Edexcel iGCSE Biology-4.16B DNA Structure- Study Notes- New Syllabus
Edexcel iGCSE Biology-4.16B DNA Structure- Study Notes- New syllabus
Edexcel iGCSE Biology-4.16B DNA Structure- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
4.16B describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G)
DNA Molecule – Structure and Base Pairing
🌱 Introduction
DNA (Deoxyribonucleic Acid) is the molecule that carries the genetic instructions used in growth, development, functioning, and reproduction.
Present in the nucleus of eukaryotic cells, stored in chromosomes.
It provides the code for making proteins, which control most functions in living organisms.
🧬 Structure of DNA
- Double Helix: DNA is shaped like a double helix → two strands coiled around each other like a twisted ladder.
Structure first described by Watson and Crick (1953). - Strands: DNA has two anti-parallel polynucleotide strands. Each strand is a chain of nucleotides.
- Nucleotide (Building Block of DNA):
- A sugar (deoxyribose)
- A phosphate group
- A nitrogenous base (A, T, C, G)
The sugar + phosphate form the backbone, while the bases stick inwards like rungs of a ladder.
🧩 Base Pairing
- The two strands are held together by complementary base pairing:
- Adenine (A) pairs with Thymine (T) → 2 hydrogen bonds
- Cytosine (C) pairs with Guanine (G) → 3 hydrogen bonds
- This is called Chargaff’s Rule of base pairing.
- Hydrogen bonds make DNA stable yet allow strands to unzip during replication and protein synthesis.
📊 Key Features Table
| Feature | Description |
|---|---|
| Shape | Double helix (twisted ladder) |
| Strands | 2 anti-parallel polynucleotide chains |
| Backbone | Sugar (deoxyribose) + phosphate |
| Bases | A, T, C, G |
| Base Pairing Rule | A ↔ T (2 bonds), C ↔ G (3 bonds) |
| Bond Holding Bases | Hydrogen bonds |
| Function | Carries genetic code for protein synthesis |
🌞 Importance of Structure
- Stability: Double helix with strong backbone protects genetic code.
- Accuracy: Complementary base pairing ensures correct copying during replication.
- Information Storage: Sequence of bases codes for amino acids → proteins.
📌 Quick Recap
– DNA = double helix with sugar-phosphate backbone.
– Built from nucleotides (sugar + phosphate + base).
– A-T (2 H bonds) and C-G (3 H bonds).
– Encodes genes → proteins.
– Stable, but can unzip for replication and protein synthesis.
✨ Mnemonic: “Apple with Tree, Car with Garage” (A–T, C–G)