Once the DNA is separated into two strands, an RNA primer is laid down to give the new strand a place to build from so that the new enzymes can mate up to the exposed bases of the DNA and continue to build until the strand is complete. The enzymes, specifically nucleotide triphosphate, must be complimentary for this to occur, which means that the A pairs with T and G with C. The base strand always dictates the replication order of the DNA strand.
DNA replication occurs in protein synthesis and cell replication. During protein synthesis, mRNA is used to pair with the DNA to translate it into a language the rest of the cell can understand so that protein can be made to fuel the processes needed in the organism.
During protein synthesis, mRNA functions as the complimentary enzymes. In the process of DNA replication, which happens in cell replication, there are six enzymes involved in creating a new DNA strand. The main pieces are topoisomerase, helicase, DNA polymerase, primase, ligase and binding proteins.
Topoisomerase is responsible for actually cutting the DNA so that it can be unwound by the helicase. The DNA polymerase is responsible for bringing the free nucleotides to form a covalent bond on the growing new strand. Primase is the enzyme that actually attaches the RNA primer that gives the new enzymes something to build on, while ligase catalyzes the formation and the proteins bind it all together.