All life shares the same molecular foundations. This HL lesson covers A1.1 Water and A1.2 Nucleic acids in depth, including strand directionality, antiparallel structure and base-pair stability.
You will study water's polarity and properties, and the detailed structure of DNA and RNA that makes accurate inheritance possible.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.
Water is polar: an uneven charge distribution lets molecules form hydrogen bonds. From this emerge:
Origin of life: water's solvent properties made it the likely medium for the first biochemistry.
Nucleotides = pentose sugar + phosphate + base. In DNA the sugar is deoxyribose; bases are purines (adenine, guanine — double ring) and pyrimidines (cytosine, thymine — single ring).
Two strands wind into a double helix. A purine always pairs with a pyrimidine, so the helix stays a constant width: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds).
Each strand has a direction set by its sugar-phosphate backbone: a 5-prime end and a 3-prime end. The two strands are antiparallel — one runs 5→3 while its partner runs 3→5.
Because G-C pairs share three hydrogen bonds and A-T only two, DNA regions rich in G-C are more thermally stable.
Sort each molecule by its type.
Match each structural feature to what it provides.
A1.1: polarity and hydrogen bonding give cohesion, solvent and thermal properties
A1.2: nucleotides; purine-pyrimidine pairing keeps a constant helix width
HL directionality: antiparallel strands; synthesis only at the 3-prime end
HL stability: G-C (3 H-bonds) more stable than A-T (2); Chargaff ratios
Press Finish to see your score.
You have worked through Theme A: Unity & Diversity — Molecules for IB Diploma Biology. 🎉
Your stars: 0 / 0
Next: test yourself in the Verify stage.