CBSE 2023 · Region 3 · Set 1 · Q2 · 1 mark

Given below is a list of steps Meselson and Stahl carried out in their experiment to prove that DNA replication is semi-conservative. Select the option that gives the correct sequence of steps followed by them.

(i)

Bacteria transferred to a $\displaystyle \mathrm{N}^{14}$ medium and sampled every $\displaystyle 20$ minutes.

(ii)

All bacteria contain hybrid DNA (N $\displaystyle { }^{14}$ DNA and $\displaystyle \mathrm{N}^{15} \mathrm{DNA}$ ).

(iii)

Bacteria grown in $\displaystyle \mathrm{N}^{15}$ medium for many generations.

(iv)

All bacteria contain $\displaystyle \mathrm{N}^{15}$ DNA.

(v)

Bacteria contain either all $\displaystyle \mathrm{N}^{14}$ DNA or all hybrid DNA.

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.