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.
A transcription unit in DNA is defined primarily by which regions in DNA?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All of the above
The passage states that “A transcription unit in DNA is defined primarily by the three regions in the DNA” and that “The promoter and terminator flank the structural gene in a transcription unit.” Put together, the three regions are the promoter, the structural gene and the terminator.
Option (1) A terminator — Correct statement, but it is only one of the correct points, so on its own it is not the answer. The terminator lies towards the 3′-end (downstream) of the coding strand and usually defines the end of transcription.
Option (2) The structural gene — Correct statement, but it is only one of the correct points, so on its own it is not the answer. The structural gene is the stretch of DNA between the promoter and the terminator. Its template strand is copied into RNA.
Option (3) A promoter — Correct statement, but it is only one of the correct points, so on its own it is not the answer. The promoter lies towards the 5′-end (upstream). It provides the binding site for RNA polymerase, and its position decides which strand is the template.