question: Between which types of DNA bases are mutational biases introduced due to directional mutation pressure?
answer: The rates of substitution mutations in two directions, v (from an AT-pair to a GC-pair) and u (from a GC-pair to an AT-pair), are usually not the same. Thereafter, the effect of mutation on a genome is not random but has a directionality toward higher or lower GC content of DNA. The net effect, v/(u + v), has previously been defined as directional mutation pressure. Thus, directional mutation pressure (GC/AT pressure) refers to mutational biases between alpha-bases (A or T) and gamma-bases (G or C).