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Biology · 2026 · 4 marks
CBSE 2026 · Region 5 · Set 1 · Q29
Read the following passage and answer the questions that follow: The process of copying genetic information from template strand of DNA into RNA is…
Read the following passage and answer the questions that follow : The process of copying genetic information from 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.
(a)
Why is the strand of DNA with $\displaystyle 3^{\prime} \longrightarrow 5^{\prime}$ polarity transcribed and not the other strand of $\displaystyle 5^{\prime} \longrightarrow 3^{\prime}$ polarity?
(b)(i)
Why is hnRNA required to undergo splicing ?
OR
(b)
(ii)
Mention the two additional processes which hnRNA needs to undergo after splicing to become functional.
(c)
Why is only one strand of the DNA transcribed ? Give two reasons.
Marking-scheme solution
(a)
Because RNA polymerase carries out polymerisation in $\displaystyle 5$' → $\displaystyle 3$' direction only.
(b)(i)
Since hnRNA contains both exons or coding sequence and introns or non
coding seqeunce undergo splicing to remove the introns or non-coding sequence
/ for the removal of introns.
OR
(b)
(ii)
.
Capping / addition of mGppp (Methyl Guanosine Triphosphate) at $\displaystyle 5$' of hnRNA.
Tailing / Adenylate residue ($\displaystyle 200$-$\displaystyle 300$) are added at the $\displaystyle 3$' end in a template strand as poly A tail.
(c)
− If a segment of DNA produces two polypeptides the genetic information machinery becomes complicated / two different polypeptides will be formed and one of which may not be useful for the cell.
− The two complementary RNA molecules produced simultaneously will form a double-stranded RNA that will not be translated into polypeptides.
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CBSE Class 12 Biology past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.