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
Section 1 of 5Bacterial 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.
Section 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.
Section 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.
| Feature | Bacterium | Virus |
|---|---|---|
| Cell structure | Cellular, prokaryotic | Non-cellular |
| Genetic material | Circular DNA, plasmids | DNA or RNA in a capsid |
| Ribosomes | Yes, 70S | None |
| Reproduction | Binary fission | Only inside host cells |
Section 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.
Section 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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