DNA synthesis is the natural or artificial creation of deoxyribonucleic acid (DNA) molecules. In nature, such molecules are created by all living cells through the process of DNA replication, with replication initiator proteins splitting the existing DNA of the cell and making a copy of each split segment, with the copied segments then being joined together into a new DNA molecule. Various means also exist to artificially stimulate the replication of naturally occurring DNA, or to create artificial gene sequences. A polymerase chain reaction is a form of enzymatic DNA synthesis, using cycles of repeated heating and cooling of the reaction for DNA melting and enzymatic replication of the DNA. Artificial gene synthesis is the process of synthesizing a gene in vitro without the need for initial template DNA samples. The main method is currently by oligonucleotide synthesis (also used for other applications) from digital genetic sequences and subsequent annealing of the resultant fragments.