IB Biology: Continuity and Change - DNA Replication

Introduction

Focus on DNA replication at the molecular level.
DNA replication is akin to making an identical copy of a painting, but with the DNA molecule.

Growth and reproduction: Essential for organism growth and reproduction.
Origin: DNA is inherited from parents, mixed from both maternal and paternal chromosomes.
Essential for cell division: Supports growth from a single cell to trillions of cells.

Semiconservative replication: Each new DNA molecule consists of one original strand and one new strand.
Template requirement: The original strand acts as a template for assembling the new complementary strand.
Base pairing rules: Adenine pairs with thymine, cytosine pairs with guanine.

Unwinding of DNA
- Helicase enzyme: Separates the two DNA strands by breaking hydrogen bonds.
Complementary Strand Synthesis
- DNA Polymerase: Adds new complementary bases to the template strand.
- Multiple DNA polymerases work simultaneously.

Polymerase Chain Reaction (PCR): Artificial replication of DNA in a lab setting.
Machine used: Thermal cycler.
Key components: Free nucleotides, primers, Taq polymerase.
PCR Steps:
1. Denaturation: Heating DNA to separate strands.
2. Annealing: Cooling to allow primers to bind to DNA.
3. Extension: Taq polymerase extends the DNA, adding nucleotides.
Exponential increase: From 1 to millions of DNA copies.
Used in virus identification (e.g., COVID-19 testing).

Purpose: Separates DNA fragments by size.
Process:
- DNA samples loaded in gel.
- Electric current moves DNA towards the positive electrode.
- Smaller fragments move faster and further than larger ones.
Use of DNA ladder: For comparison with known DNA lengths.

DNA Profiling: Identification based on DNA bands.
- Used in criminal investigations and paternity cases.
- Related individuals share similar DNA profiles.

Understanding of PCR and gel electrophoresis is crucial for the IB Biology curriculum.