CBSE 2023 · Region 3 · Set 1 · Q22 · 3 marks
(a)How many types of RNA polymerases are there in a eukaryote cell ? Mention which one of them transcribes hnRNA.(ii)Write the changes that hnRNA undergoes before it leaves the nucleus as mRNA.The length of DNA in any cell is far greater than the dimension of its nucleus. Explain how this enormous DNA is packaged in a eukaryotic cell.
(a)
How many types of RNA polymerases are there in a eukaryote cell ? Mention which one of them transcribes hnRNA.
(ii)
Write the changes that hnRNA undergoes before it leaves the nucleus as mRNA.
The length of DNA in any cell is far greater than the dimension of its nucleus. Explain how this enormous DNA is packaged in a eukaryotic cell.
Marking-scheme solution
(a)
$\displaystyle 3$ types
RNA Polymerase -II
(ii)
Splicing /Introns are removed and exons are joined in a definite order, undergoes capping /at $\displaystyle 5$' end where unusual nucleotide (methyl guanosine triphosphate) is added , tailing/ at $\displaystyle 3$' end where ($\displaystyle 200$-$\displaystyle 300$) adenylate residues are added.
A set of positively charged proteins called histones, due to presence of lysine and arginine( basic amino acids), holds the negatively charged DNA around it in a coiled manner, histones are organised to form a unit of eight molecules (histone octamer), a typical nucleosome contains $\displaystyle 200$ bp of DNA helix, Nucleosomes constitute repeating units of a structure in nucleus called chromatin thread ( like bodies as "beads on string" structure in a nucleus).
Molecular Basis of InheritanceTranscriptionRemembershort_answermedium
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CBSE Class 12 Biology past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.