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.
The template strand of the DNA in the structural gene of a transcription unit is given below. What will be the coding strand for the given template strand?
3'-ATGCATGCATGCATGCATGCATGC-5'
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5'-TACGTACGTACGTACGTACGTACG-3'
The two strands of DNA are complementary and antiparallel. A pairs with T and G pairs with C, and the strand opposite a 3′→5′ strand runs 5′→3′. The coding strand is the strand that is not copied; it carries the same sequence as the RNA (with T in place of U). So the coding strand is written by pairing each base of the template and turning the polarity round: 3′-ATGC…-5′ pairs with 5′-TACG…-3′.
Option (1) 5'-ACGTACGTACGTACGTACGTACGT-3' — Incorrect. The first base of the template is A, so the first base opposite it must be T. This sequence starts with A and is not complementary to the template at any position.
Option (2) 5'-TACGTACGTACGTACGTACGTACG-3' — Correct. Every base pairs with the template base opposite it (A with T, T with A, G with C, C with G), and the strand runs 5′→3′, opposite to the template.
Option (3) 3'-TACGTACGTACGTACGTACGTACG-5' — Incorrect. The bases are right, but the polarity is wrong. Written as 3′→5′, the strand would run parallel to the template, while the two strands of DNA are always antiparallel.
Option (4) 5'-AUGCAUGCAUGCAUGCAUGCAU-3' — Incorrect. This sequence has uracil, which is found only in RNA, never in DNA. It is also not complementary to the template: it copies the template itself with T changed to U.
Note: This exact sequence is the book’s own worked example, sitting quietly between two paragraphs. Examples like this make ready-made questions, so read them as closely as the definitions.
NCERT Extract:
Transcription Unit – “… 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. All the reference point while defining a transcription unit is made with coding strand. To explain the point, a hypothetical sequence from a transcription unit is represented below:
3′-ATGCATGCATGCATGCATGCATGC-5′ Template Strand
5′-TACGTACGTACGTACGTACGTACG-3′ Coding Strand”