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.
Assertion (A): In eukaryotes, translation can often begin much before the mRNA is fully transcribed.
Reason (R): Bacterial mRNA does not require any processing to become active, and transcription and translation take place in the same compartment.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is false but (R) is true.
Assertion (A): In eukaryotes, translation can often begin much before the mRNA is fully transcribed. — False. The statement holds for bacteria, not for eukaryotes. In eukaryotes, transcription takes place in the nucleus and the first product, hnRNA (heterogeneous nuclear RNA), is not functional. It is first spliced, capped and tailed, and only the fully processed mRNA is transported out of the nucleus for translation. So translation cannot begin before the mRNA is fully transcribed.
Reason (R): Bacterial mRNA does not require any processing to become active, and transcription and translation take place in the same compartment. — True. Bacterial mRNA needs no processing, and bacteria have no separate nucleus, so transcription and translation happen in the same compartment. This is why, in bacteria, translation can start on an mRNA that is still being made and the two processes are coupled.
A is false (it would be true only if “eukaryotes” were replaced by “bacteria”), and R is true.
NCERT Extract:
Types of RNA and the Process of Transcription – “… In bacteria, since the mRNA does not require any processing to become active, and also since transcription and translation take place in the same compartment (there is no separation of cytosol and nucleus in bacteria), many times the translation can begin much before the mRNA is fully transcribed. Consequently, the transcription and translation can be coupled in bacteria. … (ii) The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. hnRNA undergoes additional processing called as capping and tailing. … It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation (Figure 5.11).”