CBSE 2025 · Region 1 · Set 2 · Q33 · 5 marks
Student to attempt either option-(A) or (B) :(i)Haemophilia and red green colourblindness is usually observed in men. Why ?(ii)Perform a cross(es) where haemophilic daughter(s) and haemophilic son(s) are produced in same ratio.(i)Where do transcription and translation occur in bacteria and eukaryotes respectively ?(ii)Draw a labelled schematic sketch of replication fork of DNA.(iii)A DNA segment has a total of $\displaystyle 1000$ nucleotides, out of which $\displaystyle 240$ of them are Adenine containing nucleotides. How many pyrimidine bases this segment possesses ?
Student to attempt either option-(A) or (B) :
(i)
Haemophilia and red green colourblindness is usually observed in men. Why ?
(ii)
Perform a cross(es) where haemophilic daughter(s) and haemophilic son(s) are produced in same ratio.
(i)
Where do transcription and translation occur in bacteria and eukaryotes respectively ?
(ii)
Draw a labelled schematic sketch of replication fork of DNA.
(iii)
A DNA segment has a total of $\displaystyle 1000$ nucleotides, out of which $\displaystyle 240$ of them are Adenine containing nucleotides. How many pyrimidine bases this segment possesses ?
Marking-scheme solution
(i)
Both are sex-linked / X-linked recessive disorder
Males usually have only single X chromosome so disease is easily expressed.
(ii)
Cross-$\displaystyle 1$
Ratio $\displaystyle 1$:$\displaystyle 1$ Haemophilic daughter:Haemophilic son
Cross-$\displaystyle 2$
Ratio $\displaystyle 2$ : $\displaystyle 2$ / $\displaystyle 1$ : $\displaystyle 1$ Haemophilic daughter : Haemophilic son
(i)
Transcription in bacteria occurs in cytoplasm of cell
Translation in bacteria occurs in cytoplasm of cell
In Eukaryotes transcription occurs in nucleus, and translation occurs in the cytoplasm.
(ii)
(iii)
Total nucleotides = $\displaystyle 1000$
Adenine = $\displaystyle 240$
$\displaystyle \mathrm{A}=\mathrm{T}$ therefore $\displaystyle \mathrm{T}=240$
$\displaystyle \mathrm{A}+\mathrm{T}=480$
$\displaystyle \mathrm{C}+\mathrm{G}=1000-(\mathrm{A}+\mathrm{T})=1000-480=520$
Since $\displaystyle \mathrm{C}=\mathrm{G}$ Therefore $\displaystyle \mathrm{C}=520 / 2=260$
Total Pyrimidines $\displaystyle =\mathrm{T}+\mathrm{C}$ Therefore $\displaystyle 240+260=500$
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CBSE Class 12 Biology past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.