An extraterrestrial organism (ET) is discovered whose basic cell biology seems pretty much the same as that of terrestrial organisms except that it uses a different genetic code to translate RNA into protein

Question

You set out to break the code by translation experiments using RNAs of known sequence and cell-free extracts of ET cells to supply the necessary protein-synthesizing machinery. In experiments using the RNAs below, the following results were obtained when the 20 possible amino acids were added either singly or in different combinations of two or three:

RNA 1: 5?-GCGCGCGCGCGCGCGCGCGCGCGCGCGC-3?
RNA 2: 5?-GCCGCCGCCGCCGCCGCCGCCGCCGCCGCC-3?

Using RNA 1, a polypeptide was produced only if alanine and valine were added to the reaction mixture. Using RNA 2, a polypeptide was produced only if leucine and serine and cysteine were added to the reaction mixture. Assuming that protein synthesis can start anywhere on the template, that the ET genetic code is nonoverlapping and linear, and that each codon is the same length (like the terrestrial triplet code), how many nucleotides does an ET codon contain?
(a) 2
(b) 3
(c) 4
(d) 5
(e) 6

Answer 1

(d) An organism having codons with an even number of nucleotides (such as 2, 4, or 6) could read 5′-GCGCGCGCGC-3′ (RNA 1) in either of two ways, namely "GC GC GC GC …" or "CG CG CG CG …" Either of the two amino acids alone could have supported protein synthesis, so you would not need them in combination [thus eliminating choices (a), (c), and (e)]. An organism having three bases per codon could read the sequence 5′-GCCGCCGCCGCCGCC-3′ (RNA 2) in one of three ways, namely "GCC GCC GCC GCC …," "CCG CCG CCG CCG …," or "CGC CGC CGC CGC …," and so again, any one of the three amino acids could have supported synthesis of a polypeptide, and you would not need to add all three amino acids to produce a polypeptide chain, thus eliminating choice (b). Only a five-nucleotide code gives you two different consecutive codons for RNA 1 and three different consecutive codons for RNA 2.