Discussion
In the genetic code, 61 codons code for 20 amino acids. How many types of aminoacyl tRNA synthetases are required for correct matching of tRNAs to their corresponding amino acids?
Answer: D
The answer is 20.
Each aminoacyl-tRNA synthetase is specific for one amino acid and attaches it to every tRNA that carries that amino acid, including all the isoaccepting tRNAs that read its different synonymous codons. So 20 amino acids need 20 synthetases, not one per codon.
61 is the number of sense codons and 64 the total number of codons including the three stop codons; neither counts synthetases. 40 is not tied to any feature of the code. Fewer than 61 distinct tRNAs are needed in any case, because wobble pairing lets one anticodon read more than one codon.
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