(a) Meselson and Stahl carried out an experiment to prove the nature of DNA replication. Recall the experiment and answer the following questions.…

(a) (i) Normal nitrogen/ \(\displaystyle { }^{14} \mathrm{~N}\), heavy nitrogen/ \(\displaystyle { }^{15} \mathrm{~N}\) to produce two types of DNA / light and heavy DNA respectively.

(ii) E. coli has generation time of $\displaystyle 20$ minutes so the samples taken at intervals of $\displaystyle 20$ minutes, to understand the mode of replication when E.coli with \(\displaystyle { }^{15} \mathrm{~N}\) DNA was cultured in medium \(\displaystyle { }^{14} \mathrm{~N}\) (normal) nitrogen.

(iii) To distinguish or separate heavy DNA from light DNA on the basis of density.

(iv) Mode of DNA replication is semiconservative.

OR

(b) (i) TTRR (Tall round seeds) \(\displaystyle \times\) ttrr (Dwarf wrinkled seeds)

Gametes: TR and tr

\(\displaystyle F_1\) generation: TrRr (All tall round)

Selfing: TtRr \(\displaystyle \times\) TtRr

\(\displaystyle F_2\) generation -- Tall Round : Dwarf Round : Tall wrinkled : Dwarf Wrinkled = $\displaystyle 9$ : $\displaystyle 3$ : $\displaystyle 3$ : $\displaystyle 1$

(ii) 'When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters' (Law of Independent Assortment).