DNA replication is defined as the process a DNA molecule undergoes to make a complete and identical copy of itself, readying a cell for cell division. DNA replication is a semi-conservative process, and the two daughter molecules contain exact copies of the genetic material in the parent molecule. DNA replication is referred to as semi-conservative as one of the two strands is conserved/retained from one generation to the next, while the other strand is brand new. This process occurs during the S (synthesis) phase of the cell cycle and occurs in preparation for mitosis and meiosis. In eukaryotic cells, the chromosomes gain a sister chromatid and become double-stranded.

Image: Mackenzie Angell (Graphic Designer)

• DNA replication begins with the enzyme DNA helicase 'unzipping' a section of the double-stranded DNA. The enzyme does this by breaking the weak hydrogen bonds holding complementary nitrogenous bases together, hence exposing nucleotide bases. It is important to note that this separation of the parental DNA strands occurs along a small section at a time.

• The junction between the now unwound single strands of DNA and the intact double helix is called a replication fork. The replication fork moves along the parental DNA strand so that there is a continuous unwinding of the parental strands.
  

• An enzyme called RNA primase attaches a short sequence of RNA called a primer to show another enzyme (DNA polymerase) where to begin adding nucleotides. Within the nucleus, free-floating nucleotides attach to the exposed bases via complementary base pairing rules; this is catalysed by another enzyme, DNA polymerase.

• DNA ligase, a 'gluing' enzyme, seals the new short stretches of nucleotides into a continuous double strand that rewinds back into a helix shape. Ligase catalyses the formation of previously mentioned phosphodiester bonds. The nucleotides link together in a 5' to 3' direction, forming long molecules. As the DNA strands are antiparallel, DNA polymerase moves in opposite directions on the two strands during synthesis; one strand is referred to as the leading strand, while the other is called the lagging strand.

• On the leading strand, the synthesis of additional nucleotides is continuous and occurs one after the other in a 5' to 3' direction. On the lagging strand, synthesis is discontinuous as it is in a 3' to 5' direction, and DNA can only be synthesised in a 5' to 3' direction. DNA is instead synthesised in short pieces at different RNA primers, called Okazaki fragments. DNA polymerase moves in opposite directions on the two antiparallel strands and removes the RNA primers and replaces them with DNA nucleotides. DNA ligase joins the Okazaki fragments together to form a continuous strand and catalyses the formation of phosphodiester bonds.
  

The whole process is a lot to digest! For simplicity, it can be broken down into 5 essential steps.

1. DNA helicase unwinds and separates the double strands by breaking weak hydrogen bonds between complementary base pairs. Each half of the parent molecule is used as a template.

2. The enzyme RNA primase attaches a short sequence of RNA known as a primer to show DNA polymerase where to start adding nucleotides.

3. Free nucleotides are added by DNA polymerase according to complementary base pairing rules in a 5' to 3' direction.

4. DNA ligase removes and replaces the primers. The result is two identical DNA molecules that are each made up of one original strand and one new strand (semiconservative).

5. In eukaryotes, two identical sister chromatids are now ready for cell division. In prokaryotes, two identical circular chromosomes are now ready for binary fission.