MP Board · Class 12 · Biology · Molecular Basis of InheritanceExplain the process of DNA replication in detail. Describe the enzymes involved and the mechanism of leading and lagging strand synthesis.

Introduction to DNA Replication\nDNA replication is a semiconservative process where a double-stranded DNA molecule produces two identical copies. Each daughter molecule consists of one parental strand and one newly synthesized strand.

Key Enzymes Involved

• DNA Helicase: Unwinds the double helix by breaking hydrogen bonds between complementary base pairs at the replication fork.
• Topoisomerase (DNA Gyrase): Relieves the torsional strain and supercoiling ahead of the replication fork created by unwinding.
• RNA Primase: Synthesizes a short RNA primer complementary to the DNA template, providing a free 3'-OH group for DNA polymerase.
• DNA Polymerase III: The primary enzyme responsible for adding nucleotides in the 5' to 3' direction, elongating the new strand.
• DNA Polymerase I: Removes RNA primers and replaces them with DNA nucleotides, also involved in DNA repair.
• DNA Ligase: Seals the nicks in the sugar-phosphate backbone by forming phosphodiester bonds, especially joining Okazaki fragments.

Mechanism of Strand Synthesis

• 5' to 3' Polarity Rule: DNA polymerases can only catalyze synthesis in the 5' to 3' direction. Because the two parental strands are antiparallel, replication proceeds differently on each strand.
• Leading Strand Synthesis: On the template strand with 3' to 5' polarity, replication is continuous. RNA primase creates a single primer, and DNA polymerase III continuously adds nucleotides towards the moving replication fork. This newly formed strand is called the leading strand.
• Lagging Strand Synthesis: On the template strand with 5' to 3' polarity, replication is discontinuous. As the fork opens up, multiple RNA primers are synthesized at intervals. DNA polymerase extends these primers to form short segments of DNA known as Okazaki fragments. This strand is called the lagging strand.
• Joining of Fragments: DNA polymerase I removes the RNA primers, fills the gaps with deoxyribonucleotides, and DNA ligase finally seals the nicks between the Okazaki fragments to create a continuous lagging strand.

Significance\nDNA replication ensures the accurate transmission of genetic information from cell to cell and generation to generation, maintaining genetic stability across living organisms.