A portion of sequences for two types of hemoglobin are shown in the diagram.

The top sequence for wild-type hemoglobin is C-C-T, G-A-G, G-A-G. The bottom sequence for sickle-cell hemoglobin is C-C-T, G-T-G, G-A-G.

Sickle-cell anemia is caused by a mutation of one nucleotide in a gene. Using the diagram, which type of mutation is responsible for sickle-cell anemia?

A.
a frameshift mutation

B.
an insertion mutation

C.
an inversion mutation

D.
a substitution mutation