CBSE 2023 · Region 1 · Set 1 · Q31 · 5 marks
Meselson and Stahl carried out an experiment to prove the nature of DNA replication. Recall the experiment and answer the following questions.(i)Which two types of nitrogen were used by them in their experiment and why ?(ii)Why did they take samples of E. coli at definite time intervals for their observation ?(iii)State the role of caesium chloride density gradient in their experiment.(iv)Write the conclusions they arrived at.(b)A true breeding tall pea plant with round seeds is crossed with a recessive dwarf pea plant having wrinkled seeds. Work out the cross up to $\displaystyle \mathrm{F}_{2}$ generation giving the phenotypic ratios of $\displaystyle \mathrm{F}_{1}$ and $\displaystyle \mathrm{F}_{2}$ generation respectively.(ii)State the Mendelian principle that can be derived only with the help of such a cross.
Meselson and Stahl carried out an experiment to prove the nature of DNA replication. Recall the experiment and answer the following questions.
(i)
Which two types of nitrogen were used by them in their experiment and why ?
(ii)
Why did they take samples of E. coli at definite time intervals for their observation ?
(iii)
State the role of caesium chloride density gradient in their experiment.
(iv)
Write the conclusions they arrived at.
(b)
A true breeding tall pea plant with round seeds is crossed with a recessive dwarf pea plant having wrinkled seeds. Work out the cross up to $\displaystyle \mathrm{F}_{2}$ generation giving the phenotypic ratios of $\displaystyle \mathrm{F}_{1}$ and $\displaystyle \mathrm{F}_{2}$ generation respectively.
(ii)
State the Mendelian principle that can be derived only with the help of such a cross.
Marking-scheme solution
(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).
Molecular Basis of InheritanceReplicationUnderstandlong_answermedium
More from Molecular Basis of Inheritance
- Explain how the addition of lactose in the medium regulates the switching on of the lac operon in bacteria.2025 · asked 3×
- The type of bond represented by the dotted line '-----' in a schematic polynucleotide chain is:2024 · asked 3×
- Which of the options has correct identification of 'P', 'Q' and 'R' in the illustration of 'Central Dogma'…2024 · asked 3×
- Study the diagram given below and answer the questions that follows: Identify the structure shown in the…2025 · asked 3×
- What is a genetic code? Why did scientists feel the need to propose a genetic code? Genetic code is said to…2026 · asked 3×
- Assertion: Primary transcripts in eukaryotes are subjected to splicing to remove the introns. Reason (R):…2024 · asked 3×
- Given below is a heterogeneous RNA formed during Eukaryotic transcription: How many introns and exons…2025 · asked 3×
- Consider the given data of a hypothetical small portion of mRNA that codes for a functional polypeptide chain…2024 · asked 3×
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.