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1.2.51 · Cell Structure

Viruses — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 13 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

1.2.5 Viruses — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of viruses, explain why they are not cells and outline how they replicate.Non-cellular particles — viruses are not cells and not considered living organisms, so they are called part…

  • No cell machinery
  • Reproduction
  • Nucleic acid core
  • Attachment proteins
  • Retroviruses
  • Specific attachment

Part 1 of 6What viruses are

What viruses are
Non-cellular particles
  • Non-cellular particles viruses are not cells and not considered living organisms, so they are called particles.
Size
  • 20–300 nm, much smaller than bacteria, so an electron microscope is needed.
No cell machinery
  • No cell machinery no cytoplasm, surface membrane, ribosomes or metabolism.
Reproduction
  • Reproduction only inside living host cells, using the host's protein-making machinery.

Part 2 of 6Structure of a virus particle

Structure of a virus particle
Nucleic acid core
  • DNA or RNA (never both), single or double stranded.
  • Influenza has RNA, HIV has two copies of single-stranded RNA and bacteriophage T4 has double-stranded DNA.
Capsid
  • Capsid a protein coat made of subunits called capsomeres that protects the nucleic acid.
Attachment proteins
  • Attachment proteins surface proteins that bind to complementary receptors on host cells.
Envelope
  • Envelope present in some viruses, such as HIV and influenza.
  • It is a phospholipid layer taken from the host cell membrane, with viral glycoproteins.

Part 3 of 6Replication in a host cell

1
The attachment protein binds to a specific receptor on the host cell.
2
The virus or its nucleic acid enters by endocytosis, membrane fusion or injection.
3
The capsid is removed and the nucleic acid is released.
4
Host ribosomes, enzymes, amino acids and nucleotides replicate the viral genome and make viral proteins.
5
New virus particles are assembled.
6
They leave by lysis or by budding in a piece of host membrane.
  • Retroviruses
    • HIV carries reverse transcriptase, which makes DNA from its RNA so the DNA can integrate into the host chromosome.

Part 4 of 6Host range

Specific attachment
  • Specific attachment the attachment protein must fit the host receptor, so viruses infect only certain cells.
HIV
  • HIV gp120 binds to CD4 receptors on helper T cells.
Influenza
  • Influenza haemagglutinin attaches to host cells and neuraminidase helps new particles escape.

Part 5 of 6Comparing viruses and bacteria

FeatureVirusBacterium
Size20–300 nm0.5–5 µm
StructureNucleic acid in protein capsidCell wall, membrane, cytoplasm
RibosomesNone70S
ReproductionOnly inside a host cellBinary fission
TreatmentAntivirals, vaccinesAntibiotics
  • Antibiotics
    • they do not work on viruses because viruses lack bacterial walls and 70S ribosomes.

Part 6 of 6More from the specification

More from the specification
Non-cellular nature of viral particles
  • Non-cellular nature of viral particles unlike prokaryotic or eukaryotic plant and animal cells, viruses lack cytoplasm, energy metabolism,
    • and membrane-bound organelles.
Host cell organelle hijacking
  • Host cell organelle hijacking viruses inject genetic material into host cells, exploiting host mitochondria for energy and ribosomes to synthesise viral proteins.
Viral exit through membrane vesicles
  • Viral exit through membrane vesicles newly assembled virions bud through the host plasma membrane or exit via secretory vesicles during cellular lysis.
Acellular nature of viral particles
  • Viruses lack cellular structures, consisting solely of a nucleic acid genome (DNA or RNA) enclosed within a protein capsid coat.
Comparison with eukaryotic organelles
  • Unlike autonomous cellular organelles like mitochondria, viruses possess no ribosomes or metabolic enzymes, requiring host cells for replication.
Obligate intracellular parasitism
  • Viral replication involves attachment to specific host cell receptors, penetration, viral gene transcription, assembly,
  • and subsequent host cell lysis.
Micrograph
  • A photograph taken through a microscope.
Centriole
  • A microtubule structure organising the spindle in animal cells.
Animal cell
  • A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.
Endoplasmic reticulum
  • Rough and smooth (lipids, detoxification).
    • (ribosomes, proteins)
Electron micrograph
  • A photograph taken through an electron microscope.
Spindle fibres
  • Protein microtubules that move chromosomes in mitosis.
Permanent vacuole
  • Sap-filled space that keeps a plant cell firm.
Mitochondria
  • Organelles where most aerobic respiration takes place.
Organelle
  • A small structure within a cell with a specific function.
Chloroplast
  • The organelle of photosynthesis.
Vesicle
  • A small membrane-bound sac that moves materials within a cell or to the plasma membrane.
Microvilli
  • Tiny folds on the surface of villus cells.
Respiration
  • Chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
Microtubule
  • Hollow protein filament used for support and transport.
Vacuole
  • Membrane-bound store; the plant central vacuole maintains turgor.

Key terms

TermMeaning
VirusA non-cellular particle of nucleic acid in a protein coat that replicates only in host cells.
CapsidThe protein coat around viral nucleic acid.
EnvelopeA phospholipid layer from the host membrane around some viruses.
Attachment proteinA viral surface protein that binds to a host receptor.
BacteriophageA virus that infects bacteria.
Reverse transcriptaseAn enzyme that makes DNA from an RNA template.
RetrovirusAn RNA virus such as HIV that uses reverse transcriptase.

Avoid these mistakes

1
Describing viruses as cells with a cytoplasm — they have none.
2
Saying antibiotics treat viral infections — they target structures viruses lack.
3
Saying a virus contains both DNA and RNA — it has one type only.
4
Calling the capsid a membrane — it is protein; only some viruses have an envelope.
5
Saying viruses divide — host cells replicate them.

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