Explain that the sequences of bases in DNA are used as a means of classification.

1.2 Concept and Uses of Classification Systems

Objective

Explain that the sequences of bases in DNA are used as a means of classification and that classification is based on shared features of organisms.

Why Do We Classify Living Things?

• Organisms are grouped on the basis of shared characteristics (e.g., morphology, anatomy, behaviour).
• To organise the immense diversity of life.
• To provide a common language for scientists worldwide.
• To predict characteristics of organisms based on the group to which they belong.
• To show evolutionary relationships – groups are intended to reflect common ancestry.

Core Concepts Required by the Syllabus

• Species definition: a species is a group of organisms that can inter‑breed in nature and produce fertile offspring.
• Binomial naming system: every species has a two‑part Latin name – the genus name (capitalised) followed by the specific epithet (lower‑case).
  Examples: Homo sapiens (genus Homo, species sapiens); Rosa chinensis (genus Rosa, species chinensis).
• Dichotomous key: a series of paired, mutually exclusive statements that lead to the identification of an organism.
  Mini‑key – vertebrate vs. invertebrate:

  Step	Statement	Result
  1	Has a backbone (spine)?	Yes → go to 2; No → Invertebrate
  2	Has hair or fur?	Yes → Mammal; No → Other vertebrate

Activity – Construct a Dichotomous Key

Using the five organisms below, write a three‑step dichotomous key that will identify each one. (The key should start with a broad characteristic and become progressively more specific.)

1. Frog (Rana temporaria) – amphibian, smooth skin, lays eggs in water.
2. Oak tree (Quercus robur) – woody plant, broad leaves, produces acorns.
3. Housefly (Musca domestica) – insect, two wings, compound eyes.
4. Earthworm (Lumbricus terrestris) – annelid, segmented, no eyes.
5. Goldfish (Carassius auratus) – fish, scales, gills.

Answer key (teacher version)

Traditional Classification vs. Molecular Classification

Traditional systems group organisms according to observable traits such as morphology, anatomy and behaviour.

Molecular systems group organisms according to their genetic material – especially the order of nucleotides (bases) in DNA – because DNA similarity provides a direct, quantifiable measure of evolutionary relatedness.

DNA – The Molecular “Barcode”

• DNA is composed of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G).
• The specific order of these bases along a strand is called its sequence. Example of a short segment:
  $$5'‑\text{ATGCGTACGTTAGC}‑3'$$
• Because the sequence is inherited, closely related species have very similar DNA sequences, whereas more distant relatives show greater differences.

How DNA Sequences Are Used for Classification (IGCSE Level)

1. Extraction: DNA is isolated from a small sample of cells.
2. Amplification (PCR): A particular region of the genome (e.g., the mitochondrial COI gene) is copied many times.
3. Sequencing: The order of the bases in the amplified region is determined.
4. Comparison: The obtained sequence is compared with reference sequences in databases such as GenBank.
5. Phylogenetic analysis: Similarities and differences are used to draw a simple tree that shows the likely evolutionary relationships.

DNA Barcoding – A Practical Example

DNA barcoding uses a short, standardised DNA region to identify species. For most animals the cytochrome c oxidase I (COI) gene of the mitochondrial genome is used. The steps above are followed, and the resulting “barcode” is matched to a reference library. A match gives the species name (binomial) of the unknown specimen.

Taxonomic Hierarchy (Traditional) and Corresponding Molecular Groupings

Advantages of Using DNA Sequences for Classification

• Applicable to all life forms, including microorganisms and, when DNA is preserved, fossils.
• Provides objective, quantifiable data that can be stored and shared worldwide.
• Reveals hidden relationships that are not obvious from morphology alone.
• Allows rapid identification of unknown specimens (e.g., in wildlife monitoring).

Limitations and Considerations

• Requires specialised equipment and trained personnel.
• Horizontal gene transfer, especially in bacteria, can blur true evolutionary relationships.
• The choice of DNA region matters – some evolve too slowly for species‑level work, others evolve too quickly.
• Accurate identification depends on the completeness and reliability of reference databases.

Summary

The sequence of bases in DNA acts as a molecular fingerprint. By comparing these sequences, scientists can group organisms into taxa that reflect genuine evolutionary ancestry. This molecular approach complements traditional classification based on visible traits and, for many groups, provides a more precise and universally applicable system.