Certain genes located near one another in the same chromosome show linkage. These linked genes may either remain together in the process of inheritance resulting in complete linkage or drift apart during gametogenesis, resulting in incomplete linkage. The incomplete linkage occurs as a result of recombination of linked genes which is possible by a process known as crossing over. Crossing over can be defined as the exchange of strictly homologous segments between non-sister chromatids of homologous chromosomes.1 The chromatins resulting from such an interchange of chromosomal parts are known as cross-overs.
Crossing over occurs in the pachytene substage of Prophase during Meiotic cell division. It simply involves an exchange of genetic materials between non-sister chromatids by break and exchange following replication. Certain external agents, such as heat shock, chemicals, radiation, etc., are known to have significant effects on crossing over.
1.1. Mechanism of Crossing Over:
The entire process of crossing over can be explained in the following four stages:
- Synapsis
- Duplication of chromosomes
- Crossing over
- Terminalization
a. Synapsis
Synapsis is defined as the process of pairing between two homologous chromosomes (one maternal and one paternal). It occurs during the zygotene substage of Prophase I of meiosis I. Synapsis initiates when the homologous ends of two chromosomes are lined side by side in a zipper-like manner. This closeness between the chromosomes occurs due to attraction between two exactly identical or homologous regions or chromomeres. The paired chromosomes are now known as bivalents. According to the precocity theory of meiosis proposed by C.D. Darlington in 1937, single chromatids are in an unbalanced or unsaturated state in electrostatistical relations. Thus, the chromosomes must pair to become saturated or balanced.
Montrose J. Moses observed a highly organized structure of filament between the paired chromosomes of zygotene and pachytene stages which was later named as “Synaptonemal complex”. This complex has also been observed in a wide variety of species of plants and animals. This complex is found to have a significant role in chiasma formation as well as crossing over. It is used to maintain pairing in a fixed state for an extended period of time. It also provides a structural framework within which molecular recombination can occur.
b. Duplication of chromosomes:
Synapsis is followed by duplication of chromosomes which occurs in the Pachytene substage. During this stage, each homologous chromosome of bivalents splits longitudinally and results in formation of two identical sister chromatids. The sister chromatids remain held together by a centromere. The splitting of chromosomes is possible due to the separation of previously duplicated DNA molecules along with certain chromosomal proteins. At the end of this stage, each bivalent contains four chromatids and is called a tetrad.
c. Crossing over by Breakage and Union:
Homologs continue to stay in synapsis for days during the pachytene substage. The actual chromosomal crossing over occurs due to the exchange of chromosomal materials between non-sister chromatids of each tetrad.
Initially, the two non-sister chromatids break at the corresponding points due to the activity of endonuclease. Then, a segment of one side of each break connects with a segment on the opposite side of the break. The fusion of chromosomal segments with that of opposite one takes place due to ligase. There may be formation of several chiasmata in one tetrad due to when crossing over takes place at several points. In this way, the two non-sister chromatids cross each other. This process is known as chiasma formation. The longer the chromosome, the more will be the number of chiasmata formed. The farther apart the genes are located on a chromosome, the greater the probability of a chiasma occurring between them.2
d. Terminalization:
When crossing over is completed, the non-sister chromatids begin to repel each other as the force of synapsis attraction between them decreases. During the diplotene substage, the synaptonemal complex dissolves and the two homologous chromosomes in a bivalent are pulled away from each other. During diakinesis substage, bivalent is observed to contain four separate chromatids with each pair of sister chromatids linked at their centromeres. The non-sister chromatids which have crossed over are linked by chiasmata. The chromatids separate from the centromere gradually and chiasma moves towards the end of the tetrad. The movement of chiasma is known as terminalization. As a result of terminalization, homologous chromosomes are separated completely.1
1.2. Theories on the mechanism of crossing over:
Different theories have been proposed regarding the mechanism of crossing over. Some of them are described below:
i) Duplication theory:
John Beling proposed this theory. He believed that crossing over occurred due to attachments formed between newly synthesized genes. He visualized genes as beads, specially known as chromomeres which are connected by interchromomeric regions. Initially, during duplication chromosome is synthesized which remains tightly bonded to the old one. The new interchromomeric region begins to join the new chromomeres. The newly synthesized chromomeres shift to an adjacent chromomere of another homolog. This results in crossing over.
ii) Copy choice hypothesis or Switch Model:
This theory was proposed by Laderberg. According to this theory, the donor or parent chromosome acts as a template to synthesize the daughter chromosome. The daughter chromosome contains the donor segment, the original recipient’s whole chromosome, and a hybrid daughter chromosome. In successive divisions of daughter cells, the donor segment is lost. This theory has been heavily criticized on account of two facts:
a) DNA replication occurs in a semi-conservative manner rather than conservative as proposed by the theory.
b) Crossing over occurs at the tetrad stage as opposed to the involvement of any two chromatids suggested by it.2
iii) Breakage and Exchange theory:
It is the most accepted theory explaining chromosomal crossing over. According to this theory, breaks occur in the non-sister chromatids of tetrad then chromosomal segments are exchanged between them.
1.3. Types of crossing over:
According to the number of chiasma formed, crossing over is of the following types:
- Single Crossing Over: If the chiasma occurs only at one point of the chromosome pair, the crossing over is known as a single crossing over.
- Double Crossing Over: If the chiasma occurs at two points of the chromosome pair, then the crossing over is known as double crossing over.
- Multiple Crossing Over: If the chiasma occurs at more than two places in the same chromosome, then the crossing over is known as multiple crossing over. It occurs rarely. 2
1.4. Significance of Crossing Over:
- It produces a new combination of genes giving rise to variation.
- It plays an important role in determining the nature and scope of hybridization.
- Due to crossing over useful recombination can be formed which can be utilized by plant breeders.
- It helps to predict and understand inheritance patterns.
- Its study helps in the creation of genetic maps and identification of disease-causing genes.
Reference:
- B.D. Singh. Genetics. Second. Kalyani Publishers; 2018.
- Dr. P.S. Verma, Dr. C.K. Agrawal. Cell Biology, Genetics, Molecular Biology, Evolution and Ecology. fourteenth. (Bharatnagar S, Pradhan S, eds.). S.CHAND & COMPANY PVT.LTD.; 2016.
