Answer:
A. Silent, The type of mutation responsible for changing a base in the mRNA strand, without changing the coding aminoacid or protein, is called a silent mutation.
Step-by-step explanation:
In a silent mutation occurs the change of a nitrogenous base in one of the codons that encodes an aminoacid, without changing the aminoacid or altering the structure or function of the protein to be synthesized.
In this type of mutations the change of the base does not mean the change of the aminoacid, because some aminoacids can be coded with more than one codon. In the case of Leucine, the codons that encode it are CUU, CUC, CUG or CUA, so even if a base changes, the final protein will be the correct one.
For the other options:
- Missense: the change of the base in the DNA chain implies the change of the codon in the mRNA and of the encoded aminoacid, in that way a structural and functional alteration of the synthesized protein occurs.
- Nonsense: the change in the nitrogenous base in the DNA leads to the coding of a termination codon, so that the protein is ultimately incomplete.
- Insertion: in this case there is the addition of more nitrogenous bases to the DNA chain, with respect to the original one.