Biology – Structure of nucleic acids and replication of DNA | e-Consult

DNA replication is the process by which a cell duplicates its DNA before cell division. It's a semi-conservative process, ensuring each daughter cell receives a complete copy of the genetic information. The process begins with the unwinding of the DNA double helix at specific locations called origins of replication. This unwinding is facilitated by helicase, which separates the two strands.

DNA polymerase is the primary enzyme responsible for synthesizing the new DNA strands. It works by adding nucleotides to the 3' end of a pre-existing primer or DNA strand, following the base-pairing rules (A with T, and G with C). This means DNA polymerase can only add nucleotides in the 5' to 3' direction.

The 5' to 3' directionality has a significant impact on leading and lagging strand synthesis. The leading strand is synthesized continuously in the 5' to 3' direction, as the replication fork moves forward. Only one RNA primer is needed to initiate synthesis on the leading strand. As the fork progresses, DNA polymerase continuously adds nucleotides to the 3' end of the growing strand.

The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments. Because DNA polymerase can only add nucleotides in the 5' to 3' direction, the lagging strand is synthesized in short, discontinuous segments moving away from the replication fork. Each Okazaki fragment requires a new RNA primer. After the synthesis of an Okazaki fragment, the RNA primer is removed and replaced with DNA by another DNA polymerase. Finally, DNA ligase seals the gaps between the Okazaki fragments by forming a phosphodiester bond between the 3'-OH of one fragment and the 5'-phosphate of the adjacent fragment, creating a continuous DNA strand.