DNA replication is a fundamental biological process where a cell creates exact copies of its DNA. This duplication is essential for cell division, ensuring each new cell inherits complete genetic information. It plays a central role in growth, tissue repair, and the transmission of hereditary traits. The process involves several steps, utilizing specialized proteins and enzymes to achieve accurate copying.
Unwinding the DNA Double Helix
DNA replication begins with the separation of the two strands of the double helix. An enzyme called helicase facilitates this unwinding by breaking the hydrogen bonds connecting the complementary base pairs, much like unzipping a zipper. This action creates a Y-shaped structure called a replication fork, providing access to the single DNA strands. To prevent the separated strands from rejoining prematurely, single-strand binding proteins (SSBs) attach to the exposed DNA, stabilizing them for copying.
Primer Binding
Once DNA strands are separated, DNA polymerase cannot initiate a new strand from scratch; it requires a pre-existing starting point. This starting point is a short RNA segment called a primer, synthesized by primase. The primase lays down this RNA primer, which offers a free 3′-hydroxyl group. This hydroxyl group serves as the attachment site for DNA polymerase to begin adding new DNA nucleotides.
New Strand Synthesis
With the RNA primer in place, DNA polymerase starts adding complementary DNA nucleotides to the template strand, extending the new DNA in the 5′ to 3′ direction. Because the two original DNA strands are antiparallel and DNA polymerase synthesizes in only one direction, DNA synthesis occurs differently on each template strand. The leading strand is synthesized continuously, moving towards the replication fork as it unwinds, allowing DNA polymerase to add nucleotides uninterruptedly.
In contrast, the lagging strand template runs opposite to the replication fork’s movement, necessitating discontinuous synthesis. On this strand, DNA polymerase synthesizes short segments of DNA known as Okazaki fragments. Each Okazaki fragment requires its own RNA primer to initiate synthesis.
Ligation and Error Correction
After DNA polymerase synthesizes the new DNA strands, RNA primers must be removed and replaced with DNA nucleotides. Gaps remain between the newly synthesized DNA segments, especially between Okazaki fragments on the lagging strand. DNA ligase then seals these gaps by forming phosphodiester bonds, joining the DNA fragments into a continuous strand.
DNA polymerase also possesses a built-in proofreading capability. This allows the enzyme to “check its work” as it adds nucleotides, promptly removing any incorrectly paired bases. This proofreading mechanism reduces the error rate, contributing to the high fidelity of DNA replication and maintaining genetic stability.