Replication

Before a cell divides it has to replicate its DNA so that the daughter cell receives a copy of the genome. The DNA helix consists of two complementary DNA strands. Therefore, each of the two strands serves as a template for the construction of the other strand. Under normal conditions the DNA is packed into a compact structure called chromatin. To be able to replicate, the cell has to unfold and unwind the DNA, and also has to separate the two strands from each other. The cell has a complex machinery to perform these tasks. When it is time to replicate, special initiator proteins attach to the DNA at regions called replication origins. These regions are characterised by a weak bond between the two DNA strands. There are around 10,000 replication origins on the DNA in a cell; this arrangement increases the rate of replication tremendously. The initiator proteins pry the two strands apart and a small gap is created at the replication origin. Once the strands are separated another group of proteins, that carry out the DNA replication, attaches and go to work.

This group of proteins includes helicase, which serves as an unzipper by breaking the bonds between the two DNA strands. This unzipping takes place in both directions from the replication origins, creating a replication bubble. The replication is therefore said to be bi-directional. Once the two strands are separated a small piece of RNA, called an RNA primer, is attached to the DNA by an enzyme called DNA primase. These primers are the beginnings of all new DNA chains since the enzyme responsible for the copying of the DNA, DNA polymerase, can not start from scratch. It is a self-correcting enzyme and copies the DNA template with remarkable fidelity.

The DNA polymerase can only read in the 3' to 5' direction. This gives rise to some trouble since the two strands of the DNA are antiparallel. On the upper strand which runs from 3' to 5', nucleotide polymerisation can take place continuously without any problems. This strand is called the leading strand. But how does the polymerase copy the other strand then when it runs in the opposite direction, from 5' to 3'? On this so called lagging strand the polymerase produces short DNA fragments, called okazaki fragments, by using a backstitching technique. These lagging strand fragments are primed by short RNA primers and are subsequently erased and replaced by DNA.