<p>Chemical damage to DNA can occur just before DNA replication begins, not givi
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Question
<p>Chemical damage to DNA can occur just before DNA replication begins, not giving the repair system enough time to correct before the DNA is duplicated. This give rise to mutation, if the cytosine in the 5’ TCAT 3’ is deaminated and repaired, how would this sequence be mutated after two rounds of DNA replication (two cell division) Please Explain!!!<br /><br />a) 5’-TTAT-3’<br />b) 5’-TUAT-3’<br />c) 5’-TGAT-3’<br />d) 5’-TAAT-3’</p>Explanation / Answer
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a) 5'-TTAT-3'
Background: Spontaneous deamination is the hydrolysis reaction of cytosine into uracil, releasing ammonia in the process. The DNA repair system involves uracil-DNA glycosylase that removes the uracil present in DNA. Then, AP endonucleases break a phosphodiester bond in the DNA, thus creating a nick. DNA Polymerase removes the phospho-ribose without a base, then fills in the gap. DNA ligase seals the phosphoribose chain.
The question states that the cytosine is deaminated and repaired. But it does say if the repair occurs immediately after the defect or immediately after the DNA replication. I'm assuming the repair occurs immediately after the first DNA replication.
Cytosine deamination results in a uracil base as explained above. The uracil (looks like a demethylated thymine) complements with an adenine (not guanine). During the first DNA replication, DNA Polymerase is fooled by complementing the uracil with an adenine, not guanine. That's because uracil forms two complementary H-bonds with adenine, no longer three with guanine. After the first DNA replication, the base pair is thus U:A. Then the U is repaired to a T to complement the A present on the complementary strand, giving rise to the base pair T:A. The second round of DNA replication simply duplicates this point mutation.
This is how the sequence 5'-TCAT-3' becomes 5'-TTAT-3'.
/update - added some details.
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