Final answer:
A nonsense mutation that creates a premature stop codon is generally the most severe type of mutation affecting protein function, as it leads to a truncated, often nonfunctional protein. Missense mutations vary in impact but can be harmful too, as seen in diseases such as sickle cell anemia. Silent mutations typically don't affect protein function.
Step-by-step explanation:
Generally speaking, the type of mutation that would most severely affect the protein coded for by a gene is a nonsense mutation. This type of mutation converts a codon encoding an amino acid into a stop codon, leading to the production of a truncated and typically nonfunctional protein.
In contrast, a missense mutation, which changes one amino acid for another, may still yield a functioning protein, depending on the chemical properties and location of the amino acid change. However, it can also be detrimental if it disrupts a critical area such as the enzyme's active site. Meanwhile, silent mutations do not change the amino acid and generally have no effect on the protein's function.
For example, sickle cell disease is a result of a missense mutation where glutamic acid is replaced by valine, which affects the hemoglobin protein function with serious health consequences. Lastly, mutations in mRNA may only temporarily affect protein production, whereas DNA mutations are permanent if not repaired.