A cell can repair cytosine deamination damage, so it presumably has an enzyme th

Question

A cell can repair cytosine deamination damage, so it presumably has an enzyme that "recognizes" the product of this damage. The structural feature that this repair enzyme recognizes would most likely be: A1. A bulge in the duplex caused by cytosine deamination replacing a pyrimidine with a purine. A2. Non-planarity of the base, due to removal of the amine group. A3. Free radicals created by the cytosine deamination reaction. A4. C-rich sequences created by the cytosine deamination reaction. A5. None of the above would be plausible features recognized.

Explanation / Answer

Ans. Under physiological conditions, cytosine spontaneously undergoes deamination to form uracil and ammonia as follow-

                        Cytosine + H2O ----- deamination-----> Uracil + NH3

So, C (a pyrimidine base) is replaced by U (a purine base) due to deamination. Therefore, G=C base pairing becomes: G=U.

The resultant G=U pair appears as a bulged distortion in the helix, which is detected by uracil-DNA glycosylase that triggers base-excision repair pathway to correct the mutation.

So, correct option is. A.

Note that is the ‘bulge’ that is recognized as helix-distortion by the enzyme. So, all options do not have role in triggering base-excision repair of cytosine deamination.    

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