13.7
Relecture
Aperçu
La synthèse de nouvelles molécules d’ADN commence lorsque la polymérase d’ADN relie les nucléotides dans une séquence complémentaire au brin d’…
During DNA replication, free nucleotides in the cell are added to the new strand in a sequence complementary to the template. Adenine pairs with thymine, and cytosine pairs with guanine.
Sometimes, nucleotides pair incorrectly, such as when adenine pairs with cytosine. DNA polymerase proofreading helps fix these errors and reduces the chance of mutations. If they are not corrected, these errors can cause disease.
First, DNA polymerase binds more strongly to correctly paired nucleotides. This reduces the chance of incorrect pairings.
Second, as nucleotides begin pairing with the template, DNA polymerase undergoes a conformational change. This change causes incorrectly paired nucleotides to detach, allowing the correct nucleotides to be added.
Third, if an incorrect nucleotide is added to the three-prime end of the growing DNA chain, the mismatch distorts the DNA structure and pauses DNA polymerase movement.
DNA polymerase has three-prime-to-five-prime exonuclease activity, which lets it remove an incorrectly added nucleotide. It then inserts the correct nucleotide. This process is called exonucleolytic proofreading.
These steps help keep DNA replication accurate by separating nucleotide selection from error correction.
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Q1: What are the three main mechanisms DNA polymerase uses to proofread during replication?
DNA polymerase employs three proofreading mechanisms. First, it binds more strongly to correctly paired nucleotides, reducing incorrect pairings. Second, it undergoes a conformational change that causes incorrectly paired nucleotides to detach. Third, if an incorrect nucleotide reaches the three-prime end, the mismatch distorts DNA structure and pauses polymerase movement, allowing correction.
Q2: How does exonucleolytic proofreading remove and replace incorrect nucleotides?
DNA polymerase possesses three-prime-to-five-prime exonuclease activity, enabling it to reverse direction and remove incorrectly added nucleotides from the growing DNA chain. After removing the wrong nucleotide, the polymerase inserts the correct one. This exonucleolytic proofreading process separates nucleotide selection from error correction, maintaining replication accuracy.
Q3: Why do mutations in the exonuclease domain of DNA polymerase lead to cancer?
When mutations alter the exonuclease domain, DNA polymerase loses its ability to remove incorrect nucleotides. This causes mutations to accumulate rapidly throughout the genome. These uncorrected errors can disrupt normal gene function and cellular regulation, leading to uncontrolled cell growth and various types of cancer.
Q4: What happens when adenine pairs with cytosine instead of thymine during DNA replication?
When adenine incorrectly pairs with cytosine instead of thymine, a mispairing error occurs. DNA polymerase detects this incorrect pairing through its proofreading mechanisms. If not corrected, these errors can persist as mutations and potentially cause disease by altering the genetic code in newly synthesized DNA.
Q5: How do high-fidelity and error-prone polymerases differ in laboratory applications?
High-fidelity polymerases maintain strong proofreading ability and are used when accurate DNA copies are essential. Error-prone polymerases have compromised proofreading ability and are intentionally used in techniques like error-prone PCR to generate mutations in DNA sequences on purpose for research applications.
Q6: Which DNA polymerases in eukaryotes possess proofreading ability?
DNA polymerase delta and epsilon are the primary eukaryotic polymerases that possess proofreading ability during nuclear DNA replication. These polymerases read each base after it is added to the new strand and use their exonucleolytic domains to correct errors, ensuring high-fidelity replication in eukaryotic cells.
Q7: What is the relationship between base pairing rules and DNA polymerase fidelity?
DNA polymerase has higher affinity for correct base pairs, where adenine pairs with thymine and cytosine pairs with guanine. This selective binding preference ensures fidelity during replication by favoring correct nucleotide incorporation. Combined with proofreading mechanisms, this dual-level control significantly reduces replication errors and mutation rates.