Types of Mutation, DNA Repair and Recombination I

Homologous Recombination (HR) and Repair Mechanisms

• HR forms a filament with ssDNA in eukaryotes, forcing ssDNA into an unusual configuration, with Rad51 protein playing a crucial role in this process.

Role of ATP Hydrolysis and Potential Harm of HR

• ATP hydrolysis is essential for the HR process, providing the energy required for DNA strand exchange and repair.
• The potential harm of HR lies in its potential to introduce errors or gene conversions, which can lead to genetic disorders or diseases like breast and ovarian cancer, especially when proteins like BRCA1 and BRCA2 are mutated.

Role of HR in Meiosis and Crossover

• HR plays a crucial role in meiosis, as it facilitates crossover events, which increase genetic diversity by shuffling alleles and creating new combinations.
• Crossover is the exchange of genetic material between homologous chromosomes, leading to new allelic combinations and increasing genetic diversity.

Mechanisms of Homologous Recombination and Non-Homologous End Joining (NHEJ)

• HR accurately repairs double-strand breaks (DSBs) by exchanging DNA strands between homologous duplex DNA molecules.
• The hallmark of HR is the formation of a Holliday Junction, where two DNA molecules are joined together.
• Strand exchange/invasion occurs when a broken DNA strand invades a homologous template, displacing one strand, and creating a displacement loop (D-loop).
• Rad51 and other recombinases are responsible for carrying out strand exchange/invasion in HR.
• HR occurs during the S and G2 phases of the cell cycle, when sister chromatids are available as templates.
• NHEJ, on the other hand, is a faster but less accurate repair mechanism that predominates in humans, involving a three-step process: end recognition, end processing, and ligation.

Double-Strand Break (DSB) Repair Mechanisms

• DSBs are especially dangerous and can cause chromosome fragmentation.
• There are two primary mechanisms to repair DSBs: HR and NHEJ.
• NHEJ usually causes deletions and is common in mammalian somatic cells, while HR is more accurate but slower and more complex.
• HR is used when sister chromatids are available to serve as templates, while NHEJ is used when no template is available.

True or False Statements

• True: NHEJ is common and has low accuracy.
• True: DSB is mainly repaired by HR since HR is more accurate than NHEJ.
• False: The hallmark for HR is base pairing.
• False: HR happens throughout the cell’s life cycle.
• False: HR only happens between sister chromatids, not between chromosomes, since they are too far away.
• True: Although the formation of Holliday Junctions is rare, it is essential for chromosome crossover.
• False: Loss of heterozygosity (LOH) happens when HR occurs between sister chromatids.
• True: Gene conversion is an outcome of the coordination of HR and mismatch repair.
• True: Gene conversion could happen thousands of nucleotide pairs away from the DSB site due to 'branch migration'.
• True: During cell meiosis, programmed DSB is the first step of the HR process, which happens between sister chromatids.