In the first step of replication, the DNA double helix is split into two strands at the replication origins. This is done with DNA helicase, an enzyme. The DNA helicase creates a "replication fork". One strand is oriented '5 to 3' and the other 3' to 5'.
In the second step, RNA primase is used to have complementary nucleotides bond to the nucleotides on the original strand. An enzyme called DNA polymerase actually bonds and proofreads the new nucleotides, but RNA primase gets the original nucleotides ready, or "primed".
In the third step, the new strand must be sealed. DNA isn't just made of the two complementary nucleotides, after all. There is also a "backbone" made of phosphates and sugars. DNA ligase places this backbone on. The new DNA can now be used in a new cell once mitosis takes place.
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