Now that we have seen how genetic information encoded in the nucleotide sequences of DNA is translated into the structures of proteins that perform most cell functions, we can appreciate the necessity of the precise copying of DNA sequences during DNA replication.
The regular pairing of bases in the double-helical DNA structure suggested to Watson and Crick that new DNA strands are synthesized by using the existing (parental) strands as templates in the formation of new, daughter strands complementary to the parental strands.
This base-pairing template model theoretically could proceed either by a conservative or a semiconservative mechanism. In a conservative mechanism, the two daughter strands would form a new double-stranded (duplex) DNA molecule and the parental duplex would remain intact. In a semiconservative mechanism, the parental strands are permanently separated and each forms a duplex molecule with the daughter strand base-paired to it. Definitive evidence that duplex DNA is replicated by a semiconservative mechanism came from a now classic experiment conducted by M. Meselson and W. F. Stahl.
Copying of a DNA template strand into a complementary strand thus is a common feature of DNA replication and transcription of DNA into RNA. In both cases, the information in the template is preserved. In some viruses, singlestranded RNA molecules function as templates for synthesis of complementary RNA or DNA strands. However, the vast preponderance of RNA and DNA in cells is synthesized from preexisting duplex DNA.