Question

Imagine you are a student trying to decode the genetic code. An artificial mRNA molecule consisting of poly-CA (5ʹ... CACACACACACACAC..3ʹ) yields a polypeptide consisting solely of histidines and threonine, and another artificial mRNA consisting of poly-CAA (5ʹ...CAACAACAACAACAA...3ʹ) yields three different polypeptides: polythreonine, polyglutamine, and polyasparagine. On the basis of this information, which codons can you assign to which amino acids?

Answer 1

Answer:

ACA: Threonine

CAC: Histidine

Explanation:

To answer this question we need to remember that the ribosome reads every three bases or 'codon' in order to assign the right tRNA carrying the amino acid.

In the first artificial mRNA we see two patterns of three letter:

CAC and ACA.

In the second artificial mRNA we are able to identify three different patterns:

CAA

AAC

ACA

And they repeat, so we end with three different polypeptides: polythreonine, polyglutamine and polyasparagine. This will depend on the initial letter the ribosome starts reading.

The only amino acid that repeats in both artificial mRNAs is Threonine,  and we see its pattern ACA also repeated.

So, we could assign this codon (ACA) to threonine.

We can then assume that the pattern CAC codifies for histidine since we only get this two polypeptides in the first mRNA.

Lastly with the information provided we cannot determine the codons AAC and CAA for glutamine or asparagine. We would need further experiments.