Question

First evidence for triplet genetic code was given by

• A. Crick
• B. Nirenberg and Matthaei
• C. Dr. H. G. Khorana
• D. Severo Ochoa

Hint:

{Important exam point:}

• {Francis Crick} → First evidence that the genetic code is a {triplet}.
• {Nirenberg \& Matthaei} → First to identify a specific codon (\(UUU\)) coding for {phenylalanine}.
• {Khorana} → Helped decipher the {complete genetic code table}.

Remember: {Crick proved the triplet nature, while Nirenberg began decoding the codons}.

Answer 1

The Correct Option is A

Concept:
The genetic code refers to the set of rules by which the sequence of nucleotides in DNA or RNA determines the sequence of amino acids in a protein. Each amino acid is encoded by a specific sequence of nucleotides known as a codon. A key feature of the genetic code is that it is a triplet code, meaning that three nucleotides together specify one amino acid. The discovery of this triplet nature involved both genetic experiments and biochemical studies carried out by several scientists.
Step 1: Evidence provided by Francis Crick.
In 1961, Francis Crick and his colleagues conducted experiments on the T4 bacteriophage. They introduced insertion and deletion mutations into the DNA sequence and observed their effect on protein synthesis. They found that:

• When one or two nucleotides were inserted or deleted, the reading frame of the genetic message was disturbed.
• When three nucleotides were inserted or deleted, the original reading frame was restored.

This experiment clearly demonstrated that the genetic message is read in groups of three nucleotides, providing the first strong evidence that the genetic code is a triplet code.
Step 2: Contributions of other scientists.

• Marshall Nirenberg and Heinrich Matthaei performed biochemical experiments using synthetic RNA sequences (such as poly-U). They showed that the codon UUU codes for the amino acid phenylalanine. This experiment helped begin the process of deciphering the genetic code.
• Har Gobind Khorana and Severo Ochoa later developed advanced techniques using synthetic RNAs to determine many other codon–amino acid relationships, ultimately helping to complete the genetic code table.

Step 3: Identifying the correct answer.
Although several scientists contributed to decoding the genetic code, the first evidence that the code is triplet in nature was provided by Francis Crick through mutation experiments. Final Answer: \(\boxed{\text{Francis Crick}}\)