6. Which of the following mechanisms of mutagenesis can occur in deaminated cyto
ID: 3515032 • Letter: 6
Question
6. Which of the following mechanisms of mutagenesis can occur in deaminated cytosine nucleotide(s) in DNA? A. The lesion is not repaired prior to replication, resulting in a CG-TA transition. B. Base excision repair, followed by error-prone DNA polymerase fill-in of the gap created. C. Mismatch repair, followed by error-prone DNA polymerase fill-in of the gap created. D. Choices A and B are both possible. E. All of the above are possible 7. Describe what happens to B cells and their BCRs within a germinal center.Explanation / Answer
Q6.
There are two answers in two possible scenarios.
Scenario number 1: Cytosine is non-methylated but deaminated. After deamination it will be converted to Uracil.
In this scenario, the answer is B.
Reasoning: In this scenario, it is enough to remove the existing base (Uracil) and replace it by an appropriate one (Cytosine). Uracil-glycosylase and AP endonucleases help DNA polymerase enzyme in this repair process.
Scenario number 2: Cytosine is methylated as well as deaminated. After deamination it will be converted to 5-methylcytosine, which spontaneously deaminates to thymine and ammonia.
In this scenario, the answer is C.
Reasoning: In this scenario, it is not possible to repair by base-excision pathway, as it is a mismatch which has been created to due deamination (Earlier it was C-G (C on one DNA strand and G on the other). Now it is T-G (T on strand and G on other), which is a mismatch, as usually the base pairing happens as A-T and G-C). Thus in this case, it will attract the mismatch repair (MMR) system enzymes which along with DNA polymerase will repair the mismatch.
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