What cause DNA to split and replicate?

in the body, DNA is “unzipped” by an enzyme called DNA helicase, which requires energy to function, so really energy is put into the system to allow the DNA to split. If left alone at physiological temperature it is reasonable to assume that it never would. It is also possible to denature DNA by heating, as in Polymerase Chain Reaction, or PCR, which is used in the lab to rapidly copy strands of DNA for various applications.

as far as the replication, there are alot of other enzymes involved, including DNA polymerase, which attaches complimentary nucleotides to the unzipped strands, DNA ligase, which joins together sections of newly placed complimentary nucleotides called okazaki fragments(only exist on one side of the replication fork), and topoisomerase, which can be thought of as an enzyme which untangles the DNA strand to allow the helicases to work.