Protein coding genes are composed of multiple codons, sequences of three nucleotides that each code for a specific amino acid. Given that multiple codons may encode a single amino acid, which of the following nucleotide substitutions in a codon would be LEAST likely to affect protein structure?

Question

Protein coding genes are composed of multiple codons, sequences of three nucleotides that each code for a specific amino acid.  Given that multiple codons may encode a single amino acid, which of the following nucleotide substitutions in a codon would be LEAST likely to affect protein structure?

Quizplus · Accepted Answer

11ecba2d_103b_77d4_a3bf_535dc1c9654e_MD0008_00 Protein-coding genes consist of multiple codons, sequences of three nucleotides that each code for a specific amino acid. Together, 64 total codons comprise the genetic code. However, because these 64 codons code for only 20 amino acids, the genetic code is referred to as degenerate, meaning multiple codons may code for the same amino acid. During translation, each mRNA codon (except the stop codon) base-pairs with a corresponding tRNA that is charged with a specific amino acid. DNA mutations in the protein-coding regions of a gene may alter the sequences of transcribed mRNAs, leading to changes in the amino acid structure of encoded proteins. Nucleotides in the first or second position of a codon require traditional Watson-Crick base pairing with the tRNA anticodon. Because mutating either of these positions also changes the base pairing, these mutations are likely to alter tRNA binding, and therefore are most likely to change the encoded amino acid. However, silent mutations alter the sequences of DNA and mRNA but do not affect protein structure. Mutations in the third position often code for the same amino acid, making them silent mutations. This is largely due to less stringent base pairing (wobble) at the third position of the mRNA codon and tRNA anticodon. Because mutating nucleotides in the third codon position often result in silent mutations, substitution of the third nucleotide in a codon is least likely to affect protein structure. (Choices A, B, and D) Because the first two nucleotides in a codon require traditional (Watson-Crick) base pairing with a tRNA anticodon, mutations altering the first or second nucleotide in a codon are more likely to alter the amino acid structure of the encoded protein. Educational objective: Because multiple codons may code for the same amino acid, the genetic code is called degenerate. This degeneracy is largely due to less stringent (wobble) base pairing between the third nucleotides in the mRNA codon and tRNA anticodon.

Protein Coding Genes Are Composed of Multiple Codons, Sequences of Three