A change as small as a single nucleotide in the DNA sequence can have highly significant consequences. Thus it is crucial that DNA be reproduced accurately. The structure of DNA has all the information needed to perpetuate its sequence. Recall that DNA is two strands of nucleotides held together in a double helix by specific hydrogen bonds between bases.
           During DNA replication, the strands of the helix separate and each strand serves as a template for synthesis of new DNA. The new DNA is assembled on each parental strand one nucleotide at a time by an enzyme called DNA polymerase. The sequence of the new DNA is directed by the parental strand and depends on the specificity of base pairing. If there is a thymine (T) in the template strand, then an adenine (A) is added opposite it in the new strand; if there is a G then a C is added and so on. DNA replication is semiconservative- each replicated DNA molecule consists of one parental strand and one newly synthesized strand.