6.9
Mismatch Repair
Organisms are capable of detecting and fixing nucleotide mismatches that occur during DNA replication. This sophisticated process requires identifying…
Some errors in DNA replication, such as incorrect base pairing between adenine and cytosine, can remain even after proofreading. Cells fix these errors after replication through a process called mismatch repair.
During mismatch repair, proteins detect distortions in DNA caused by mispaired bases. In bacteria, one of these proteins is MutS.
Other mismatch repair proteins, such as MutL, help distinguish the newly synthesized DNA strand from the original template strand, ensuring that only the incorrect strand is targeted for repair.
Different organisms identify the new DNA strand in different ways. In E. coli, the parental DNA strand is already methylated, while the newly synthesized strand remains temporarily unmethylated, allowing mismatch repair proteins to recognize and repair this new strand.
In eukaryotes, the newly synthesized DNA strand often contains small breaks, called nicks. Mismatch repair proteins recognize these nicks and target that strand.
Once the incorrect strand is identified, exonuclease enzymes remove a section of DNA that contains the error.
DNA polymerase then fills in the missing nucleotides, and DNA ligase seals the remaining gaps, restoring the correct DNA sequence.
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Q1: What is mismatch repair and why is it important in DNA replication?
Mismatch repair is a DNA proofreading mechanism that identifies and corrects base-pairing errors missed during DNA replication. These errors occur when incorrect nucleotides are incorporated opposite template bases. The system recognizes mismatched base pairs and removes the incorrect nucleotide, allowing DNA polymerase to insert the correct one. This process significantly reduces mutation rates and maintains genomic stability.
Q2: How do mismatch repair proteins recognize errors in newly replicated DNA?
Mismatch repair proteins scan newly synthesized DNA strands for distortions caused by incorrect base pairing. Recognition proteins bind to mismatched sites and recruit additional enzymes to the error location. The system distinguishes the newly synthesized strand from the template strand, allowing targeted removal of the incorrect nucleotide. This directional recognition is essential for accurate correction without damaging the template strand.
Q3: What happens when mismatch repair systems fail or are defective?
Defective mismatch repair leads to accumulation of mutations because replication errors persist uncorrected. Cells with non-functional mismatch repair systems exhibit microsatellite instability and increased mutation rates. This genomic instability can result in cancer predisposition, as seen in Lynch syndrome, where inherited mismatch repair gene mutations increase colorectal cancer risk significantly.
Q4: How does mismatch repair differ from other DNA repair mechanisms?
Unlike excision repair that removes damaged bases caused by environmental factors, mismatch repair specifically targets replication errors involving incorrect base pairing. Mismatch repair operates immediately after DNA synthesis, while other repair pathways address damage from radiation or chemicals. The system's directional specificity for newly synthesized strands distinguishes it from general DNA repair processes.
Q5: What are the key steps in the mismatch repair process?
Mismatch repair proceeds through recognition, excision, resynthesis, and ligation stages. Recognition proteins identify mismatched bases and recruit endonucleases to cleave the newly synthesized strand. Exonucleases remove nucleotides surrounding the error, and DNA polymerase resynthesizes the correct sequence. DNA ligase seals the remaining nick, restoring strand integrity and completing the repair cycle.
Q6: Which organisms possess mismatch repair systems?
Mismatch repair systems are conserved across bacteria, archaea, and eukaryotes, reflecting their fundamental importance for genomic stability. Prokaryotes use MutS, MutL, and MutH proteins, while eukaryotes employ MLH and MSH protein families. This evolutionary conservation demonstrates that accurate DNA replication and error correction are essential for all cellular life.
Q7: How does mismatch repair contribute to maintaining fidelity during DNA replication?
Mismatch repair provides a final proofreading layer beyond DNA polymerase's intrinsic 3' to 5' exonuclease activity. While polymerase catches most errors during synthesis, mismatch repair corrects the remaining mistakes that escape initial proofreading. Together, these mechanisms reduce error rates to approximately one mistake per billion nucleotides, ensuring high-fidelity transmission of genetic information across generations.