DNA is a molecule made up of two strands twisted around each other in a double helix shape. Each strand is made up of a sequence of four chemical bases represented by the letters A, C, G and T. The two strands are complementary. This means that wherever there's a T in one strand there will be an A in the opposite strand and wherever there's a C there will be a G in the other strand. Each strand has a five prime end and a three prime end. The two strands run in opposite directions. This determines how each strand of DNA is replicated. The first step in DNA replication is to separate the two strands. This unzipping is done by an enzyme called helicase and results in the formation of a replication fork. The separated strands each provide a template for creating a new strand of DNA. An enzyme called primase starts the process. This enzyme makes a small piece of RNA called a primer. This marks the starting point for the construction of the new strand of DNA. An enzyme called DNA polymerase binds to the primer and will make the new strand of DNA. DNA polymerase can only add DNA bases in one direction from the five prime end to the three prime end. One of the new strands of DNA, the leading strand, is made continuously. The DNA polymerase adding bases one by one in the five prime to three prime direction. The other strand, the lagging strand, cannot be made in this continuous way because it runs in the opposite direction. The DNA polymerase can therefore only make this strand in a series of small chunks called okazaki fragments. Each fragment is started with an RNA primer. DNA polymerase then adds a short row of DNA bases in the five prime to three prime direction. The next primer is then added further down the lagging strand. Another okazaki fragment is then made and the process is repeated again. Once the new DNA has been made, the enzyme exonuclease removes all the RNA primers from both strands of DNA. Another DNA polymerase enzyme then fills in the gaps that are left behind with DNA. Finally, the enzyme DNA ligase seals up the fragments of DNA in both strands to form a continuous double strand. DNA replication is described as semi-conservative because each DNA molecule is made up of one old, conserved strand of DNA and one new one.