Lesson 2: DNA Structure & Replication
DNA (deoxyribonucleic acid) carries the genetic blueprint for every living organism. Its double-helix structure — two strands wound around each other — allows it to store and copy information with remarkable accuracy.
Key Concepts
Double Helix
DNA looks like a twisted ladder. The sides (backbone) are alternating sugar (deoxyribose) and phosphate groups. The rungs are pairs of nitrogenous bases.
Base Pairing Rules
Adenine (A) always pairs with Thymine (T). Cytosine (C) always pairs with Guanine (G). These complementary pairs hold the two strands together via hydrogen bonds.
Nucleotides
Each unit of DNA is a nucleotide: one phosphate + one sugar + one base. Millions of nucleotides link together to form a single DNA strand.
DNA Replication
Before a cell divides, DNA unzips down the middle. Each original strand serves as a template. DNA polymerase reads each template and adds complementary nucleotides, making two identical double helices.
Why It Matters
The base sequence in DNA spells out genes — instructions for building proteins. Errors in replication (mutations) can change traits. This is the foundation of heredity and evolution.
🔬 Virtual Lab: DNA Base-Pair Builder
Click a base on the left strand. The correct complementary base snaps into place on the right strand.
✅ Check Your Understanding
1. Which base pairs with Adenine in DNA?
2. What enzyme builds new DNA strands during replication?
3. What is the shape of DNA called?