question_answer

Describe the Meselson and Stahl's experiment in detail.

Or

(i) Single Nucleotide Polymorphisms (SNPs) identified in human genome, can bring out revolutionary changes in biological and medical sciences. Give reason.

(ii) Write any four applications of Human Genome Project (HGP).

(iii) What was the first genetic material? Give reasons in support of your answer.

Answer:

Meselson and Stahl in 1958, proved that DNA replicates in a semiconservative way. This semiconservative DNA replication suggests that, after the completion of replication, each DNA molecule will have one parental and one newly - synthesised strand. Steps of the experiment conducted by them are as follows

· They grew E. coli in a medium containing \[^{15}N{{H}_{4}}Cl\](\[^{15}N\]is the heavy isotope of nitrogen) for many generations. As a result, \[^{15}N\]got incorporated into the newly synthesised DNA. This heavy DNA can be differentiated from normal DNA by centrifugation in caesium chloride (CsCl) density gradient.

· Then, they transferred the cells into the medium containing \[^{14}N{{H}_{4}}Cl\]and took the samples at various definite time intervals.

· The extracted DNAs were centrifuged and measured to get their densities, The parent bacteria had fully labelled DNA\[({{N}^{15}}\text{ }{{N}^{15}})\]. The second generation of bacteria, contained two types of DNA 50% light \[({{N}^{14}}{{N}^{14}})\]and 50% intermediate\[({{N}^{15}}\text{ }{{N}^{14}})\]. The third generation had intermediate and light DNA in 1 : 3 ratio and so on.

· It was possible only if DNA replicates in a semiconservative mode.

Or

(i) Scientists have identified about 1.4 million locations, where single base DNA differences, Single Nucleotide Polymorphism (SNP) occurs in humans. This information can bring out revolutionary changes in biological and medical science by following ways

(a) By tracing human history.

(b) By finding chromosome location for disease associated sequences.

(ii) Application of HGP

These are as follows

(a) Knowledge of DNA sequence will define research leading to our knowledge of biological system.

(b) It will enable a radically new approach to biological research.

(c) It can be used to study genes in a genome.

(d) It can create new ways to diagnose, treat and prevent the disorders that affect humans.

(iii) RNA is the first genetic material. This is because

I. Essential life processes have evolved around RNA.

II. It acts both as catalyst and genetic material.