Final answer:
A mutation occurs when one nitrogen base replaces another in a segment of DNA, specifically called a substitution mutation. This change is permanent and can lead to various effects, some silent and others consequential.
Step-by-step explanation:
When one nitrogen base replaces another in a segment of DNA, the result is a mutation. Specifically, this type of mutation is known as a substitution mutation. A mutation is a permanent variation in the nucleotide sequence of a genome.
Substitution mutations involve a change where one nitrogenous base is replaced by another, impacting the DNA molecule at a single base pair level. This could either be a transition, where a purine is replaced with a purine or a pyrimidine with a pyrimidine, or a transversion, where a purine is replaced with a pyrimidine, or vice versa. These mutations can have a range of effects, from silent, where there is no change in the organism's traits, to significant changes which can potentially lead to diseases or adaptations.