DNA Replication Requires Unwinding and Synthesis
- DNA replication copies the entire genome so each daughter cell receives identical genetic information.
- Two key enzymes drive this process:
- Helicase: Unwinds the DNA double helix.
- DNA polymerase: Synthesizes new DNA strands.
Helicase doesn’t break the covalent bonds in the DNA backbone, it only disrupts the weaker hydrogen bonds between the bases.
The Role of Helicase: Unwinding the Double Helix
Definition
Helicase
An enzyme that separates the two strands of the DNA double helix.
How Helicase Works
- Step 1: Breaking Hydrogen Bonds
- Helicase moves along the DNA molecule.
- It breaks the hydrogen bonds between complementary base pairs (A-T and C-G).
- This separates the two strands.
- Step 2: Forming the Replication Fork
- As helicase unwinds the DNA, it creates a Y-shaped structure called the replication fork.
- The two separated strands serve as templates for new DNA synthesis.
Key Features
- Ring-shaped enzyme: Helicase encircles one strand of DNA and moves along it.
- Only breaks hydrogen bonds: It does NOT break the covalent bonds in the sugar-phosphate backbone, only the weaker hydrogen bonds between bases.
- Directional movement: Helicase moves in one direction (typically 3' to 5' on the template strand), continuously unwinding DNA ahead of the replication machinery.
- Helicase is like unzipping a jacket.
- The zipper (helicase) separates the two sides (DNA strands) by breaking the connections (hydrogen bonds) between the teeth (base pairs).
The Role of DNA Polymerase: Synthesizing New DNA Strands
Definition
DNA Polymerase
An enzyme that synthesizes new DNA strands by adding nucleotides to a growing chain, using the original strand as a template.
How DNA Polymerase Works
- Step 1: Reading the Template
