Problem 19 - Biology

Question

The initial mechanism for repairing nucleotide errors in DNA is ______. a. mismatch repair b. DNA polymerase proofreading c. nucleotide excision repair d. thymine dimers

Step-by-Step Solution

Answer

b. DNA polymerase proofreading

Step 1 - Understand DNA Repair Mechanisms

Recognize that the question is about different mechanisms for repairing errors in DNA.

Step 2 - Identify the Initial Repair Mechanism

Determine which mechanism comes into play immediately during DNA replication to fix nucleotide errors.

Step 3 - Analyze DNA Polymerase Proofreading

DNA Polymerase adds nucleotides to the growing DNA strand and also has proofreading ability that corrects errors immediately as they occur.

Step 4 - Evaluate Other Repair Mechanisms

Mismatch repair, nucleotide excision repair, and thymine dimers are other mechanisms that fix errors but act after initial DNA synthesis.

Step 5 - Select the Correct Answer

Given the understanding, DNA polymerase proofreading is the mechanism that immediately repairs nucleotide errors during DNA replication.

Key Concepts

DNA polymerase proofreading

DNA polymerase is an essential enzyme in DNA replication. It adds nucleotides to a growing DNA strand, ensuring accurate copying of the genetic material. One of its crucial functions is proofreading. As DNA polymerase adds each nucleotide, it also checks whether the newly added nucleotide is correctly base-paired with the template strand. If an incorrect nucleotide is incorporated, the enzyme detects the mismatch almost immediately.

This proofreading process involves a 3' to 5' exonuclease activity. The enzyme briefly pauses, removes the incorrect nucleotide, and replaces it with the correct one. This immediate error-checking significantly increases the fidelity of DNA replication.
Understanding DNA polymerase proofreading helps to see why it is the initial mechanism for repairing nucleotide errors during DNA replication. Other repair mechanisms, while vital, act later if the proofreading process misses any errors.

Mismatch repair

Mismatch repair (MMR) is an essential DNA repair mechanism that deals with mistakes missed by the DNA polymerase proofreading process. This system corrects mismatches, small insertions, and deletions that occur during DNA replication.

The mismatch repair system identifies the error due to a distortion in the DNA double helix. Proteins in this system, such as MutS, recognize and bind to the mismatched DNA region. After this initial recognition, additional proteins like MutL help remove the faulty segment of DNA.

The gap left by this removal is then filled with the correct nucleotides by DNA polymerase and sealed by DNA ligase. This repair mechanism thus serves as a critical secondary line of defense to maintain genetic stability after the proofreading activity of DNA polymerase.

nucleotide excision repair

Nucleotide excision repair (NER) is a versatile and important mechanism for fixing bulkier DNA lesions, such as those caused by UV radiation. This can include thymine dimers, where thymine bases bond abnormally, distorting the DNA structure.

NER operates by identifying distortions in the DNA helix. Once such a lesion is detected, a set of proteins excise a small segment of the DNA strand that includes the damage. The DNA helicase unwinds the DNA to allow specific endonucleases to cut out the damaged section.

This gap is then filled in by DNA polymerase, which synthesizes the correct sequence using the undamaged strand as a template. Finally, DNA ligase seals the new strand into the existing DNA, completing the repair process. NER is crucial for preventing mutations that could lead to serious conditions like cancer.