base analogue A purine or pyrimidine base (see BASE PAIR) that differs slightly in structure from the normal base, but that because of its similarity to that base may act as a mutagen when incorporated into DNA. Once in place, these bases, which have pairing properties unlike those of the bases they replace, can produce mutations by causing insertions of incorrect nucleotides opposite them during replication. Though the original base analogue exists only in a single strand, it can cause a nucleotide-pair substitution that is replicated in all DNA copies descended from that original strand. An example is 5-bromo-uracil (5BU), an analogue of thymine that has bromine at the C-5 position in place of the CH₃ group found in thymine.

base analogue A purine or pyrimidine base that differs slightly in structure from the normal base, but that because of its similarity to that base may act as a mutagen when incorporated into DNA. Once in place, these bases, which have pairing properties unlike those of the bases they replace, can produce mutations by causing insertions of incorrect nucleotides opposite them during replication. Though the original base analogue exists only in a single strand, it can cause a nucleotide-pair substitution that is replicated in all DNA copies descended from that original strand. An example is 5-bromo-uracil (5BU), an analogue of thymine that has bromine at the C-5 position in place of the CH₃ group found in thymine.