MCAT Biochemistry Question 86: Answer and Explanation

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Question: 86

11. Why might uracil be excluded from DNA but NOT RNA?

  • A. Uracil is much more difficult to synthesize than thymine.
  • B. Uracil binds adenine too strongly for replication.
  • C. Cytosine degradation results in uracil.
  • D. Uracil is used as a DNA synthesis activator.

Correct Answer: C

Explanation:

One common DNA mutation is the transition from cytosine to uracil in the presence of heat. DNA repair enzymes recognize uracil and correct this error by excising the base and inserting cytosine. RNA exists only transiently in the cell, such that cytosine degradation is insignificant. Were uracil to be used in DNA under normal circumstances, it would be impossible to tell if a base should be uracil or if it is a damaged cytosine nucleotide.

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