DNA is double stranded. Each strand has a 5 prime end and a 3 prime end. The strands oppose each other in direction
3-------------5
5-------------3
The DNA bases A,T,C and G are pair A-T, C-G
3-----------------------5
ATACGTAATACG
TATGCATTATGC
5-----------------------3
In addition to being double stranted, molocule is also twisted into a helix.
In order to replicate the DNA the cell must first unwind the helix which requires an enzyme known as DNA helicase. Once the DNA has been unwound the replication can begin. The DNA polymerace, the molocule which inserts new bases, can only place bases of the new strand in the 5' to 3' direction. This poses a problem because when the replication fork opens you have both an exposed 3' end and an exposed 5' end.
As demostrated in the graphic below, replication occurs on the open 3' end in the 5' to 3' direction without interuption. On the open 5' end however, base insertion must also occur in the same direction. The polymorace must replicate small segments at a time. These are called Okazaki fragments.. After completing each fragment polymorace must jump to the next section before continuing. DNA ligase then binds the Okazaki fragments together on the lagging strand.
