Question

Nirenberg synthesized an mRNA containing 34 poly-Adenine (A-A-A-A-A-A-----) and found a polypeptide formed of 11 poly-lysine this proved that genetic code for lysine was

• A. One-adenine
• B. A-A doublet
• C. A-A-A triplet
• D. Many adenines

Answer 1

The correct option is C A-A-A triplet

Nirenberg and Matthei found one enzyme called polynucleotide phosphorylase that when used in cell free system made uridine diphoshates into a poly-U- messenger RNA (mRNA). This was then used for cracking the genetic code by them, i.e, to understand which codon codes for which amino acid. They used a cell-free system in a test tube where they put together all the things they needed for protein synthesis - RNA template (poly U mRNA), ribosomes, nucleotides, amino acids, stabilizing agents and energy. They found phenylalaline residues being incorporated into the polypeptide as mentioned in the question. Ribosomes read the information in the mRNA and incorporate the specific amino acid. Each three letter nucleotide code is read as one codon and a specic amino acid is added. No amino acid is incorporated for the three stop codons. Similarly a 34 poly adenine mRNA ( i.e (A-A-A-A-A-A-----) sequence will encode a 11 amino acid peptide. Since 34 when divided by 3 gives 11 (approx), therefore it shows the genetic code for lysine is a triplet i.e AAA. Thus, the correct option is C.

Had the genetic code been a doublet i.e if we suppose AA codes for lysine, then a polypeptide of 17 amino acids should have been formed which is not given in question. Similarly for the genetic code for lysine to be only A, a polypeptide of 34 amino acids should form (again not in question).