When the amino acid sequences of insulin isolated from different organisms were determined, some differences were noted. for example, alanine was substituted for threonine, serine was substituted for glycine, and valine was substituted for isoleucine at corresponding positions in the protein.
which of the following single base changes would result in alanine being substituted for threonine?
a) a changed to c in the third position of the codon.
b) c changed to g in the second position of the codon.
c) a changed to g in the first position of the codon.

C) A changed to G in the first position of the codon.

Explanation:

In the translation of DNA,  triplets  or codons of mRNA  (a group of three nucleotides) are read according to a genetic code, to build proteins.

Proteins are composed of amino acids. In the translation process, each codon codes for an particular amino acid according to the  genetic code as illustrated  in the DNA codon table (attached here).  

In this table we can see that if the first position  of any of the codons coding for Threonine ( ACT, ACC, ACA, ACG) is changed for G, Adenine is going to be encoded instead of threonine.

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