DNA replication occurs within the nucleus of a cell. During the cell cycle S phase, any time that a cell needs to divide, DNA replication occurs. Prior to cell division, the DNA replication process ensures the copying of chromosomes.
Helicases and SSB proteins partially unwind the double helix strand. The two strands of DNA unzip in anti-parallel directions and DNA polymerase goes to where synthesis starts. The primase, which is the leading strand, lays down an RNA primer that works to build a second strand with the help of polymerase III. This second strand is built in the same direction the double helix opens. Okazki fragments result when RNA primers fill in the remaining gaps, and DNA nucleotides take the place of the DNA polymerase that RNA primer strips away. One long continuous strand results when the sealing of the fragments occurs due to DNA ligase, a type of enzyme.
Certain cells are constantly dividing and undergoing DNA replication such as the ones in the bone marrow, the hair and the fingernails. Other cells go through several division cycles and then just stop such as the specialized cells in the muscle, heart and brain. Still other cells cease dividing completely such as liver cells and skin cells.