7.10
Gene Conversion
Other than maintaining genome stability via DNA repair, homologous recombination plays an important role in diversifying the genome. In fact, the reco…
Unlike mitosis where double-strand breaks are accidental, in meiosis, they are created by an enzyme called Spo11, which cleaves the phosphodiester backbone. The broken helical ends are trimmed by a protein complex called MRX and the damage is repaired by a process called gene conversion.
Here the damaged “acceptor” DNA strand invades a homologous “donor” DNA duplex to form a displacement loop. This creates regions of heteroduplex DNA where a strand from the donor DNA pairs with a complementary strand from the acceptor DNA.
DNA polymerase extends the invading strand, and the extended D-loop then pairs with the free 3′ tail. DNA synthesis at the newly captured strand, results in the formation of an intermediate with two four-strand structures called Holliday junctions.
This double Holliday junction intermediate is resolved by DNA repair enzymes called resolvases and there are two orientations in which the junctions can be cleaved. In the first, resolvase nicks each junction horizontally so that the parental strands are still intact.
This results in a non-crossover product, named because the strands after the break remain with their original partner, and there is no major crossing over of the donor and acceptor strands.
Alternatively, if cleavage occurs vertically the regions flanking the damage are switched leading to a crossover product, where the donor strand following the break recombines with the acceptor strand.
Gene conversion has a significant impact on genomic diversity. In sexually reproducing organisms the offspring inherits one set of genes from the father and one set from the mother.
The parental DNA sets recombine and the sister chromatids undergo gene conversion. This results in the offspring having novel chromosomes compared to those of the parents.
Q1: How does gene conversion repair double-strand breaks during meiosis?
Gene conversion repairs double-strand breaks created by Spo11 enzyme during meiosis. The damaged acceptor DNA strand invades a homologous donor DNA duplex, forming a displacement loop. DNA polymerase extends the invading strand, creating heteroduplex DNA regions where donor and acceptor strands pair. This process ultimately resolves into either crossover or non-crossover products through Holliday junction cleavage.
Q2: What are Holliday junctions and how are they resolved?
Holliday junctions are four-strand DNA structures formed during gene conversion when the extended D-loop pairs with the free 3' tail. Two Holliday junctions create an intermediate structure that resolvase enzymes cleave in two possible orientations. Horizontal cleavage produces non-crossover products where parental strands remain intact, while vertical cleavage generates crossover products with recombined donor and acceptor strands.
Q3: What is the difference between crossover and non-crossover products in gene conversion?
Non-crossover products result from horizontal Holliday junction cleavage, keeping parental strands with their original partners without major strand exchange. Crossover products arise from vertical cleavage, causing regions flanking the damage to switch, so donor strands recombine with acceptor strands. Both outcomes contribute to genomic diversity but through different recombination patterns.
Q4: How does gene conversion contribute to genomic diversity in offspring?
Gene conversion increases genomic diversity by recombining parental DNA sets during meiosis. Offspring inherit one gene set from each parent, and sister chromatids undergo gene conversion, creating novel chromosomes different from either parent. This recombination of sequences forms the molecular basis of genomic evolution, generating new amalgamated sequences that determine cell fitness and survival.
Q5: What role do the MRX protein complex and Spo11 enzyme play in gene conversion?
Spo11 enzyme deliberately creates double-strand breaks during meiosis by cleaving the phosphodiester backbone, unlike accidental breaks in mitosis. The MRX protein complex then trims the broken helical ends, preparing them for repair. Together, these proteins initiate the gene conversion process that repairs damage while promoting genetic recombination and diversity.
Q6: What is heteroduplex DNA and how does it form during gene conversion?
Heteroduplex DNA forms when a strand from donor DNA pairs with a complementary strand from acceptor DNA during gene conversion. This occurs after the acceptor strand invades the donor duplex and creates a displacement loop. The heteroduplex regions represent mismatched base pairing between the two DNA sources, which DNA polymerase extends to complete the repair process.
Q7: How do alleles and phenotypes change through gene conversion in offspring?
Gene conversion alters the distribution of alleles between homologous chromosomes, changing a gene's form or manifestation in offspring. Minor sequence differences between homologous chromosomes don't affect gene function but determine which allele is expressed. For example, a hair color gene's allele determines whether hair is black, blonde, or red, and gene conversion redistributes these allelic variants among offspring.