Biosynthesis of cellular and viral DNA and RNA has been a major topic in molecular biology and biochemistry. The studies by Arthur Kornberg and his colleagues on the in-vitro synthesis of DNA have opened new avenues to understanding the processes controlling the duplication of the genetic information encoded in the DNA and RNA of bacterial and mammalian cells. Viral nucleic acids are replicated in infected cells (bacterial, plant, and animal) by virus coded enzymes with or without the involvement of proteins and enzymes coded by the host cells. The ability of the virus to replicate its genome within a relatively short period in the infected cell makes it an excellent biological tool for studying the molecular events in nucleic acid replication. Indeed, the identification of a number of virus-coded proteins that participate in the biosynthesis of X174 and SV40 DNA has led to the construction of in-vitro systems for the study of nucleic acid biosynthesis. Similarly, studies on the replication of other phage, animal and plant viruses have provided an insight into the nucleic acid sequences from which DNA synthesis is initiated, as well as the proteins and enzymes that regulate the catalyse biosynthetic processes. Investi gation of the molecular processes involved in the replication of cellular and mitochondrial genomes has gained momentum from the rapid developments in the analyses of viral nucleic acid biosynthesis.