A Level Biology Practice Exam

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What replaces thymine in the base sequence of mRNA?

Adenine
Cytosine
Uracil
Guanine

The correct answer is: Uracil

The correct answer is indeed uracil. In the process of transcription, when DNA is converted into mRNA, thymine, which is found in DNA, is replaced by uracil in the mRNA sequence. This is due to the structural similarity between these two nitrogenous bases, but uracil does not pair with adenine in the same way thymine does. Instead, uracil forms complementary base pairs with adenine during the synthesis of mRNA from a DNA template. This substitution is a key characteristic of RNA, distinguishing it from DNA, where thymine is present.