DNA Replication

DNA replication occurs by synthesizing new strands of DNA using existing strands as a template. The two new double helices each contain an original template strand and a newly synthesized daughter strand, which is why this process is known as semiconservative replication.  

To begin replication, an enzyme, helicase, unwinds the DNA helix and breaks the hydrogen bonds between the two strands. It then separates the individual strands forming a Y-shaped structure, known as the replication fork, where the template strands can be accessed by additional enzymes.

Another enzyme, primase, adds short RNA fragments called primers onto each template strand. These primers are essential for the synthesis of DNA as DNA polymerase can only add nucleotides to an existing strand. 

DNA polymerase adds to the growing daughter strands on both template DNA strands. The addition of nucleotides is guided by the sequence of the original DNA strand according to the DNA pairing rules.

Synthesis of one of the daughter strands, the leading strand, occurs in the direction of the replication fork movement. The other strand, the lagging strand, is synthesized in the opposite direction.

This leads to the leading strand being synthesized as a continuous polymer, whereas the lagging strand is synthesized as short fragments. This occurs because DNA can be synthesized only in the 5’ to 3’ direction.

Before their addition to the growing polymer, nucleotides exist as deoxyribonucleoside triphosphates, with three phosphates attached to the fifth carbon on the sugar.

This free nucleotide triphosphate reacts with the 3’ hydroxyl groups. This is the OH that is attached to the third carbon of the sugar at the end of the growing strand.  The reaction causes the release of pyrophosphate and the formation of a phosphodiester bond between the two nucleotides.

After the synthesis of the new strands, RNase H or additional variants of DNA polymerase remove the primers and synthesize DNA in their place. The gaps between the fragments are then sealed by DNA ligase to generate a continuous strand.

The addition of nucleotides continues until two replication forks meet each other, resulting in the completed replication.