(a) Why did Messe

(a) Messelson and Stahl use two different isotopes of nitrogen in their experiments because nitrogen is a major constituent of DNA. And also they are the most abundant isotope of nitrogen.

¹⁵N is not a radioactive isotope, and it can be separated from ¹⁴N based on densities. That is easily separated.

(b) They collected their samples of E. coli from the culture medium for testing at various definite time intervals as the cells multiplied.

E.coli is collected after 20 minutes as well as 40 minutes.

Because the DNA extracted after 20 minutes experiment had a hybrid density.

The DNA extracted after 40 minutes had equal amounts of hybrid and light densities.

(c) The importance of centrifugation in their experiments is to distinguish heavy DNA molecule from the normal DNA.

(d) The following conclusion was drawn at the end of the experiment:

The experiment proves the semi-conservative replication of DNA. In this type of replication, one strand of parent is new and one strand is old. It means, one strand of daughter duplex is derived from the parent, while the other strand is formed new.

Since E. coli divides every 20 minutes, the DNA extracted after 20 minutes experiment had a hybrid density.

The DNA extracted after 40 minutes had equal amounts of hybrid and light densities.

OR

(a) 3:1 is phenotypic ratio, 1:2:1 is genotypic ratio.

Monohybrid cross is a cross where two forms of a single traits are hybridised.

After undercrossing the hybrids of F₁ generations, F₂ progeny was obtained.

As per 3:1 ratio, it means – 3/4^th of the plants are tall and 1/4^th of the plants are dwarf.

As per 1:2:1 ratio, it means – 1/4^th are TT, 1/2 is Tt and 1/4^th are tt.

(b)

In the monohybrid cross, Mendels selected pure line parents and they have one dominant and one recessive allele (this is finding the genotype of the parents).

Next emasculation (removal of stamens before the formation of pollen grains) is carried out.

This is followed by dusting, i.e., pollens from selected male flower are dusted on the stigma of an emasculated female flower.

Then Mendel crossed many flowers, collected seeds and raised F₁ generation.

The F₁ progeny so obtained were all tall.

Punnett square for F₁ generation:

Mendel allowed natural pollination in each F₁ hybrid; collected seeds separately and F₂ generation is obtained.

The ratio of tall plants to dwarf plants in F₂ generation is found to be 3:1.

Punnett square for F₂ generation:

The genotype ratio (pure tall: hybrid tall: pure dwarf) is 1:2:1.

In this way these ratios are obtained.

(c) The two laws of inheritance Mendel derived from such a cross are as follows:

- Law of Dominance: In this the characters are controlled by discrete units called factors. Factors occur in pairs. And in dissimilar pairs of factors one member of the pair dominates the other.

This law is used to explain the expression of only one of the parental characters in a monohybrid cross in the F₁ and the expression of both in the F₂. This law also explains the proportion 3:1 obtained at F₂.

- Law of segregation: Parents contains two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.

A homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes each having one allele with equal proportion.