Before DNA can be replicated, the double stranded molecule must be “unzipped” into two single strands. DNA has four bases called adenine (A), thymine (T), cytosine (C) and guanine (G) that form pairs between the two strands. Adenine only pairs with thymine and cytosine only binds with guanine. In order to unwind DNA, these interactions between base pairs must be broken. This is performed by an enzyme known as DNA helicase. DNA helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork. This area will be the template for replication to begin.
DNA is directional in both strands, signified by a 5’ and 3’ end. This notation signifies which side group is attached the DNA backbone. The 5’ end has a phosphate (P) group attached, while the 3’ end has a hydroxyl (OH) group attached. This directionality is important for replication as it only progresses in the 5’ to 3’ direction. However, the replication fork is bi-directional; one strand is oriented in the 3’ to 5’ direction (leading strand) while the other is oriented 5’ to 3’ (lagging strand). The two sides are therefore replicated with two different processes to accommodate the directional difference.
Initiation of replication occurs within the double helix and rarely at the end. Opening up of DNA molecule creates replication bubble. Replication bubble progresses in the form of replication fork in one direction in the case of unidirectional replication and in both directions in bi-directional replications.
Bidirectional Replication
In circular DNA of bacteria and linear DNA of eukaryotes, DNA replication proceeds bidirectionarlly starting from a fixed origin of replication. The two replication forks move in opposite directions. Bidirectional replication may have multiple replication forks. They speed up the process of replication.
Eukaryotic chromosomes are very long. In order to speed up the process of replication, a chromosome may have thousands of points of origin of replication (O). They tremendously speed up the process of replication. Bidirectional replication forks proceed in opposite directions and meet the neighboring replication units, thereby opening the entire chromosome by separating two strands.
