DNA replication is a fundamental biological process where a cell creates an exact copy of its DNA. This process is essential for all living organisms, playing a crucial role in vital functions like cell division, growth, and tissue repair. Every time a cell divides, its complete genetic information must be precisely duplicated to ensure that each new daughter cell receives an identical set of instructions. It is also essential for heredity, ensuring genetic traits pass consistently from one generation to the next.
Initiating the Process: Unwinding the DNA
DNA replication begins with the unwinding of the DNA double helix. This exposes the genetic information contained within the DNA strands. An enzyme called DNA helicase is responsible for this unwinding, breaking the hydrogen bonds that hold the two DNA strands together.
As DNA helicase separates the strands, a Y-shaped structure known as the replication fork is formed. This replication fork is where DNA synthesis occurs. To prevent the separated single strands from rejoining, single-strand binding proteins (SSBs) quickly attach to them, stabilizing the unwound DNA.
Preparing for Synthesis: Primer Binding
After the DNA strands are unwound and stabilized, the cell prepares for new DNA synthesis. DNA polymerase, the enzyme that builds new DNA, cannot initiate synthesis from scratch; it requires a starting point.
This starting point is provided by a short segment of RNA called a primer. An enzyme known as primase synthesizes these RNA primers, attaching them to the template DNA strands. These primers offer a free 3′-hydroxyl group, which DNA polymerase can then extend by adding new DNA nucleotides. Primers are necessary for the synthesis of both the leading and lagging strands.
Building the New Strands: Elongation
Elongation is the stage where new DNA strands are synthesized. DNA polymerase, the enzyme involved, moves along the template strands, adding complementary nucleotides. This enzyme builds the new DNA strands exclusively in the 5′ to 3′ direction.
Replication proceeds differently on the two template strands due to their antiparallel orientation. One template strand, oriented 3′ to 5′ towards the replication fork, allows for continuous synthesis of a new strand called the leading strand. Here, DNA polymerase adds nucleotides continuously as the DNA unwinds.
Conversely, the other template strand is oriented 5′ to 3′ away from the replication fork, requiring discontinuous synthesis. On this lagging strand, primase adds multiple RNA primers at intervals. DNA polymerase then synthesizes short DNA segments, known as Okazaki fragments, between these primers.
Each Okazaki fragment is synthesized in the 5′ to 3′ direction, moving away from the replication fork until it reaches the previously synthesized fragment. DNA Pol III is primarily responsible for DNA synthesis during elongation.
Concluding Replication: Ligation and Termination
After new DNA strands are synthesized, the replication process concludes to ensure complete and accurate copies. The RNA primers must be removed. DNA polymerase I removes these RNA primers and fills the resulting gaps with DNA nucleotides.
After the primers are replaced with DNA, small gaps or nicks remain in the sugar-phosphate backbone, particularly between the Okazaki fragments on the lagging strand. An enzyme called DNA ligase then seals these nicks, creating a continuous DNA strand. This ligation creates complete, functional DNA molecules.
Replication terminates when the replication forks meet or when specific termination sequences are encountered. The newly synthesized DNA molecules, each consisting of one original and one new strand (semi-conservative replication), are then complete.