D1.1.3 Role of helicase and DNA polymerase in DNA replication Notes

DNA Replication Requires Unwinding and Synthesis

1. DNA replication copies the entire genome so each daughter cell receives identical genetic information.
2. Two key enzymes drive this process:
   1. Helicase: Unwinds the DNA double helix.
   2. DNA polymerase: Synthesizes new DNA strands.

The Role of Helicase: Unwinding the Double Helix

How Helicase Works

1. Step 1: Breaking Hydrogen Bonds
   1. Helicase moves along the DNA molecule.
   2. It breaks the hydrogen bonds between complementary base pairs (A-T and C-G).
   3. This separates the two strands.
2. Step 2: Forming the Replication Fork
   1. As helicase unwinds the DNA, it creates a Y-shaped structure called the replication fork.
   2. The two separated strands serve as templates for new DNA synthesis.

Key Features

1. Ring-shaped enzyme: Helicase encircles one strand of DNA and moves along it.
2. Only breaks hydrogen bonds: It does NOT break the covalent bonds in the sugar-phosphate backbone, only the weaker hydrogen bonds between bases.
3. Directional movement: Helicase moves in one direction (typically 3' to 5' on the template strand), continuously unwinding DNA ahead of the replication machinery.

The Role of DNA Polymerase: Synthesizing New DNA Strands

How DNA Polymerase Works

1. Step 1: Reading the Template
   1. DNA polymerase reads the template strand (one of the separated strands).
   2. It determines which nucleotide to add next based on complementary base pairing:
      1. A pairs with T
      2. C pairs with G
2. Step 2: Adding Nucleotides
   1. DNA polymerase adds free nucleotides to the growing strand, one at a time.
   2. Each nucleotide is added to the 3' end of the growing strand.
3. Step 3: Forming Covalent Bonds
   1. DNA polymerase catalyzes the formation of covalent bonds between the sugar of one nucleotide and the phosphate of the next.
   2. This creates the sugar-phosphate backbone of the new DNA strand.

Directionality: 5' to 3' Synthesis

1. DNA polymerase can only add nucleotides in the 5' to 3' direction.
2. This has important consequences:
   1. Leading strand: Synthesized continuously in the 5' to 3' direction as helicase unwinds the DNA.
   2. Lagging strand: Synthesized discontinuously in short segments called Okazaki fragments, which are later joined by the enzyme DNA ligase.

Proofreading Function

1. DNA polymerase has a proofreading ability.
2. It can detect when an incorrect nucleotide has been added.
3. It removes the incorrect nucleotide and replaces it with the correct one.
4. This dramatically reduces replication errors.

Comparing Helicase and DNA Polymerase