DNA replication is among the most tightly regulated, controlled, and precise processes in nature. During cell division, all of the information stored in the cells must be cloned, so that daughter cells can receive one copy each and remain ‘genetically identical’ to the original cell. This is done by DNA replication. During this process, the entire genome - the whole set of DNA and genes in a cell - is duplicated, and any modifications present in the parent DNA (such as methylation) are also copied over.
Since DNA is a double stranded helix, the process is initiated by the enzyme helicase, that opens the DNA into two single strands (called a replication fork). An enzyme called DNA polymerase goes along each strand and adds nucleotide bases complementary to the original strand.
The precision of the enzymes that synthesise new DNA strands is extremely high, since errors in DNA replication can result in mutations, which may be harmful for the organism. By the time the replication is complete, we have two identical DNA molecules, each containing one old strand and a newly synthesised one – thus making the process semi-conservative.
A quote from Watson and Crick's landmark 1953 paper suggests the motivation behind this discovery:
_"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanisms for the genetic material."_
DNA replication can also be performed artificially, outside the cell. One such way of doing this is by Polymerase Chain Reaction (PCR). This is useful when you have a limited amount of DNA sample and need to analyse or use it further.