Question

Read the following passage and answer the questions that follow:

The process of copying genetic information from the template strand of DNA into RNA is called transcription. It is mediated by RNA polymerase. Transcription takes place in the nucleus of eukaryotic cells. In transcription, only a segment of DNA and only one of the strands is copied into RNA.

1. Why is the strand of DNA with 3' → 5' polarity transcribed and not the other strand of 5' → 3' polarity?     [1]
2. Why is hnRNA required to undergo splicing?     [1]
   OR
3. Mention the two additional processes which hnRNA needs to undergo after splicing to become functional.     [1]
4. Why is only one strand of the DNA transcribed? Give two reasons.     [2]

Answer 1

1. Because RNA polymerase builds RNA in the 5' → 3' direction, it must use the DNA strand that runs 3' → 5' as the template so nucleotides can be added to the growing RNA's 3' end. Polymerisation therefore requires the template to be read 3' → 5' (absence of a 3' OH blocks further chain extension).
2. hnRNA contains non-coding intervening sequences (introns) that must be removed and the coding sequences (exons) joined to form a continuous, translatable mRNA; splicing also enables alternative exon combinations (isoform diversity).
   OR
3. Additional processes which hnRNA needs to undergo-
   • Addition of a 5' methylated guanosine cap (for ribosome binding/translation initiation).
   • Addition of a 3' poly-A tail (for mRNA stability/longevity).
4. Only one strand of DNA is transcribed because RNA polymerase synthesizes RNA in the 5' → 3' direction. The DNA template strand that runs 3' → 5' provides the correct orientation for RNA polymerase to synthesize RNA in the 5' → 3' direction.
   The two strands of DNA serve distinct purposes. One strand functions as the template strand, which is transcribed, and the other is the coding strand, which shares the same sequence as the RNA (except that the RNA substitutes uracil for thymine). Transcription only requires the template strand.