Crossing Over (Genetic Recombination)

Discovery of Crossing Over:

Belgian Cytologist Janssens (1909) noticed chiasmata formation during the prophase of meiosis I. Morgan (1910) found that chiasmata formation leads to the crossing over of alleles by breakage and reunion of homologous chromosomes.

What is Crossing Over?

Crossing Over occurs during meiosis I while homologous chromosomes have synapsed. Crossing Over may be defined as the exchange of a part of a chromatid from one homologous chromosome with an equivalent part of a chromatid from the other homologous chromosome. This exchange results in a new gene combination. Remember that a chromosome is a double strand of DNA. To break a chromosome, bonds between sugars and phosphates are broken. This is done at the same spot on both chromatids, and the two pieces switch places. After switching places, the two pieces of DNA are bonded together by re-forming the bonds between the sugar and the phosphate molecules.

Mechanism of Crossing Over:

The mechanism of crossing over is also called modern breakage reunion theory. The various steps are-

• Synapsis- In this phase the chromosome form pairs. The pairing takes place from centromere to centromere, arms to arms. Such chromosomes are called Bivalent. The synaptic forces are responsible for the formation of pairs.
• Tetrad- After the formation of the bivalent stage the chromatids of each chromosome separate and form two visible separated thread-like structure as a result four chromatids are formed from two chromosomes. One chromosome is maternal and the other is paternal.
• Crossing Over- After the formation of the tetrad stage, the two chromatids of a chromosome come close to each other either at 1, 2 or 3 points. Such point of contacts called chiasmata. The chiasmata formation may lead to the breakage and reunion theory of Darlington. In this stage, the chromosome mutual exchange their segments and there is no loss or addition of genes.
• Disjunction- This is the last step and in this stage, only two chromatids have new recombinants and such chromatids are called crossover chromatids and the remaining two are called non-crossover chromatids.

Types of Crossing Over:

Crossing Over is of three types-

• Single Crossing Over- The type of crossing over that involve the formation of single chiasmata is called a single crossing over. It results in the formation of two crossover chromatids and two non-crossover chromatids. But the new segments are attached at the ends.
• Double Crossing Over- The type of crossing over that involve the formation of two chiasmata is called a double crossing over. It results in the formation of two crossovers chromatids and two non-crossover chromatids. But new segments are attached at middle.
• Multiple Crossing Over- This type of crossing over involves the formation of more than two chiasmata and it is very rare in nature but however it may results due to mutations or exposure of chromosome to mutagens.

Importance of Crossing Over:

• It brings a new combination of genes that are different from parents. This introduces variations. The variations are helpful in struggle for existence and adaptability to changes in the environment.
• Variations form a platform for natural selection to act.
• It has helped in establishing the concept of a linear arrangement of genes.
• The frequency of crossing over helps in the mapping of chromosomes, i.e., determining the locations of the genes in the chromosomes.
• Useful recombinations are picked up by breeders for the development of improved varieties.