Which of the following repair mechanisms is most likely to correct a double-strand break without error?
Answer
a. Nonhomologous end joining
b. Active sites within the DNA polymerase
c. It cannot be corrected; the cell will die
d. Homologous recombination
e. Excision repair
The correct answer and explanation is:
The correct answer is d. Homologous recombination.
Explanation:
Double-strand breaks (DSBs) are among the most severe forms of DNA damage, as they can lead to chromosomal rearrangements, mutations, or even cell death if not repaired accurately. There are several mechanisms the cell uses to repair DSBs, but not all are error-free.
- Nonhomologous End Joining (NHEJ): This repair mechanism involves directly ligating the broken ends of DNA together. While this process is relatively quick and commonly used, it is prone to errors, such as the loss or addition of nucleotides at the break site. This can result in mutations, which makes it less accurate than other mechanisms.
- Active sites within DNA polymerase: DNA polymerase is involved in adding nucleotides during DNA replication and repair. However, its role in correcting double-strand breaks is indirect. The polymerase itself does not directly repair DSBs, and its involvement is more related to processes like DNA replication or repair synthesis. It is not the primary mechanism for fixing double-strand breaks without error.
- Homologous recombination (HR): This mechanism is the most accurate repair method for double-strand breaks. HR relies on the use of a homologous sequence, typically a sister chromatid (in the case of cells in S or G2 phase), to guide the repair process. This allows the cell to use an intact copy of the genetic material to perfectly repair the break, ensuring the correct sequence is restored without loss or modification of genetic information. This makes homologous recombination a high-fidelity repair mechanism, especially in comparison to others like NHEJ.
- Excision repair: Excision repair mechanisms, such as nucleotide excision repair (NER) or base excision repair (BER), are designed to fix damaged bases or small lesions within a strand of DNA, not double-strand breaks. Therefore, excision repair is not directly involved in fixing DSBs.
Thus, homologous recombination is the most accurate and error-free repair mechanism for double-strand breaks, particularly when a homologous sequence is available to guide the repair.