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18.1.217-18 · Selection, Evolution, Classification and Conservation

The Three Domains- Archaea, Bacteria and Eukarya — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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18.1.2 The Three Domains: Archaea, Bacteria and Eukarya — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the three domains and the molecular differences between Archaea and Bacteria.Taxonomy — the practice of biological classification into categories (taxa).…

  • Taxonomic ranks
  • Extremophile
  • Peptidoglycan
  • Binary fission
  • Protoctista
  • Grouping into taxa makes evolutionary relationships easier to see

Section 1 of 10Taxonomy and domains

Taxonomy
  • Taxonomy the practice of biological classification into categories (taxa).
Grouping into taxa makes evolutionary relationships easier to see.
The domain is the highest rank in the hierarchy.
Cell type matters: prokaryotes lack a nucleus, while eukaryotes have their genetic material in a nucleus and other compartments.
Molecular analysis of RNA genes showed prokaryotes divide into two groups, so there are three domains.
  • Archaea (prokaryotes), Bacteria (prokaryotes) and Eukarya (eukaryotes).

Section 2 of 10Archaea

Archaea
  • Archaea single-celled organisms with no nucleus.
First discovered in extreme environments (extremophiles), but not all live there.
  • ExampleHalobacterium salinarum lives in high-salt environments such as the Dead Sea.
Initially classified as bacteria until unique properties were found:
  • Unique lipids in cell membranes.
  • No peptidoglycan in cell walls.
  • Small ribosomal subunit more similar to the eukaryotic ribosome.
  • DNA transcription more similar to eukaryotes.
Their size range is similar to bacteria and their metabolism is similar.

Section 3 of 10Bacteria and Eukarya

Bacteria
  • Bacteria single-celled prokaryotes with no nucleus.
  • They vary widely in sizethe smallest are bigger than the largest viruses.
  • They divide by binary fission, e.g. Streptococcus pneumoniae causes pneumonia.
Eukarya
  • Eukarya cells with a nucleus and membrane-bound organelles.
  • They range from single cells several micrometres across to blue whales.
  • Cells divide by mitosis and organisms reproduce sexually or asexually.
  • Examplethe wolf, Canis lupus.

Section 4 of 10Differences between Archaea and Bacteria

Differences between Archaea and Bacteria
Membrane lipids
  • Archaea have branched hydrocarbon chains bonded to glycerol by ether linkages, unique among organisms.
  • Bacteria have unbranched chains bonded to glycerol by ester linkages.
Ribosomal RNA
  • Both have 70S ribosomes.
  • The archaeal small subunit, rRNA sequences and ribosomal proteins are more similar to eukaryotes than to bacteria.
Cell walls
  • Bacterial walls always contain peptidoglycan.
  • Archaeal walls are always present but lack peptidoglycan.

Section 5 of 10Comparing the three domains

FeatureArchaeaBacteriaEukarya
Cell typeProkaryoticProkaryoticEukaryotic
ChromosomeCircularCircularLinear plus circular mtDNA and cpDNA
Membrane lipidsGlycerol-etherGlycerol-esterGlycerol-ester
Ribosomes70S, small subunit like eukaryotes70S80S in cytosol, 70S in mitochondria and chloroplasts
Cell wallAlways, no peptidoglycanAlways, with peptidoglycanSometimes, no peptidoglycan
HistonesSometimesNoYes
IntronsSometimesRarelyYes

Section 6 of 10Applying the domain system

1
Ask whether there is a nucleus; if yes the organism is in Eukarya.
2
If not, it is a prokaryote; check the wall and lipids.
3
Peptidoglycan walls and ester lipids mean Bacteria.
4
No peptidoglycan with ether-linked branched lipids and histones means Archaea.

Section 7 of 10Importance of the domains

1
Penicillin blocks cross-linking of peptidoglycan, so archaea are not affected by it.
2
Heat-stable enzymes from thermophiles, e.g. Thermus aquaticus, are used in PCR.
3
Methanogens in ruminant guts produce methane, a greenhouse gas.
4
Mitochondria and chloroplasts have 70S ribosomes and circular DNA, supporting the origin of eukaryotic organelles from bacteria.

Section 8 of 10More from the specification

Protoctista
  • Kingdom of eukaryotes that are not fungi, plants or animals.
Taxonomic ranks
  • Taxonomic ranks below domain come kingdom, phylum, class, order, family, genus and species;
    • higher ranks hold more organisms with less in common.

Section 9 of 10Key terms

TermMeaning
TaxonomyPractice of biological classification.
DomainHighest rank in the taxonomic hierarchy.
ArchaeaProkaryotes with ether-linked lipids and no peptidoglycan in the wall.
ExtremophileOrganism found in extreme environments such as high salt or heat.
PeptidoglycanCell wall polymer found in bacteria.
Binary fissionDivision of a bacterial cell into two.

Section 10 of 10Exam technique

1
Saying all archaea live in extreme environments — many do not.
2
Saying archaea have peptidoglycan walls — only bacteria do.
3
Using cell type alone for domains — archaea and bacteria are both prokaryotic but separate domains.
4
Saying archaea are closest to bacteria in ribosome structure — their ribosomal RNA and proteins resemble eukaryotes.

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