What starts DNA replication?

DNA replication begins when the enzyme helicase separates the two strands in a small section of the DNA molecule. The strands are kept separated during the replication process by the binding of proteins called single stranded binding proteins (SSB). 

The enzyme primase attaches short RNA sequences to the exposed DNA called primers. The enyzme DNA polymerase extends the primers by adding DNA nucleotides that are complementary to the original DNA strand. DNA replication can only move in one direction. Because of this, the strand of DNA whose sequence runs in the opposite direction must be replicated using multiple RNA primers. The resulting fragments of replicated DNA are called Okazaki fragments. The Okazaki fragments are later connected by the enzyme DNA ligase.

Once new DNA strands have be created, the RNA primers are removed by the enzyme RNAse H. The remaining gaps in the DNA are then filled in by DNA polymerase.