DNA REPAIR

DNA repair mechanisms are essential processes that organisms use to correct damage to their DNA. DNA can be damaged by various factors, including exposure to ultraviolet (UV) radiation, chemicals, and errors that occur during DNA replication. Here are some key notes on DNA repair mechanisms:

Types of DNA Damage:
- Chemical Damage: DNA can be damaged by exposure to various chemicals, such as those found in certain drugs or environmental pollutants.
- UV Radiation Damage: UV radiation from the sun can cause thymine dimers, where two adjacent thymine bases bond together, leading to distortions in the DNA structure.
- Replication Errors: During DNA replication, mistakes can occur, leading to mismatches or the insertion of incorrect nucleotides.
DNA Repair Pathways:
- Direct Repair: Involves the direct reversal of the DNA damage without removing or replacing nucleotides. For example, photolyases can repair thymine dimers induced by UV radiation.
- Base Excision Repair (BER): Targets damaged or incorrect bases and involves the removal of the damaged base by a DNA glycosylase enzyme, followed by the excision and replacement of the damaged segment.
- Nucleotide Excision Repair (NER): Repairs a wider range of DNA damage, including bulky lesions and thymine dimers. It involves the excision and replacement of a segment of nucleotides surrounding the damaged site.
- Mismatch Repair (MMR): Corrects errors that arise during DNA replication by recognizing and removing mismatched bases. It is crucial for maintaining the fidelity of DNA replication.
Double-Strand Break Repair:
- Homologous Recombination (HR): Involves the exchange of genetic material between homologous DNA strands. It is a high-fidelity repair mechanism and is often used during the S and G2 phases of the cell cycle.
- Non-Homologous End Joining (NHEJ): Joins broken DNA ends together, often with the loss or addition of a few nucleotides. It is quicker than HR but is more error-prone.
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