in the chromosome.
As a result of crossing over, new gene combinations are produced, which play an important role in the process of evolution.
It is necessary for the natural selection, because, due to this, the chances of variation increase.
Due to crossing over, useful recombinations can be formed which might be used in breeding programmes.

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I am Shailesh Koirala