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Consider an mRNA strand that has a sequenceuuu of base pairs of GUU CUA ACU. Suppose a substitution mutation occurred uuuin the DNA, resulting in the last base in the first codon uuuof the mRNA to be replaced by wCytosine. Using the codon chart below, a) identify which amino acid will now be formed, b) identify whether or not there will be a 9phenotypic change, and a brief explanation.
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a) GUC codes for valine.
b) No (most likely) phenotypic change — this is a silent (synonymous) substitution. GUU → GUC changes the third base but both codons specify valine (the genetic code is degenerate/wobble at the third position), so the amino‑acid sequence remains Val–Leu–Thr. (Rarely, silent changes can affect expression or splicing, but normally there is no change in protein function.)
b) No (most likely) phenotypic change — this is a silent (synonymous) substitution. GUU → GUC changes the third base but both codons specify valine (the genetic code is degenerate/wobble at the third position), so the amino‑acid sequence remains Val–Leu–Thr. (Rarely, silent changes can affect expression or splicing, but normally there is no change in protein function.)
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