CBSE · Class 12 · Biology · Molecular Basis of InheritanceExplain the process of DNA replication in detail. Describe the enzymes involved and the mechanism of formation of leading and lagging strands.

Introduction to DNA Replication\nDNA replication is a biological process by which a cell makes an identical copy of its DNA. It is a semiconservative process, meaning each daughter molecule inherits one parental strand and one newly synthesized strand.

Key Enzymes Involved

1. DNA Helicase: This enzyme unwinds the double helix by breaking the hydrogen bonds between complementary base pairs, creating a replication fork.
2. Topoisomerase: It relieves the torsional strain or supercoiling ahead of the replication fork caused by unwinding.
3. DNA Primase: It synthesizes a short RNA primer complementary to the DNA template strand, providing a free 3'-OH group necessary for DNA polymerase to initiate synthesis.
4. DNA Polymerase III: The primary enzyme responsible for adding nucleotides to the growing DNA chain in the 5' to 3' direction.
5. DNA Polymerase I: It removes the RNA primers and replaces them with DNA nucleotides.
6. DNA Ligase: It seals the nicks in the sugar-phosphate backbone by forming phosphodiester bonds, particularly joining the Okazaki fragments on the lagging strand.

Mechanism of Strand Formation

• Directionality: DNA polymerase can only synthesize DNA in the 5' to 3' direction. Because the two strands of DNA are antiparallel, replication proceeds differently on each template strand.
• Leading Strand: On the template strand with 3' to 5' polarity, replication is continuous in the same direction as the moving replication fork. This newly synthesized strand is called the leading strand and requires only a single RNA primer at the origin.
• Lagging Strand: On the template strand with 5' to 3' polarity, replication is discontinuous because it must move away from the replication fork. This results in short segments of DNA known as Okazaki fragments. This strand is called the lagging strand, and it requires multiple RNA primers. Eventually, RNA primers are removed, and Okazaki fragments are joined together by DNA ligase.