Base analogs

Base analogs are chemicals that are structurally very similar to the bases
normally found in DNA. Base analogs can get incorporated into DNA during
replication because of their structural similarity to normal bases. One base
analog, 5-Bromouracil, is almost identical to the base thymine. Most often,
5-bromouracil (also known as 5BU), which is pictured in Figure 13-5, gets
incorporated as a substitute for thymine and as such is paired with adenine.
The problem arises when DNA replicates again with 5-bromouracil as part of
the template strand; 5BU’s mistaken for a cytosine and gets mispaired with
guanine. The series of events looks like this: 5-bromouracil is incorporated
where thymine used to be, so T-A becomes 5BU-A. After one round of replication,
the pair is 5BU-G because 5BU is prone to chemical changes that make
it a mimic of cytosine, the base normally paired with guanine. After a second
of replication, the pair ends up as C-G because 5BU isn’t found in normal DNA.
Thus, an A-T ends up as a C-G pair.