# Explain how advances in microscopy and DNA analysis have changed the way scientists classify living organisms, including why the three-domain system was introduced.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 3 Classification and Biodiversity · Explain · 5 marks

> Early classification systems placed all bacteria and other microscopic organisms together in a single kingdom. As technology improved, scientists were able to examine organisms in far greater detail, leading to major revisions in how living things are classified.

## Mark scheme (5 marks)

1. Early microscopes had limited resolution, so scientists could not distinguish fine structural differences between microscopic organisms
2. Electron microscopes allowed scientists to see ultrastructure/fine internal detail of cells, revealing differences between cell types (e.g. presence or absence of a nucleus/membrane-bound organelles)
3. DNA/molecular analysis allowed scientists to compare genetic sequences between organisms, revealing evolutionary relationships not visible from physical appearance
4. Evidence showed that what had been grouped as bacteria contained two fundamentally different groups (Bacteria and Archaea), which are more different from each other than either is from Eukarya
5. The three-domain system (Bacteria, Archaea, Eukarya) was introduced to reflect these true evolutionary relationships more accurately than the five-kingdom system

## Key terms

- [three-domain system](https://www.gradenine.co.uk/glossary/three-domain-system)
- [DNA analysis](https://www.gradenine.co.uk/glossary/dna-analysis)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-advances-in-microscopy-and-d4864610) · Published by Druglandscape Ltd.