Question

Which residue is removed from DNA by base excision repair process?

• A. Uracil
• B. Adenine
• C. Cytosine
• D. Thymine

Hint:

Base excision repair commonly removes abnormal bases such as uracil from DNA. Remember: Uracil belongs to RNA, not DNA.

Answer 1

The Correct Option is A

Concept:
Base excision repair is a DNA repair mechanism. It repairs damaged or abnormal bases in DNA without removing a large portion of the DNA strand.

Step 1: Understand why repair is required.
DNA can undergo chemical changes. One common change is the deamination of cytosine. When cytosine undergoes deamination, it gets converted into uracil. \[ \text{Cytosine} \longrightarrow \text{Uracil} \]

Step 2: Why uracil is abnormal in DNA?
Uracil is normally present in RNA, not in DNA. DNA normally contains thymine instead of uracil. Therefore, if uracil appears in DNA, it is considered an abnormal base.

Step 3: Enzyme involved in removal.
The enzyme uracil-DNA glycosylase recognizes uracil present in DNA and removes it. This creates an abasic site, which is later repaired by other enzymes.

Step 4: Final conclusion.
Since uracil is not a normal DNA base and is removed during base excision repair, the correct answer is Uracil. \[ \therefore \text{Correct Answer is (A)} \]