General Properties of Viruses
What is a virus? ★★★
The smallest infectious and ACELLULAR microbe, consisting of only ONE kind of nucleic acid (DNA or RNA), which replicates in host cells.
The complete mature viral particle — the intact infectious virus particle.
Distinctive features
- Pass through 0.2 µm filters
- Acellular microbes
- Contain EITHER DNA OR RNA
- Obligate intracellular parasites
- Self-replication
Viruses were first described as “filterable agents”; they are the smallest infectious particles, 20–300 nm in diameter, observed under the electron microscope. More than 40 genera of viruses are related to human disease, and new ones emerge: MERS-CoV, highly pathogenic avian influenza (H5N1, H7N9), Ebola virus, SARS-CoV, 2019-CoV and Zika virus. Four of the world's top ten health threats in 2019, as announced by WHO, are viral diseases.
- Define a virus. → The smallest infectious, acellular microbe, containing only one kind of nucleic acid (DNA or RNA), replicating in host cells
- Define a virion. → The complete mature viral particle — the intact infectious particle
- Name the five distinctive features. → Pass 0.2 µm filters · acellular · either DNA or RNA · obligate intracellular parasites · self-replication
- What were viruses first called? → Filterable agents
Size and shape ★★
The unit of measurement is the nanometre. Viruses are too small to be seen by light microscopy — except the poxvirus — so the electron microscope is the most direct and reliable method for size, shape and structure.
| Organism / virus | Approximate size |
|---|---|
| Staphylococcus | ~1000 nm |
| Rickettsia | ~450 nm |
| Chlamydia | ~390 nm |
| Poxvirus (vaccinia) | 300 × 250 nm — the largest |
| Bacteriophage | 10–100 nm |
| Influenza virus | 100 nm |
| Adenovirus | 70 nm |
| Encephalitis B virus | 40 nm |
| Poliovirus | 30 nm |
| Shape | Example |
|---|---|
| Spherical | HBV, HIV — most viruses |
| Rod-shaped | Tobacco mosaic virus |
| Bullet-shaped | Rabies virus |
| Brick-shaped | Poxvirus |
| Tadpole-shaped | Bacteriophage |
| Filamentous | Ebola virus |
- How are viruses measured and seen? → In nanometres, by electron microscopy — they are too small for light microscopy except poxvirus
- Name six viral shapes with examples. → Spherical (HBV, HIV) · rod (tobacco mosaic) · bullet (rabies) · brick (poxvirus) · tadpole (bacteriophage) · filamentous (Ebola)
- Which is the largest virus, and which the smallest listed? → Poxvirus 300 × 250 nm; poliovirus 30 nm
⭐ Structure ★★★
| Component | Content and function |
|---|---|
| Core | The viral nucleic acid — DNA or RNA. dsDNA · ssDNA · dsRNA · ssRNA (+ssRNA and −ssRNA). Function: carries the genetic information and determines viral properties |
| Capsid | The protein shell, an accumulation of capsomers (the MORPHOLOGICAL subunit), each composed of polypeptide molecules (the CHEMICAL subunit). Functions: protects the viral nucleic acid · associates with viral attachment (adsorption) · determines the antigenicity of the virus · determines the geometric symmetry of the nucleocapsid |
| Nucleocapsid | Core + capsid complex |
| Envelope | A lipid-containing envelope surrounding the nucleocapsid, DERIVED FROM THE HOST CELL |
| Spikes (peplomers) | Participate in viral attachment and penetration · determine viral antigenicity · have specific activities, e.g. HA and NA |
| Special structures | e.g. the penton fibres of adenoviruses |
Geometric symmetry — determined by capsomer arrangement
| Symmetry | Example |
|---|---|
| Helical | Tobacco mosaic virus |
| Icosahedral | Adenovirus |
| Complex | Poxviruses, bacteriophage |
That gives five basic structural forms in nature: naked helical · enveloped helical (influenza virus) · naked icosahedral · enveloped icosahedral · complex.
| What the virion is | |
|---|---|
| Naked virus | Virion = nucleocapsid |
| Enveloped virus | Virion = nucleocapsid + envelope complex |
Derived from host cells. Four words with consequences all through Modules D and E.
An envelope is a lipid membrane, so an enveloped virus is destroyed by anything that dissolves lipid — ether, alcohol, detergent, soap, drying, heat, bile. Naked viruses are not. That single difference predicts transmission: naked viruses (polio, hepatitis A, the enteroviruses) survive the stomach and spread faecal–oral; enveloped viruses (influenza, HIV, herpes) are fragile and need respiratory droplets, blood or sexual contact.
It also determines how the virus leaves the cell — §5's release step. A naked virus must lyse the cell; an enveloped virus buds, wrapping itself in host membrane as it goes, and can leave the cell alive. Persistent and chronic infections are overwhelmingly enveloped viruses for exactly this reason.
And it is why hand-washing with soap works so well against respiratory viruses: soap dissolves the envelope directly.
- Name the basic structural components. → Core (nucleic acid) + capsid = nucleocapsid; some have an envelope with spikes
- What is a capsomer, and what is the chemical subunit? → The capsomer is the morphological subunit of the capsid; polypeptide molecules are the chemical subunit
- Name the four functions of the capsid. → Protects the nucleic acid · associates with attachment · determines antigenicity · determines geometric symmetry
- Where does the envelope come from? → The host cell
- Name the three symmetries with examples. → Helical (tobacco mosaic) · icosahedral (adenovirus) · complex (poxvirus)
- What are spikes for? → Attachment and penetration, antigenicity, and specific activities such as HA and NA
Subviruses — viroids and prions ★★★
| Detail | |
|---|---|
| Viroid | A circular, self-complementary ssRNA particle containing 250–400 nucleotides — a single circular RNA molecule WITHOUT a protein coat. Causes plant disease by interfering with plant cell metabolism |
| Prion | A proteinaceous infectious particle — an infectious agent composed of a single glycoprotein of 27–30 kDa. Human diseases: Kuru · Creutzfeldt-Jakob disease (CJD) · Gerstmann-Sträussler-Scheinker syndrome (GSS) · fatal familial insomnia (FFI) Animal diseases: scrapie · bovine spongiform encephalopathy (BSE, mad cow disease) |
Go back to the definition: the smallest infectious and acellular microbe consisting of only one kind of nucleic acid.
A prion has no nucleic acid at all — it is protein, and nothing else. It is infectious because the abnormal form induces the normal cellular protein to misfold into the same abnormal shape, which then does the same to its neighbours. Information propagates as conformation, not as sequence.
That is why prion diseases are untreatable and why prions are not destroyed by ordinary autoclaving, formalin or UV — every method in Unit 7 targets nucleic acid or ordinary protein, and there is no nucleic acid to hit.
The viroid is the mirror image: nucleic acid and no protein. Between them they mark the two edges of what infectivity can be, which is why the deck groups them as subviruses.
- Define a viroid. → A circular, self-complementary ssRNA of 250–400 nucleotides with no protein coat, causing plant disease by interfering with plant cell metabolism
- Define a prion. → A proteinaceous infectious particle — a single glycoprotein of 27–30 kDa with no nucleic acid
- Name four human and two animal prion diseases. → Kuru, CJD, GSS, fatal familial insomnia; scrapie and BSE (mad cow disease)
⭐ Viral replication — the six steps ★★★
In the host cell, a virus replicates its nucleic acid and synthesises its proteins, then assembles them into progeny viral particles that are released from the host cell. Six steps, and every later virology unit refers back to them.
| Step | Detail |
|---|---|
| 1 · Adsorption / attachment | Specific binding of a viral attachment protein (VAP) to a receptor on the host cell surface. VAP — a viral surface glycoprotein: the spike for an enveloped virus, the capsid protein for a naked virus. Viral receptor — a glycoprotein, carbohydrate or glycolipid on the host cell: CD4 (HIV) · CD46 (measles virus) · sialic acid (influenza virus). A virus may have more than one receptor. This specific recognition determines the TISSUE TROPISM of the virus — the susceptible host cell is the one containing the viral receptor |
| 2 · Penetration | The process of viral internalisation into the host cell. Three mechanisms: (a) Endocytosis — some enveloped viruses, most naked viruses (b) Direct fusion of the cell membrane with the viral envelope — some enveloped viruses (c) Viral translocation across the cell membrane — some bacteriophages and naked viruses |
| 3 · Uncoating | The process of removing the capsid and releasing the viral nucleic acid into the cytoplasm. Proteases are needed |
| 4 · Biosynthesis | Viral genome replication and viral protein synthesis. The eclipse phase is the early stage of biosynthesis. The process depends on the viral genome: dsDNA · ssDNA · dsRNA · +ssRNA · −ssRNA · +ssRNA with a DNA intermediate (HIV) · dsDNA with an RNA intermediate (HBV) |
| 5 · Assembly | Viral genome and proteins are packaged into a nucleocapsid. Site: DNA viruses (except poxvirus) in the cell NUCLEUS; RNA viruses and poxvirus in the CYTOPLASM. Manner: (a) assemble as an empty protein shell (procapsid), then the genome fills in; (b) capsomers array around the genome to form a helical nucleocapsid For a naked virus the nucleocapsid is the complete virion; for an enveloped virus it is still incomplete |
| 6 · Release | Naked viruses: by CELL LYSIS. Enveloped viruses: by BUDDING — during which they acquire their envelope. Defective measles virus: released from cell to cell via cell bridges → SSPE (subacute sclerosing panencephalitis) |
Biosynthesis in a dsDNA virus — the worked example (HSV)
Viral dsDNA (template) → transcribed by the host cell's DNA-dependent RNA polymerase → early mRNA → translated to early proteins (non-structural: enzymes, inhibitors, regulatory factors, including viral DNA-dependent DNA polymerase) → semi-conservative replication → progeny viral DNA → transcribed to late mRNA → translated to late proteins (viral structural proteins) → assembly → progeny nucleocapsid.
The susceptible host cell is the one containing the viral receptor. That sentence explains why each virus causes the disease it does.
HIV binds CD4 — so it infects helper T cells and macrophages, and the disease is immunodeficiency. It could not cause hepatitis if it tried.
Influenza binds sialic acid, which coats respiratory epithelium — so it is a respiratory disease. And because the sialic-acid linkage differs between birds and humans, receptor specificity is precisely what makes an avian influenza virus like H5N1 hard to transmit between people, and what would make it dangerous if it changed.
Rabies binds the acetylcholine receptor at the neuromuscular junction — so it is a neurological disease.
Two therapeutic consequences follow. Blocking attachment blocks infection entirely, which is what neutralising antibody does and what entry-inhibitor drugs try to do. And the NA (neuraminidase) spike of influenza, which cleaves sialic acid so progeny virus can detach, is the target of oseltamivir — attacking step 6 instead of step 1.
- Name the six steps of replication. → Adsorption/attachment · penetration · uncoating · biosynthesis · assembly · release
- What is a VAP, and what are the receptors? → Viral attachment protein — the spike of an enveloped virus or the capsid protein of a naked one; receptors are host glycoproteins, carbohydrates or glycolipids such as CD4 (HIV), CD46 (measles), sialic acid (influenza)
- What determines tissue tropism? → The specific binding of VAP to receptor — the susceptible cell is the one bearing the receptor
- Name the three mechanisms of penetration. → Endocytosis · direct fusion of cell membrane with viral envelope · translocation across the membrane
- What is uncoating, and what does it need? → Removal of the capsid and release of the nucleic acid into the cytoplasm; proteases
- Where does assembly occur? → Nucleus for DNA viruses (except poxvirus); cytoplasm for RNA viruses and poxvirus
- How are naked and enveloped viruses released? → Naked by cell lysis; enveloped by budding, which is when they acquire the envelope
⭐ Bacterium versus virus ★★★
“Please compare bacterium and virus (as a table).” — 7 marks, 2020 Section III. This is slide 49 of this deck, and the paper's instruction to answer as a table is the deck's own layout. Reproduce it.
| Virus | Bacterium | |
|---|---|---|
| Size | 0.02–0.3 µm | 0.5–3.0 µm |
| Structure | Non-cellular microorganism | Prokaryotic microorganism |
| Nucleic acid | DNA OR RNA | DNA AND RNA |
| Growth on cell-free medium | Cannot grow | Can grow |
| Mode of multiplication | Replication | Binary fission |
| Ribosome | None | Has |
| Antibiotic | Resistant | Sensitive |
| Interferon | Sensitive | Resistant |
Size · structure · nucleic acid · growth on cell-free medium · mode of multiplication · ribosome · antibiotic sensitivity · interferon sensitivity.
The last two rows are the ones that show understanding, and the deck's speaker notes explain them:
Why antibiotics work on bacteria but not viruses. Antibiotics exploit the metabolic, structural and molecular differences between bacteria and animal cells — penicillin attacks peptidoglycan, which is unique to bacteria; streptomycin attacks the bacterial 30S/50S ribosome, which differs from the eukaryotic 40S/60S. A virus uses the host cell's own machinery to make its components, so there is no target that is not also the patient's.
Why interferon works on viruses. Interferon is a glycoprotein with antiviral effect, produced by the infected host cell — see Unit 19 §3.
- Give eight differences between a virus and a bacterium. → Size (0.02–0.3 vs 0.5–3.0 µm) · non-cellular vs prokaryotic · DNA or RNA vs DNA and RNA · cannot vs can grow on cell-free medium · replication vs binary fission · no ribosome vs ribosome · antibiotic-resistant vs sensitive · interferon-sensitive vs resistant
- Why do antibiotics not work on viruses? → Viruses use the host cell's own machinery, so there is no unique target to attack without harming the host
- What is interferon? → A glycoprotein with antiviral effect produced by infected host cells
Revision
The whole unit on one screen
| Question | Answer |
|---|---|
| Virus? | The smallest infectious acellular microbe with one kind of nucleic acid, replicating in host cells |
| Virion? | The complete mature (intact infectious) viral particle |
| Size range? | 20–300 nm |
| Structure? | Core + capsid = nucleocapsid; ± envelope with spikes |
| Capsid subunits? | Capsomer (morphological) of polypeptides (chemical) |
| Symmetries? | Helical · icosahedral · complex |
| Envelope origin? | The host cell membrane |
| Six steps? | Adsorption · penetration · uncoating · biosynthesis · assembly · release |
| Tropism decided by? | VAP–receptor binding at adsorption |
| Assembly site? | Nucleus — DNA viruses except poxvirus; cytoplasm — RNA viruses and poxvirus |
| Release? | Naked → lysis · enveloped → budding |
| Viroid / prion? | Circular ssRNA without protein / infectious protein without nucleic acid |
- Answer the 2020 comparison. → Size · non-cellular vs prokaryotic · one vs both nucleic acids · cannot vs can grow on cell-free medium · replication vs binary fission · no ribosome · antibiotic-resistant vs sensitive · interferon-sensitive vs resistant
- Define virus and virion. → The smallest infectious acellular microbe with only one kind of nucleic acid, replicating in host cells; the complete mature infectious particle
- Describe viral replication. → Adsorption (VAP to receptor, determining tropism) → penetration (endocytosis, fusion or translocation) → uncoating (proteases) → biosynthesis (genome replication and protein synthesis, after the eclipse phase) → assembly (nucleus or cytoplasm) → release (lysis or budding)
- Why does an enveloped virus behave differently from a naked one? → The lipid envelope is fragile — destroyed by ether, alcohol, detergent and drying — so transmission is by droplets, blood or sex rather than faecal–oral; and it buds rather than lysing the cell
- Define a prion and a viroid. → Prion: a proteinaceous infectious particle, a 27–30 kDa glycoprotein with no nucleic acid. Viroid: a circular self-complementary ssRNA of 250–400 nucleotides with no protein coat