Question

In an in vitro translation experiment, poly(UC) RNA template produced poly(Ser-Leu), while poly(AG) RNA template produced poly(Arg-Glu) polypeptide. Which one of the following options represents correct interpretations of the codon assignments for Ser, Leu, Arg, and Glu.
(a)Ser - UCU, Leu - CUC, Arg - AGA, Glu - GAG
(b)Ser - CUC, Leu - GAG, Arg - UCU, Glu - AGA
(c)Ser - AGA, Leu - UCU, Arg - GAG, Glu - CUC
(d)Ser - GAG, Leu - AGA, Arg - CUC, Glu - UCU

Answer 1

Hint: A codon is a sequence of three nucleotides that form a unit of the genetic code in an RNA or a DNA molecule. One codon codes for a specific amino acid only. In this case, based upon the results of the in vitro experiment, it can be determined that Serine is coded using two Uracil bases, while Arginine and Glutamate are not coded using a codon with uracil.

Complete answer:
A poly(UC) RNA has a nucleotide sequence with only Uracil and Cytosine, i.e., UCUCUCUCUCUC. It codes for the amino acid sequence poly(Ser-Leu).
Codons are present as a group of three nucleotides. So according to that, the first codon which consists of the first three nucleotides code for Serine while the next three nucleotides or the next codon codes for Leucine.
So, codon UCU codes for Ser while CUC codes for Leu.
In a similar fashion, the poly(AG) RNA, AGAGAGAGAGAG codes for poly(Arg-Glu).
So, Arg is coded by the codon AGA while Glu is coded by GAG.
So, the correct answer is, “Ser - UCU, Leu - CUC, Arg - AGA, Glu - GAG”

Note: According to the rules of the genetic code
-A single codon always codes for only one amino acid. In no circumstances, a codon codes for two separate amino acids. In the above experiment, UCU will always code for Ser and not any other amino acid.
-However, since there are four nucleotides and only three of them are present in a codon, there are 64 different codons.
Accordingly, a single amino acid may be coded using two different codons.
For example- The amino acid Leucine may be coded by four different codons CUU, CUC, CUA, and CUG none of which will code for any other amino acid.