Question

Nirenberg used an artificial mRNA poly U (UUU_U) which gives rise to polyphenylalanine. This proved that
A. Artificial mRNA was not useful for the study of genetic code.
B. Genetic code is triplet
C. Triplet UUU code for phenylalanine
D. Genetic code is non overlapping

Answer 1

Hint: Nirenberg synthesizes the RNA by using polyifridylic acid (poly U). This resulted in poly U along the length of mRNA with possible triple UUU codon. This proved the triplet UUU code of phenylalanine.

Complete answer:
When poly U (RNA) was used for cell free synthesis of polypeptide by using cell free extract of E.coli that supplied all the components of protein synthesizing machinery, polyphenylalanine was synthesised.
It was considered that genetic code should be triplet so that codons must be assigned for 20 amino acids. First approach was made to find out the codons for amino acids by using sequences of mRNA in vitro, and secondly to prove the same in vivo.
Each genetic codon consists of three nitrogenous bases. In the artificial mRNA poly U chain, only the uracil base was present. Its translation led to accumulation of phenylalanine.
(a)This option is not correct according to the question as Nirenberg only gave the study of triplet codons.
(b)Genetic code is a set of rules used by living cells to translate information encoded within genetic material (sequence of nucleotide triplets), but this was not given by Nirenberg. So, this is not a right option according to the question.
(c)This is a correct option as the study done by Nirenberg proved that UUU codes for phenylalanine.
(d)This is not a correct option in accordance with the question.

So, the correct option is C

Note: A codon is a triple sequence of DNA and RNA that corresponds to a specific amino acid whereas anticodon are the section of a transfer RNA ( t RNA) which corresponds to codons in mRNA. As UUU codes for phenylaniline similarly CCC codes for proline and AAA codes for lysine.