What is Crossing Over?

Gregor John Mendel established in his experiment that characteristics are determined by various variables. When gametes form, these variables segregate independently and are stable. But he didn’t know where these elements were in the cell, so he couldn’t tell what their physical counterparts were. The discovery of chromosomes, the chromosomal theory of heredity, and the mechanism of cell division were all aided by the development of new and improved techniques. Linkage Crossing Over is a phenomenon that has been clarified by geneticists’ substantial work on the chromosomal theory of inheritance.

What is Linkage?

A physical connection between two genes is called a linkage. It is also known as the process of pairing together nearby genes on the same chromosome. Linked genes are more likely to be inherited jointly since they are located close to one another on a chromosome. Complete linkage and incomplete linkage are the two types of linkage.

Crossing Over

When paired homologous chromosomes (one from each parent) exchange DNA during the development of egg and sperm cells, this is referred to as “crossing over” (meiosis). The gametes (egg or sperm) created as a result of this procedure contain new allele combinations, ensuring genetic variety in any progeny. They are called chromosomal crossover as well.

Theoretically, Thomas Hunt Morgan described crossing over. He was relying on Frans Alfons Janssens’ discovery of the phenomena, which he had named “chiasmatypie” and had described in 1909. Chromosome crossover and the term “chiasma” are related, if not the same. Morgan recognized right once how crucial Janssens’ cytological interpretation of chiasmata was to the experimental findings of his work on the heredity of Drosophila. Barbara McClintock and Harriet Creighton presented the physical foundation of crossing over for the first time in 1931.

The crossing-over value has then correlated with the frequency of crossing-over between two gene loci (markers). Recombination in a specific region of a linkage structure (chromosome) tends to be constant for fixed genetic and environmental variables. The same is true for the crossing-over value utilized in the creation of genetic maps.

Origin

Morgan is the originator of the term “crossing over” (1912). It happens at the pachytene stage of meiotic prophase I. The origin of crossing over is explained by the two popular and overlapping theories that are-

• Crossing-over is a creative technique to replace potentially damaged DNA regions since meiosis occurred as another mechanism of DNA repair.
• Meiosis developed from bacterial transformation with the goal of propagating diversity.

Chiasmata

During meiosis, two homologous non-sister chromatids cross across and exchange genetic material at the chiasmata (Singular chiasma): Although the actual crossing-over takes place in the earlier pachytene stage, chiasmata start to show up in the meiotic prophase I diplotene stage. Each tetrad, which is made up of two pairs of sister chromatids, begins to split. The primary point of contact is at the chiasmata.

Non-Homologous Crossover

• Crossovers happen between homologous regions of corresponding chromosomes, but alignments might be off because of similarities in sequence and other variables. 
• Base pair sequences that are repeated a great deal make up the majority of DNA. 
• These repetitive segments, also known as satellites, are rather uniform within a species. 
• Each DNA strand is employed as a template for the construction of new strands during DNA replication utilizing a largely conserved mechanism. 
• When this process is carried out correctly, two identical, paired chromosomes—often referred to as sisters—are produced. 
• In eukaryotes, it is known that sister chromatid crossover events take place at a rate of multiple crossover events per cell per division. 
• The majority of these interactions involve an equal exchange of genetic data, although sequence mismatches can lead to unequal exchanges. 
• These occur in the insertion or deletion of genetic material from the chromosome and go by a number of names, including non-homologous crossover, uneven crossover, and imbalanced recombination. 
• Although uncommon in comparison to homologous crossover occurrences, these mutations are severe and affect numerous loci at once. 
• They are a common source of mutation in the genome and are thought to be the primary cause of gene duplications.

Types of Crossing Over

There are three different types of crossing over, depending on how many chiasmata are formed.

• A single cross-over: Only two out of the four chromatids are involved in the formation of a single chiasma.
• Double cross over: Two chiasmata are formed, involving perhaps two, three, or all four strands
• Multiple cross-over: The frequency of crossing-over is very low and more than two chiasmata are formed.

Theories of Crossing Over

The connection between crossing over and chiasma development is explained by two theories:

• Classical Theory: It is often referred to as a two-plane theory. L.W. Sharp came up with this theory. According to this theory, chiasma causes crossing over rather than being a result of it. Before the genetic crossing over, the chiasma is formed.
• Chiasma-type Theory: It is often referred to as the theory of one plane. Janssen came up with the concept while Belling and Darlington later developed it. This theory implies that crossing over leads to the creation of the chiasma and that crossing over happens first. 

Mechanism of Crossing Over

A double-strand break in one of the DNA molecules signals the start of crossing over at the molecular level. Double-strand breaks can develop naturally as a result of radiation, carcinogens, or the activity of particular proteins. Exonucleases, which eliminate nucleotides from the 5′ end of DNA, then react to this break and eliminate brief intervals of nucleotides from both strands that are oriented in the 5′ -> 3′ direction. This results in two hanging single-stranded areas that become covered with recombinases, which are proteins that catalyze recombination. These enzymes facilitate the invasion of single-strand regions into base-pairing-friendly sequences. This single-stranded region can utilize the sequence on the homologous chromosome because of the proximity of non-sister chromatids during prophase I. As a primer, the first strand that invades creates a double-stranded area for itself utilizing one of the strands from its non-sister chromatid as a template. This results in the displacement of its complementary strand and base pairing with the second single-stranded region that the exonuclease originally produced. In the end, the development of the Holliday junction—a cross-like structure—occurs as a result of the interchange of two strands. This event is known as gene conversion when a heterozygous gene locus turns homozygous during cell division. This phenomenon is named after the scientist who first hypothesized that such a junction could explain both crossing over and another phenomenon known as crossing over. Holliday junctions are also visible under a microscope as “chiasma” towards the conclusion of prophase I, and they are still visible at the end of anaphase I. The proteins MSH4 and MSH5, which regulate genomic manipulation, stabilize and resolve Holliday junctions.

Significance of Crossing Over

1. Crossing Over has significant genetic implications, according to one study. A common occurrence called crossing over offers solid evidence that genes are organized linearly. The knowledge gained from the study of crossing over has tremendously aided in the construction of chromosomal maps and the tracing of linkage groups.
2. It increases variation frequency, an important factor in evolution. Numerous combinations are formed as a result, and these combinations can be affected by natural selection. The established linkage groups and linear arrangement of the genes revealed a significant amount of information on the structure and operation of genes.

Importance 

All organisms cross over, including bacteria, fungi, yeast, higher plants, and mammals. Its importance is 

• Its significance lies in the fact that segment interchange results in novel gene pairings, which are crucial for evolution.
• Genes are ordered linearly on chromosomes, according to studies on crossing over.
• Based on the frequency of cross-over, genetic maps are created.
• Understanding the nature and mechanism of gene action is made easier by crossing over.
• If a useful new combination emerges, it may be applied to plant breeding.

Factors Affecting Crossing Over

Several genetic, physiological, and environmental factors can affect Crossing Over:

1. Sex-the crossing over in male is more than female
2. Mutation-mutation decreases the chances of crossing over 
3. Age-older age are more prone to crossing over
4. Inversion
5. X-Radiation-increase the chances of crossing over 
6. Temperature-High temperatures increase the chances of crossing over
7. Centromere region
8. Nutrition
9. Distance between 2 genes

Functions of Crossing Over

There is a situation known as Muller’s Ratchet that affects organisms that can only reproduce asexually and have no opportunity for recombination. That is, each generation of that species has at least as many, if not more, genetic alterations as the one before it. In other words, there is no chance for genetic mistakes to be fixed or for novel, advantageous combinations to emerge when all the progenies are genetically identical to one another.

A population’s variability is increased by crossing, which also allows some parental combinations to be passed on to the progeny while preventing the accumulation of harmful allele combinations. This achieves a balance between preserving potentially advantageous allelic combinations and allowing for variation and change.

Evolution

By enabling genetic variation on the same chromosome to evolve independently, crossover significantly increases an organism’s capacity for evolution. If there were no cross-over, every genetic variant on a chromosome would be inherited as a block. Imagine a chromosome copy with a good variant say, let’s flu resistance at one gene, and a bad variant, let’s say, tapeworm susceptibility at a different gene. If they don’t cross over, the populace must choose between the flu and tapeworms. By crossing over, the population can advance toward a better solution, resulting in a chromosome with the advantageous variant and without the unfavorable one. This speeds up the adaption process.

Recombination

In an organism known as a recombinant, crossing across leads to the development of new character combinations. To create new allele combinations, DNA segments are split and recombined in this process. Recombination is the name given to this process.

Recombination Frequency Calculation (RF)

Recombination frequency is the proportion of recombinant offspring in a cross. Utilizing the following formula, the recombination frequency (cross-over frequency) (RF) is determined. The information came from alleles involved in coupling confirmation.

Genetic Map

Along the chromosome, genes are found in a linear arrangement. They can be found in a place known as the locus (plural: loci). Genetic mapping is the diagrammatic depiction of gene positions and the corresponding distances between adjacent genes. It varies in direct proportion to how frequently they recombine. Additionally known as a connection map. Alfred H. Sturtevant, a student of Morgan’s, introduced the idea of gene mapping in 1913. It gives hints as to where the genes are located on that chromosome.

Uses of Gene Mapping

• Finding the position, order, and linkages between the genes on a chromosome is helpful.
• Predicting the outcomes of dihybrid and trihybrid crossings.

FAQs On Crossing-Over

Question 1: Define linkage and crossing over.

Answer:

• Linkage: A physical connection between two genes is called a linkage. It is also known as the process of pairing together nearby genes on the same chromosome. Linked genes are more likely to be inherited jointly since they are located close to one another on a chromosome. Complete linkage and incomplete linkage are the two types of linkage.
• Crossing over:When paired homologous chromosomes (one from each parent) exchange DNA during the development of egg and sperm cells, this is referred to as “crossing over” (meiosis). The gametes (egg or sperm) created as a result of this procedure contain new allele combinations, ensuring genetic variety in any progeny. They are called chromosomal crossover as well.

Question 2: What are the uses of genetic mapping?

Answer:

Uses of gene mapping

• Finding the position, order, and linkages between the genes on a chromosome is helpful.
• Predicting the outcomes of dihybrid and trihybrid crossings.

Question 3: What is RF?

Answer:

Recombination frequency is the proportion of recombinant offspring in a cross. Utilizing the following formula, the recombination frequency (cross-over frequency) (RF) is determined. The information came from alleles involved in coupling confirmation.

Question 4: Who invented the classical theory of crossing over?

Answer:

L.W. Sharp invented the classical theory of crossing over.

Question 5: Write the importance of crossing over.

Answer:

 All organisms cross over, including bacteria, fungi, yeast, higher plants, and mammals. Its importance is 

1. Its significance lies in the fact that segment interchange results in novel gene pairings, which are crucial for evolution.
2. Genes are ordered linearly on chromosomes, according to studies on crossing over.
3. Based on the frequency of cross-over, genetic maps are created.
4. Understanding the nature and mechanism of gene action is made easier by crossing over.
5. If a useful new combination emerges, it may be applied to plant breeding.