During cellular division (mitosis), DNA undergoes DNA replication. Simply put, DNA replication is when the two strands of DNA unzip. In order for this to occur an enzyme called helicase must hack the hydrogen bonds. The place where the DNA unzips is called the replication fork. Primase then adds RNA primers so that nucleotides can be attached to the exposed nucleic acids on each of the unzipped strands. DNA polymerase then replaces the RNA primers, adds the nucleotides, and proofreads for mistakes. However, DNA polymerase only reads DNA the 3' --> 5' direction and synthesizes a new complementary strand in the 5' ---> 3' direction. Thus, DNA polymerase works in opposite direction on each of the DNA strands. The enzyme called ligase then links and seals the phosphate-sugar backbones.