Read the passage given below and answer the following five questions.
The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. A transcription unit in DNA is defined primarily by the three regions in the DNA. There is a convention in defining the two strands of the DNA in the structural gene of a transcription unit. Since the two strands have opposite polarity, the strand that has the polarity 3'→5' acts as a template, and is also referred to as template strand. The other strand which has the polarity (5'→3') and the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription. Strangely, this strand (which does not code for anything) is referred to as coding strand. The promoter and terminator flank the structural gene in a transcription unit. The promoter is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands.
In transcription, the principle of complementarity is followed. With which base does adenosine pair in the RNA formed?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Uracil
The passage states that in transcription “the adenosine complements now forms base pair with uracil instead of thymine.”
Transcription follows the same rule of complementary base pairing as replication. The one difference is that the new strand is RNA, and RNA has uracil in place of thymine. So wherever the DNA template has adenine, uracil is placed in the RNA.
Option (1) Thymine — Incorrect. Thymine pairs with adenine in DNA, for example during replication. RNA does not contain thymine, so it cannot be the partner of adenosine in the RNA formed.
Option (2) Uracil — Correct. Uracil is the RNA base that takes the place of thymine, so adenosine pairs with uracil.
Option (3) Guanine — Incorrect. Guanine pairs with cytosine, both in DNA and in RNA. It never pairs with adenine.
Option (4) Cytosine — Incorrect. Cytosine pairs with guanine. It is not the partner of adenine.