Recombination: Videos & Practice Problems

Homologous recombination involves the exchange of genetic material between homologous chromosomes at equivalent positions, initiated by single or double strand breaks. Single strand breaks create a cross bridge structure allowing DNA strands to invade each other, while double strand breaks form a Holliday Junction for template repair. Gene conversion, a nonreciprocal exchange, can alter allele ratios, leading to unexpected genetic outcomes. Understanding these processes is crucial for grasping genetic variation and inheritance mechanisms.

Recombination after Single Strand Breaks

Homologous recombination is a crucial biological process involving the exchange of genetic material between homologous chromosomes at equivalent positions, specifically at the same genes. This process can be initiated by single-strand breaks, often referred to as nicks, or by double-stranded breaks in the DNA.

When a single-strand break occurs in one strand of each homologous chromosome, it creates an opportunity for the strands to invade each other. This invasion leads to the formation of a cross-bridge structure, which allows for the exchange of genetic information. The cross-bridge can undergo branch migration, where it moves along the chromosome, facilitating further interactions between the strands.

As the blue strand invades the red strand, complementary hydrogen bonds form between the two, establishing a stable connection. This interaction is often represented by an 'X' shape in diagrams, indicating the crossover point where the exchange occurs. An enzyme then introduces another nick at the cross-bridge, resulting in two homologous chromosomes that now contain a mixture of genetic material from both the blue and red strands.

Ultimately, this process leads to genetic diversity, as the resulting chromosomes are no longer identical but instead are hybrids containing segments from both parental chromosomes. This recombination is essential for processes such as meiosis, where genetic variation is critical for evolution and adaptation.

Recombination after Double Strand Breaks

Double strand breaks in DNA, similar to single strand breaks, follow a specific series of steps for repair. In the context of homologous chromosomes, which consist of two DNA strands forming a double helix, a double strand break typically occurs in one of the strands. This break triggers the recruitment of a protein known as RecA, which facilitates the removal of nucleotides, resulting in a structure called a 3' overhang.

Once the 3' overhang is established, a process known as strand invasion occurs. In this process, the broken strand (often referred to as the red strand) invades the intact homologous strand (the blue strand) to access complementary nucleotides necessary for repair. This is possible because homologous chromosomes carry the same genetic information, allowing the blue strand to serve as a template for the repair of the broken red strand. The interaction between these strands leads to the formation of a structure called a Holliday junction, which can migrate along the chromosome.

During this repair process, two Holliday junctions may form, with the intact blue strand continuously invading the broken red strand. As the repair progresses, the genetic information is restored, resulting in a combination of red and blue strands. Specifically, the repaired strand will consist of segments from both the blue and red strands, leading to a new genetic configuration. This recombination can introduce variations, such as different alleles, dominant or recessive traits, and even point mutations, thereby altering the genetic makeup of the homologous chromosomes.

In summary, the process of double strand break repair through homologous recombination not only restores the integrity of the DNA but also contributes to genetic diversity by allowing the exchange of genetic material between homologous chromosomes.

Gene Conversion

Gene conversion is a fascinating process involving the nonreciprocal genetic exchange between two closely linked genes, which are located near each other on a chromosome but are not identical. Unlike recombination, which occurs at the same genetic loci, gene conversion involves a mismatch of base pairs during the formation of a DNA duplex. This mismatch can lead to a situation where a segment of one chromosome aligns with a segment of another chromosome, resulting in the conversion of one genetic allele into another.

To illustrate, consider a scenario where an organism has a heterozygous genotype, such as AA. In typical gamete formation, one would expect a 1:1 ratio of dominant to recessive alleles. However, gene conversion can skew this ratio, leading to a situation where three-fourths of the gametes carry the dominant allele and one-fourth carry the recessive allele. This alteration occurs because the gene conversion process can transfer alleles from one homologous chromosome to another, even if they are not at the same position on the chromosome.

For example, if you have two homologous chromosomes, both carrying the allele 'A', gene conversion can result in the transfer of the 'A' allele from one chromosome to another, leading to an unexpected distribution of alleles in the resulting gametes. This non-genetically precise conversion can significantly impact allele ratios, demonstrating the unique nature of gene conversion in genetic inheritance.

Homologous recombination is defined as what?

• A. Exchange of alleles from two different genes
• B. Exchange of regulatory regions from two different genes
• C. Exchange that occurs at equivalent positions in two non-homologous chromosomes
• D. Exchange that occurs at equivalent positions in two homologous chromosomes

Answer: (D)

Homologous recombination repairs double-strand breaks only

• A. True
• B. False

Answer: (B)

Gene conversion can cause a Aa genotype to be sorted into gametes in which way?

• A. All gametes get A allele
• B. All gametes get a allele
• C. ½ gametes get A and ½ gametes get a
• D. ¾ gametes get a and ¼ gametes get A

Answer: (D)

Here’s what students ask on this topic:

What is homologous recombination and how does it occur?

Homologous recombination is the exchange of genetic material between homologous chromosomes at equivalent positions. It can be initiated by single or double strand breaks. In single strand breaks, a nick in one strand allows the DNA to invade the homologous chromosome, forming a cross bridge structure. This structure can move along the chromosome in a process called branch migration. In double strand breaks, a protein called RecA creates a 3' overhang, leading to strand invasion and the formation of a Holliday Junction. This junction can also migrate, facilitating the repair of the broken DNA strands using the homologous chromosome as a template.

What is the difference between single strand breaks and double strand breaks in homologous recombination?

Single strand breaks involve a nick in one strand of the DNA double helix, allowing the DNA strand to invade the homologous chromosome and form a cross bridge structure. This structure can move along the chromosome through branch migration. Double strand breaks, on the other hand, involve breaks in both strands of the DNA double helix. A protein called RecA creates a 3' overhang, leading to strand invasion and the formation of a Holliday Junction. This junction can migrate along the chromosome, facilitating the repair of the broken DNA strands using the homologous chromosome as a template.

What is gene conversion and how does it differ from homologous recombination?

Gene conversion is a nonreciprocal genetic exchange between two closely linked genes, meaning they are close together on the chromosome but not at the same position. It occurs due to a mismatch of base pairs during duplex formation, leading to the conversion of one genetic allele into another. Unlike homologous recombination, which involves the exchange of genetic material at equivalent positions on homologous chromosomes, gene conversion can result in unexpected allele ratios. For example, instead of a 1:1 ratio of dominant to recessive alleles, gene conversion can lead to a 3:1 or 1:3 ratio.

What is a Holliday Junction and what role does it play in homologous recombination?

A Holliday Junction is a cross-shaped structure that forms during homologous recombination when there is a double strand break. It occurs when the broken DNA strands invade the homologous chromosome and use it as a template for repair. The Holliday Junction can migrate along the chromosome, facilitating the exchange of genetic material between the homologous chromosomes. This structure is crucial for ensuring that the genetic information is accurately repaired and exchanged, contributing to genetic diversity and proper chromosome segregation during cell division.

How does branch migration occur during homologous recombination?

Branch migration occurs during homologous recombination when the cross bridge structure, formed after a single strand break, moves along the chromosome. This movement is facilitated by enzymes that help the DNA strands slide past each other, allowing the exchange of genetic material to extend along the chromosome. Branch migration ensures that the homologous chromosomes can accurately align and exchange genetic information over a larger region, contributing to genetic diversity and proper chromosome segregation during cell division.