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.
A1.1 Water
Water — polarity and emergent properties
Water is polar: an uneven charge distribution lets molecules form hydrogen bonds. From this emerge:
Cohesion & adhesion — continuous water columns in xylem; surface tension.
Solvent — dissolves ions and polar solutes (the medium of metabolism and transport).
Thermal buffering — high specific heat and heat of vaporisation; evaporative cooling.
Buoyancy and habitat — supports aquatic life; ice floats.
Origin of life: water's solvent properties made it the likely medium for the first biochemistry.
Quick check
Question
?Water's high specific heat capacity is useful to organisms because it...
A1.2 Nucleic acids
Nucleotides and the double helix
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).
A1.2 Nucleic acids
Directionality and stability
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.
%A = %T · %G = %CChargaff's rule in double-stranded DNA
Calculate
Your turn — calculate
1A double-stranded DNA sample contains 32% thymine. Calculate the percentage that is guanine.
%
Hint: %T = %A = 32, so G + C = 100 − 64 = 36%, shared equally.
Calculate
Your turn — calculate
2A DNA fragment is 1200 base pairs long. Calculate the total number of nucleotides in the double-stranded molecule.
nucleotides
Hint: each base pair is 2 nucleotides, so 1200 × 2.
Sort it
Sort each item into its group
Sort each molecule by its type.
Purine
Pyrimidine
Not a DNA base
Quick check
Question
?New nucleotides are added to a growing DNA strand only at the...
Quick check
Question
?A DNA region rich in G-C base pairs is more thermally stable because G-C pairs have...
Match it
Match each item to its partner
Match each structural feature to what it provides.
Item
Partner
Quick check
Question
?In DNA, adenine always pairs with...
Recap
The big ideas to take away
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.
🏆
Mini-lesson complete!
⭐⭐⭐
You have worked through Theme A: Unity & Diversity — Molecules for IB Diploma Biology. 🎉
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