Life's shared code, endless forms

Unity and Diversity: The Double Helix of Life

Why do humans share 60 % of their DNA with bananas? How does a single genetic code generate millions of species? This unit unpacks the molecular unity and evolutionary diversity that define biology.

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  • IB BIOLOGY TOPIC A

This unit

Biology
Topics
10
Unit of Biology
1 / 4

Overview

A — Unity and Diversity

Topic A anchors the entire IB Biology course by establishing the chemical and evolutionary foundations of life. Students investigate water as the medium of life, the structure and function of nucleic acids and proteins, the origin of cells, and the evidence for common ancestry that unites all living things.

Assessment draws on Paper 1 (multiple-choice and data-based questions) and Paper 2 (short-answer and extended-response questions). Practical work on enzyme catalysis, membrane properties, and phylogenetic analysis feeds directly into the Internal Assessment and the experimental skills tested in Paper 1A.

Key ideas

What A — Unity and Diversity covers

  1. 01

    Water — The Universal Solvent

    Hydrogen bonding gives water its cohesion, thermal stability, and solvent power — properties that make metabolism possible.

  2. 02

    Nucleic Acids — Information Storage

    DNA and RNA structure explains replication, transcription, and the universal genetic code shared by all life.

  3. 03

    Proteins — Molecular Workhorses

    Amino-acid sequences fold into precise 3-D shapes, enabling catalysis, transport, signaling, and structural roles.

  4. 04

    Origin of Cells

    Endosymbiosis and membrane evolution explain the leap from prokaryotes to complex eukaryotes.

  5. 05

    Evidence for Evolution

    Homologous structures, molecular clocks, and biogeography reveal common ancestry across domains.

  6. 06

    Classification & Phylogeny

    Modern taxonomy uses DNA sequences to build evolutionary trees that reflect true relatedness.

Outcomes

What you will learn

  • Explain water's biological significanceLink hydrogen bonding to cohesion, adhesion, high specific heat, and solvent properties in living systems.
  • Model DNA and RNA structureDraw nucleotides, describe base-pairing rules, and outline how directionality enables replication and transcription.
  • Predict protein structure–function relationshipsConnect primary, secondary, tertiary, and quaternary structure to enzyme active sites and allosteric regulation.
  • Evaluate evidence for endosymbiosisCompare mitochondrial and chloroplast genomes to free-living bacteria to support the origin of eukaryotes.
  • Construct phylogenetic treesUse molecular sequence alignments to infer evolutionary relationships and estimate divergence times.
  • Apply classification principlesDistinguish domains, kingdoms, and clades; justify reclassification based on new molecular data.

Topics

Topics in this unit

10 topics, each with notes to preview.

  1. 01A1.1 Water
  2. 02A1.2 Non-Coding RNA and Chromatin OrganisationHigher Level
  3. 03A1.2 Nucleic Acids
  4. 04A2.1 Origins of CellsHigher Level
  5. 05A2.2 Cell Structure
  6. 06A2.3 VirusesHigher Level
  7. 07A3.1 Diversity of Organisms
  8. 08A3.2 Classification and CladisticsHigher Level
  9. 09A4.1 Evolution and Speciation
  10. 10A4.2 Conservation of Biodiversity

Ready to master A — Unity and Diversity?

Why do humans share 60 % of their DNA with bananas? How does a single genetic code generate millions of species? This unit unpacks the molecular unity and evolutionary diversity that define biology.

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