Classification Of Virus

Question 1

What are the Basic Properties of Viruses

Answer 1

• Obligate intracellular parasites
  Cannot replicate independently—must hijack host machinery.
• Structure:
  Genetic material (DNA or RNA) + protein coat (capsid)
  Some have an envelope (lipid membrane from host).
• Host range:
  Infect animals, plants, bacteria (bacteriophages), and archaea.
• No metabolic activity:
  Cannot produce ATP or proteins on their own.

Question 2

What are the Effect of Physical & Chemical Agents such as
Disinfectant with e.g
Temperature
pH sensitivity
Lipid Solvent
And radiation

Answer 2

1. Disinfectants

• Viruses more resistant than bacteria.
• Effective agents:
  • Hydrogen peroxide
  • Potassium permanganate
  • Hypochlorite
  • Organic iodine compounds
  • Formaldehyde, β-propiolactone (used in vaccines)
• Phenol: less effective
• Chlorination kills most viruses except HAV, poliovirus

2. Temperature

• Most are heat-labile:
  • Inactivated at 56°C (seconds), 37°C (minutes)
• Stable at low temperatures (can be frozen)
• Exceptions:
  • HBV resists 60°C for 1 hr
  • Scrapie virus resists autoclaving (121°C for 15 mins)
  • Poliovirus doesn’t tolerate freeze-drying

3. pH Sensitivity

• Most stable at pH 5–9
• Rhinovirus: sensitive to acid
• Enteroviruses: acid-resistant

4. Lipid Solvents

• Ether, chloroform, detergents:
  • Destroy enveloped viruses
  • No effect on naked viruses

5. Radiation

• Inactivated by UV, ionizing radiation, sunlight

Question 3

How does Viral Replication occur?

Answer 3

• No binary fission or mitosis
• Relies entirely on host cell machinery
• Produces hundreds of progeny
• Not truly alive: lacks metabolism, doesn’t grow/divide independently

Question 4

Ubiquity of Viruses

• Present everywhere — in food, air, water
• Humans constantly inhale/ingest billions of viral particles
• 8.3% of human genome = LTR retrotransposons, likely remnants of ancient viruses

Question 5

List the Viral Composition

Answer 5

• Nucleic acid (DNA or RNA) — never both
• Surrounded by capsid made of capsomeres
• Together form the nucleocapsid
• Some viruses have an outer lipid envelope

Question 6

What are the functions of Capsid

Answer 6

Capsid Functions

1. Protects viral genome from enzymes
2. Enables attachment to host cells
3. Aids in entry and nucleic acid injection

Question 7

What’s the structure of a viral envelope?

Answer 7

Viral Envelope

• Found in enveloped viruses
• Lipid bilayer from host + viral proteins
• Spikes (glycoproteins) assist in host cell binding
• Formed during budding from host cell membrane

Question 8

What are the viral genome types?

Answer 8

Viral Genome Types

• Type: DNA or RNA
• Strand: Single or double
• Sense: Positive or negative (in RNA viruses)
• Shape: Linear or circular
• Segmentation: Segmented (e.g. influenza) or non-segmented

Question 9

Size and Shape

• Much smaller than bacteria (20–450 nm)
• what are the 3 types of capsid symmetry:?

Answer 9

• Icosahedral (e.g. adenovirus)
  
  • Helical (e.g. rabies)
  • Complex (e.g. poxvirus, bacteriophage)

Question 10

What are the ** Functions:** of capsid?

Answer 10

1. Protection – Shields viral nucleic acid from enzymes that might degrade it.
2. Attachment – Has surface sites that help the virus stick to host cells.
3. Penetration – Provides proteins to enter the host cell and inject genetic material.

Question 11

Viral Nucleic Acid Core is made up of?

Answer 11

• Type: Either DNA or RNA (never both)
• Strand: Can be single-stranded (ss) or double-stranded (ds)
• Polarity: Positive-sense (like mRNA) or Negative-sense
• Shape: Circular or Linear
• Segmentation: May be segmented (split into parts) or non-segmented (one piece)

Question 12

What are the composition of a viral envelope? And it’s function

Answer 12

Viral Envelope

• Viruses are either enveloped or non-enveloped
• Envelope Composition:
  • Two lipid layers + proteins (lipoprotein bilayer)
  • Derived from host cell membrane during budding
• Function:
  • Virus inserts its own proteins into the host membrane to create a hybrid envelope
  • Spikes (glycoproteins) help the virus recognize and bind to specific host cells

Question 13

Here’s a structured and simplified summary of Viral Classification using mnemonics and essential points for fast revision:

Primary Basis for Viral Classification

1. Type of nucleic acid: DNA or RNA
2. Capsid symmetry: Icosahedral, helical, or complex
3. Envelope: Present or absent
4. Virion size and capsid dimensions

Extra criteria:

• Disease type
• Host range and tissue tropism
• Transmission mode

Baltimore Classification of Viruses

Based on genome type and replication strategy
(Developed by David Baltimore — 7 groups)

Group 1: dsDNA viruses

• Examples: Pox, Herpes, Papilloma, Polyoma, Adeno
• Mnemonic: Please House Her Pretty And Properly

Group 2: ssDNA viruses

• Example: Parvo (only ssDNA virus)
• Mnemonic: PARVO = Positive Alone (ss)

Group 3: dsRNA viruses

• Example: REO
• Mnemonic: Retired

Group 4: (+) ssRNA viruses

• Examples: Astro, Calici, Picorna, Flavi, Corona, Toga
• Mnemonics:
  • ACP FCT
  • All are linear +RNA
  • Capsid types:
    • Corona = helical
    • Flavi = spherical
    • Others = icosahedral
  • Envelope:
    • ACP = naked
    • FCT = enveloped

Group 5: (–) ssRNA viruses

• Examples: Paramyxo, Rhabdo, Orthomyxo, Filo, Bunya, Arena
• Mnemonic: PROF BA
  • All enveloped, helical/filamentous capsids
  • Orthomyxo, Bunya, Arena are segmented
  • BA = ambisense

Group 6: ssRNA (RT)

• Example: Retroviruses (e.g., HIV)
• Mnemonic: Rich
• Enveloped, spherical, linear genome

Group 7: dsDNA (RT)

• Example: Hepadnaviridae (e.g., Hepatitis B)
• Mnemonic: House = Hepadna
• Circular, partial dsDNA, uses reverse transcription

Dr Bello’s Helpful Mnemonics

DNA Viruses

• “Please House Her”
  • Pox (Complex, enveloped, ovoid)
  • Hepadna (Enveloped, circular genome)
  • Herpes (Enveloped, icosahedral)
• “PAP” = Papova, Adeno, Parvo
  • Naked
  • Parvo = ssDNA
  • Papilloma & Polyoma = circular genome
  • Others = linear

RNA Viruses (Dr Banimoh Mnemonics)

• ACP FCT = +ssRNA
  • Astro, Calici, Picorna (naked)
  • Flavi, Corona, Toga (enveloped)
• PROF BA = –ssRNA
  • Paramyxo, Rhabdo, Orthomyxo, Filo
  • Bunya, Arena (ambisense, segmented)
• Retired = REO = dsRNA, naked
• Rich = RETRO = +ssRNA, uses RT, enveloped

Virus Taxonomy (ICTV)

• Official classification body: ICTV
• Groups viruses into taxa based on:
  • Genome
  • Structure
  • Replication
  • Host range
• Baltimore classification is functional (replication focused)
• ICTV taxonomy is formal (scientific naming and grouping)