(a) What is Central dogma? Who proposed it?

(b) Describe Meselson and Stahl’s experiment to prove that the DNA replication is semi-conservative.

OR

(a) A couple with blood groups ‘A’ and ‘B’ respectively have a child with blood group ‘O’. Work out a cross to show how it is possible and the probable blood groups that can be expected in their other off-springs.

(b) Explain the genetic basis of blood groups in human population.

(a) Central dogma is a phrase in molecular biology, proposed by Francis Crick. Central dogma states that the genetic information flows from DNA to RNA and then to protein, but the revvese is not possible, i.e., proteins cannot pass the information back to DNA. It is represented as

(b) Messelson and Stahl’s experiment is as follows:

- They first grew E. coli in a medium containing ¹⁵N. The ¹⁵N was incorporated in a newly synthesised DNA. This heavy DNA can be separated from normal DNA by centrifugation in a cesium chloride (CsCl) density gradient.

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

- They collected their samples of E. coli from the culture medium with ¹⁴N for testing at various definite time intervals as the cells multiplied and extracted the DNA that remained as double-stranded helices.

- Hence the DNA was extracted from the culture one generation after the transfer from ¹⁵N to ¹⁴N medium.

- 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.

Note: They used 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.

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) Note: A and B are dominant genes. O is recessive. If A and B are equally dominant they create a new blood type: AB. And a person with blood type A can have genotype: AA, AO, a person with blood type B can have genotype: BB, BO.

Given couple with blood groups ‘A’ and ‘B’ respectively,

So father = I^Ai and mother = I^Bi

Parents: I^Ai × I^Bi

Cross:

Hence a couple with blood groups ‘A’ and ‘B’ respectively can have a child with blood group ‘O’ i.e., ii.

The probable blood groups that can be expected in their other off-springs are: AB, A, B and O

(b) The blood groups are controlled by gene I.

The gene (I) has three alleles I^A, I^B and i.

Humans are diploid in nature, so they possess any two of the three I gene alleles.

As there are 3 alleles there will be 6 different combinations possible in human ABO blood types.

The Genetic Basis of Blood Groups in Human Population is represented as follows:

A and B is co-dominant as A and B is equally dominant they create a new blood type: AB, so equally expressed.