DNA Replication Step 4: Elongation and Synthesis of DNA Strands Explained

Step 4 of DNA replication

Step 4 of DNA replication is the elongation or synthesis of the new DNA strands

Step 4 of DNA replication is the elongation or synthesis of the new DNA strands. Once the DNA double helix is unwound and the replication fork is created, the actual process of DNA replication begins.

During elongation, each of the separated single DNA strands acts as a template for the synthesis of a complementary strand. The enzyme responsible for this process is called DNA polymerase, which moves along the template strands in the 3′ to 5′ direction.

The leading strand is the one that is synthesized continuously, while the lagging strand is synthesized in short fragments called Okazaki fragments, which are later joined together.

For the leading strand, DNA polymerase synthesizes a complementary strand in the 5′ to 3′ direction, following the unwinding of the DNA helix. It does this by adding nucleotides to the 3′ end of the growing strand. The continuous synthesis of the leading strand occurs in the same direction as the replication fork.

On the lagging strand, synthesis occurs in the opposite direction of the replication fork, therefore, DNA polymerase encounters some challenges. As the replication fork unwinds, short stretches of the lagging strand become exposed. DNA polymerase can only add nucleotides to the 3′ end of a growing strand, so it synthesizes the lagging strand in short sections.

Special RNA primers are synthesized by another enzyme called primase, which is then used by DNA polymerase to initiate the synthesis of Okazaki fragments. These fragments are typically around 1,000-2,000 nucleotides long in eukaryotes.

Once the RNA primers are added, DNA polymerase III begins synthesizing the fragments, elongating them by adding nucleotides to the 3′ end. As it reaches the next RNA primer, DNA polymerase III detaches and moves to the next Okazaki fragment, leaving a small gap between the fragments.

Later, another DNA polymerase, called DNA polymerase I, replaces the RNA primers with the appropriate DNA nucleotides. It then seals the gaps between the Okazaki fragments by forming a covalent bond between adjacent nucleotides, a process known as DNA ligation.

Overall, step 4 of DNA replication involves the meticulous synthesis of new DNA strands using DNA polymerase and RNA primers, resulting in the elongation of both the leading and lagging strands. This process ensures accurate and complete replication of the entire DNA molecule.

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