Read the given passage and answer the following question.
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. However, unlike in the process of replication, which once set in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA. The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5' -end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It 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. By switching its position with terminator, the definition of coding and template strands could be reversed. The terminator is located towards 3' -end (downstream) of the coding strand and it usually defines the end of the process of transcription.
The passage states that in transcription only one of the strands of a DNA segment is copied into RNA. Consider the following statements:
A. If both strands acted as templates, they would code for RNA molecules with different sequences, and the proteins coded would have different sequences of amino acids.
B. If both strands were copied, the two RNA molecules produced simultaneously would be complementary to each other and would form a double stranded RNA, which would prevent translation.
C. The coding strand has the polarity 3′→5′ and acts as the template for the RNA formed.
Which of the above give the reason why both strands of DNA are not copied during transcription? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and B only
The passage says that only one strand of a DNA segment is copied in transcription, but it does not give the reason. In a transcription unit, the strand with polarity 3′→5′ is the template strand. The other strand, with polarity 5′→3′ and the same sequence as the RNA (T in place of U), is the coding strand.
Statement A: If both strands acted as templates, they would code for RNA molecules with different sequences, and the proteins coded would have different sequences of amino acids. — True. The two strands are complementary, not identical. So one DNA segment would code for two different proteins, which would complicate the transfer of genetic information. This is the first reason the book gives.
Statement B: If both strands were copied, the two RNA molecules produced simultaneously would be complementary to each other and would form a double stranded RNA, which would prevent translation. — True. The two RNAs would pair with each other into double stranded RNA. This RNA could not be translated into protein, so transcription would be a futile exercise. This is the second reason.
Statement C: The coding strand has the polarity 3′→5′ and acts as the template for the RNA formed. — False. The strand with polarity 3′→5′ is the template strand. The coding strand has polarity 5′→3′ and is displaced during transcription; it does not act as the template. Statement C is also not a reason for copying only one strand.
NCERT Extract:
Transcription – “… Why both the strands are not copied during transcription has the simple answer. First, if both strands act as a template, they would code for RNA molecule with different sequences (Remember complementarity does not mean identical), and in turn, if they code for proteins, the sequence of amino acids in the proteins would be different. Hence, one segment of the DNA would be coding for two different proteins, and this would complicate the genetic information transfer machinery. Second, the two RNA molecules if produced simultaneously would be complementary to each other, hence would form a double stranded RNA. This would prevent RNA from being translated into protein and the exercise of transcription would become a futile one.”
Transcription Unit – “… Since the two strands have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction, that is, 5′→3′, 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.”