Crossing Over

Crossing over

Crossing over is a process of exchange of genetic materials or segments between non-sister chromatids of homologous chromosomes. The chromatids in which crossing over has occurred are called crossovers or recombinants and the chromatids that remain intact are called non-cross or parental chromatids. As a result of crossing over, new or non-parental combinations are formed.

Crossing over occurs in pachytene stage of prophase I of meiosis during a process called synapsis. Hence, crossing over is one of the final stages of genetic recombination. Before the synaptonemal complex develops, the process of synapsis begins and is not completed until near the end of prophase I. Crossing over usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

Crossing over was first described by T.H. Morgan. He first described crossing over in theory. He depended on the findings/research of Belgian Professor Frans Alfons Janssens of the University of Leuven who described the phenomenon in 1909 and had termed the process “chaismatypie”. The term chaisma is identical to the term chromosomal crossover. Morgon, seeing the importance of this interpretation of chaisma, researched on the heredity of Drosophila. The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931.

As stated above, crossing occurs during prophase I of the first meiotic division. Generally, two non-sister chromatids cross each other at one or more points. Such points of crossing over are known as chaisma. There is a break at the point of crossing over which is followed by the reunion of corresponding segments of chromatids.

Mechanism of Crossing Over-

The process of crossing over takes place in following steps:

1) Synapsis

2) Duplication of chromosomes

3) Crossing over

4) Terminalizataion

1) Synapsis- Synapsis is the process in which the two homologous chromosomes pair up in the meiosis division. It is also called syndesis. The pairing of the homologous chromosomes allows the matching-up of homologous pairs existing before their segregation. Synapsis takes place during the prophase I stage of meiosis division. When homologous chromosomes (paternal and maternal) come together and lay side by side, they coil around each other, until the matching ends of the chromosomes have been paired, forming a bivalent. Then the coiled regions of the chromosomes are brought together and are connected by a protein called synaptonemal complex. Every chromosome except sex chromosomes undergoes synapsis. Sex chromosomes don't undergo synapsis.

2) Duplication of chromosome-After the process of synapsis, this process occurs. In this process, the chromosomes which are laying side by side forming a bivalent splits longitudinally to form two sister chromatids. This occurs during the diplotene stage of meiosis cell division. Now, the bivalent consists of four chromatids. This stage is called a tetrad. This tetrad is now ready for the process of crossing over.

3) Crossing over- This stage of crossing over is the important region in which the genetic information (materials) are interchanged between the paternal and maternal chromosomes. The formed tetrad, which is arranged in a bivalent side to side state, and is attached and coiled around each other, start to uncoil. The chromatids remain attached at one or more points and this establishes one or more exchange of genetic material per coiling. This process takes place during the pachytene stage of meiosis cell division. The point of attachment of coiling between the chromatids is called chaisma. Hence, chaisma is the point in crossing over from where genetic material is exchanged between paternal and maternal chromosomes. The crossing over starts at synapsis stage but it is clearly visible at this stage as the chromatids start to uncoil and the changes are more notable.

4) Terminalization-After the completion of the exchange of genetic materials between the chromosomes (crossing over), the non-sister chromatids do not start repelling each other. Slowly, the chromosomes, start to get away from each other and separate from the centromere. The chromatids now shift towards the terminal ends. The constant repulsion between the chaisma now causes the chromosomes to be free, The movement of chaisma towards the end of the chromosome is known as terminalization.

Kinds of crossing over-

The number of chaisma usually depends on upon the length of the chromosome. Hence, depending upon the number of chaisma, crossing over can be of three types:

1) Single crossing over -In single crossing over there is only one point of contact or chaisma in the chromosome. This means, the chromatids of homologous chromosome contact and break only at one point along their entire length.

2) Double crossing over-In double crossing over there is two point of contact or chaisma in the chromosome. This means, the chromatids of homologous chromosome contact and break at two points along their entire length.

3) Multiple crossing over-In multiple crossing over there are a number of contact points or chaisma formed in the chromosome. This means, the chromatids of homologous chromosome contact and break at multiple points along their entire length,

Factors affecting crossing over

The frequency of chaisma formation may be influenced by a number of physiological and environment factors. Following are the factors that affect the process of crossing over:

a) Age-It was found that the frequency of crossing over decreases with the age. This was found in femaleDrosophila.

b) Temperature-The increase in temperature increases the amount of crossing over.

c) Chemical-Certain chemicals also do affect the rate of crossing over.

d) The distance between two genes-The distance between two genes is the main factor that affects the process of crossing over. If two genes are distant apart, they have better chances of separation than those which are closely set together. Thus, the longer the distance between two genes, the better chances of crossing over.

Stages of crossing over-

During leptotene substage of prophase I, the chromosomes appear as a thread. In zygotene substage, homologous chromosomes pair to form bivalent.

In zygotene substage, homologous chromosomes pair to form bivalent.

In pachytene substage, each homologous chromosome splits up into two chromatids.

In diplotene substage, these chromatids start separating out.

This means that between pachytene and diplotene substages, the bivalent is composed of four chromatids. This is known as tetrad stage. At this substage, crossing over occurs between homologous chromosomes,

The crossing over or recombination occurs at tetrad stage during pachytene stage of meiosis. It has been well demonstrated in the mould (fungi)Neurospora.

Significance of crossing over-

Crossing over is significant in the following ways:

1) It produces new combinations of genes.

2) Due to crossing over, useful recombinations can be formed which might be used by plant breeders.

3) Various organisms have been created through recombination of genes.

Non-disjunction

During meiosis, homologous chromosomes separate and move to opposite pole. Sometimes homologous chromosomes do not separate or disjoin during meiosis into individual gametes. This phenomenon is called non-disjunction. It is mainly due to lack of proper pairing between homologous chromosomes during metaphase. As a result, centromeres of two homologous chromosomes are not on opposite side of metaphase phase and two unpaired homologous then separate randomly, moving either to opposite poles or to the same pole. This movement of both homologous chromosomes during meiosis may lead to the presence of both chromosomes in one gamete and absence in the other.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.