MP Board · Class 12 · Biology · Molecular Basis of InheritanceExplain the process of DNA replication in detail. Describe the enzymes involved and the mechanism of continuous and discontinuous synthesis on the template strands.

Introduction to DNA Replication\nDNA replication is a semi-conservative process by which a DNA molecule makes a copy of itself during cell division. It occurs during the S-phase of the cell cycle. The two strands of the DNA double helix unwind and separate, each acting as a template for the synthesis of a new complementary strand.

Key Enzymes Involved

• DNA Helicase: Unwinds the DNA double helix by breaking the hydrogen bonds between complementary base pairs at the replication fork.
• Topoisomerase (DNA Gyrase): Relieves the torsional strain and supercoiling ahead of the replication fork caused by unwinding.
• RNA Primase: Synthesizes a short RNA primer complementary to the DNA template, providing a free 3'-OH group necessary for DNA polymerase to initiate synthesis.
• DNA Polymerase III: The primary enzyme responsible for adding nucleotides to the growing DNA chain in the 5' to 3' direction.
• DNA Polymerase I: Removes the RNA primers and replaces them with DNA nucleotides.
• DNA Ligase: Joins the newly synthesized DNA fragments (Okazaki fragments) on the lagging strand by forming phosphodiester bonds.

Mechanism of Synthesis: Leading and Lagging Strands

• Template Polarity and Direction: DNA polymerase can only synthesize DNA in the 5' to 3' direction. Because the two strands of DNA are antiparallel (one runs 5' to 3' and the other 3' to 5'), replication proceeds differently on each strand.
• Continuous Synthesis (Leading Strand): On the template strand with a 3' to 5' polarity, replication proceeds continuously in the direction of the moving replication fork. This newly synthesized strand is known as the leading strand.
• Discontinuous Synthesis (Lagging Strand): On the template strand with a 5' to 3' polarity, replication must occur away from the replication fork. This results in short segments of DNA called Okazaki fragments, synthesized discontinuously. This strand is known as the lagging strand.
• Joining of Fragments: Later, DNA ligase seals the gaps between the Okazaki fragments to form a continuous, unbroken DNA strand. Thus, semi-conservative replication ensures genetic continuity with high fidelity.