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6.1.5Unit 6: Practical Skills in Biology II

Comparison of Bacterial and Viral Structure — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 48 Unit quiz

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Part 1 of 5Bacterial Structure

6.1.5 Comparison of Bacterial and Viral Structure
Prokaryote
  • A bacterium is a single-celled prokaryote with no nucleus and no membrane-bound organelles in its cytoplasm.
Genetic material
  • It has a single circular chromosome free in the cytoplasm, and often small loops of DNA called plasmids.
Ribosomes and wall
  • Its 70S ribosomes are smaller than eukaryotic ribosomes, and its cell wall is made of murein (peptidoglycan).
Optional features
  • Many have a slime capsule, flagella for movement, and pili for attachment and gene transfer.

Part 2 of 5Viral Structure

Viral Structure
Non-cellular
  • Viruses are non-cellular particles much smaller than bacteria, with no membrane, cytoplasm or ribosomes.
Genome
  • The core is DNA or RNA, which may be single or double stranded.
Capsid
  • A protein coat made of repeating subunits called capsomeres surrounds the nucleic acid.
Envelope
  • Some viruses have an outer lipid layer from the host cell membrane, and attachment proteins enable them to bind to host cells.

Part 3 of 5Types of Virus

DNA viruses
  • The viral DNA is a template for new viral DNA and for mRNA, for example bacteriophages.
RNA viruses
  • These have RNA genomes and do not make DNA, for example tobacco mosaic virus.
Retroviruses
  • These are RNA viruses that use reverse transcriptase to make DNA from their RNA, and HIV is an example.
Host dependence
  • All viruses can reproduce only inside living host cells, using the host machinery to make new particles.
FeatureBacteriumVirus
Cell structureCellular, prokaryoticNon-cellular
Genetic materialCircular DNA, plasmidsDNA or RNA in a capsid
RibosomesYes, 70SNone
ReproductionBinary fissionOnly inside host cells

Part 4 of 5Lytic and Lysogenic Pathways

1
A virus enters a host cell, for example a bacteriophage injecting its genetic material into a bacterium.
2
In the lysogenic pathway the viral DNA joins the host DNA and a repressor protein stops it being expressed.
3
During this latent period the viral DNA is copied whenever the host DNA is copied.
4
Cell damage or poor nutrition can activate the virus, which then enters the lytic pathway.
5
In the lytic pathway new viral parts are made and assembled, and the host cell bursts open (lysis), releasing many viruses.

Part 5 of 5Exam Checklist & Tips

6.1.5 – Key Points for the Exam

  • Prokaryote A cell with no nucleus and no membrane-bound organelles.
  • Capsid The protein coat of a virus.
  • Retrovirus An RNA virus that makes DNA using reverse transcriptase.
  • Lysis Bursting of a cell.

Common Mistakes

  • Describing viruses as cells; they have no cytoplasm or ribosomes.
  • Saying that antibiotics work on viruses, which lack cell walls and ribosomes.
  • Mixing up lysogenic and lytic pathways; lysogenic is the dormant stage and lytic causes the cell to burst.
  • Forgetting that a retrovirus needs reverse transcriptase to make DNA from RNA.

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