Final answer:
A mutation from a U to an A in the third position of a Cys codon could lead to a nonsense mutation, creating a premature stop codon, or a missense mutation,
Step-by-step explanation:
If the DNA sequence that coded for Cys (Cysteine) was mutated in the third position from a U (uracil) to an A (adenine), the corresponding mRNA codon would change. This could result in a different amino acid being incorporated into the peptide chain during translation because codons specify particular amino acids. Cys is typically coded by the codons UGU and UGC. A third-position mutation to A would change the codon to either UGA or UCA, which would code for a stop codon and serine, respectively. The impact of this mutation depends on if it occurs in the UGU or UGC codon. If it is in the UGU codon, it could create a premature stop codon, leading to truncation of the resulting protein. This type of mutation is known as a nonsense mutation and can have serious consequences on the protein's function. If it occurs in the UGC codon, the mutation would result in a missense mutation, potentially altering the protein's properties or function if serine gets incorporated instead of cysteine. However, the effects of a missense mutation vary; some can be benign, while others can be deleterious depending on the role of the amino acid in the protein structure and function.possibly substituting serine for cysteine, potentially affecting the protein's function.