The base sequence of DNA is conserved through replication through complementary base pairing. The first phase of DNA replication is the unwinding of the double-stranded, or double helix, DNA. This is done by breaking the hydrogen bonds between the bases of the double strands through the action of the enzyme helicase. Each of these separated strands now becomes a template for the synthesis of a new strand. The loose nucleotides will form hydrogen bonds with their complementary base pair, so adenine will join with thymine and guanine will pair with cytosine. The strands then rewind into the double helix forming new DNA molecules. This complementary base pairing guarantees that the two DNA molecules will be identical. One original strand and one new strand make up each DNA molecule. The new strands are complimentary to the original template strands and therefore identical to the other template.
Complementary base pairing conserves DNA base sequence.