(a) What is a peptide bond? How it is formed?
(b) Explain the steps involved in the elongation of polypeptide chain during protein synthesis.
OR
What did Meselson and Stahl observe when
(a) They cultured E. coli in a medium containing ¹⁵NH₄Cl for a few generations and centrifuged the content.
(b) What did Meselson and Stahl conclude from this experiment? Explain with help of diagram.
(c) Which is the first genetic material? Give reason in support of your answer.

(a) Peptide bond is formed between two amino acids where one water molecule is liberated. 

(b) Major steps of protein synthesis are: 
i. Activation of amino acids 
ii. Transfer of amino acids to tRNA 
iii. Initiation of polypeptide chain 
iv. Translocation of ribosomes 
v.Chain Termination 
OR
(a) After culturing E.coli cells in  ¹⁵NH₄Cl for a few generations and centrifugation of the content Meselson observed that all the DNA formed were heavy DNA (¹⁵N¹⁵N).

(b)

Meselson and Stahl's experiment proved that DNA replication is semi-conservative, which means that each of the two resulting DNA molecules contains one original strand and one newly synthesized strand.

In their experiment, Meselson and Stahl grew E. coli bacteria in a medium containing a heavy isotope of nitrogen (¹⁵N) for several generations until all the DNA in the cells contained this isotope. They then transferred the bacteria to a medium containing a lighter isotope of nitrogen (14N) and allowed them to grow for one or more generations. They collected DNA samples from the bacteria at different time points and analyzed the DNA by density gradient centrifugation.

They found that after one generation of growth in the ¹⁴N medium, the DNA had a density intermediate between that of the ¹⁵N and ¹⁴N DNA. This result was consistent with the semi-conservative mode of replication, where each daughter DNA molecule contains one 15N-labeled strand and one newly synthesized ¹⁴N-labeled strand.

After two generations of growth in the 14N medium, the DNA had two distinct bands: one at the intermediate density and one at the ¹⁴N density. This result was consistent with semi-conservative replication, as it showed that the original ¹⁵N-labeled DNA had been completely replicated and the resulting DNA molecules were now composed of one ¹⁵N-labeled strand and one ¹⁴N-labeled strand.

Overall, Meselson and Stahl's experiment provided strong evidence for the semi-conservative mode of DNA replication and helped to establish the fundamental mechanisms of DNA replication.

(c) The first genetic material is believed to have been RNA (ribonucleic acid). There are several reasons to support this hypothesis:

(i) RNA can perform both genetic and catalytic functions: RNA has the ability to store genetic information like DNA, and also has catalytic activity like enzymes. This makes RNA a good candidate for the first genetic material because it could have performed both functions necessary for the early evolution of life.

(ii) RNA can self-replicate: RNA molecules can act as templates for the synthesis of new RNA molecules through a process called RNA replication. This self-replication ability could have allowed RNA to reproduce and evolve over time, leading to the development of more complex organisms.

(iii) RNA is simpler than DNA: RNA is simpler in structure than DNA, which makes it more likely to have arisen spontaneously. DNA requires complex enzymes for its synthesis, whereas RNA can be synthesized using simpler chemical processes.

Overall, while the exact origins of life on Earth are still not fully understood, the hypothesis that RNA was the first genetic material is supported by several lines of evidence and is currently widely accepted in the scientific community.