Introduction to medical virology

B.Mishra26.09.012

What is viruses(virus: poisonous or slimy fluid)

• Strictly intra cellular

• Potentially pathogenic

• Possesing only one type of nucleic acid

• Multiplying in the form of their genetic material

• Unable to grow in ordinary media

Lwoff (1957): Defines:-

“Viruses should be considered as viruses

because viruses are viruses”

 Growth on artificial

media binary

fission Both DNA and

RNA sensitivity to antibiotics

>300 nm diameter

Bacteria   Yes Yes Yes YesYes

Mycoplasma   Yes Yes Yes Yes +/-

Rickettsia   No Yes Yes Yes Yes

Chlamydia No Yes Yes Yes+/-

Viruses No No No NoNo

Relative size of viruses and bacteria

Sizes Object1mm

100m

10m

1m

100nm

10nm

1nm

1A°

ProtozoaProtozoa

Bacteria

viruses

Macromolecules

Blood cells

Molecules

Atoms

Microscopes

Light

UV

Electron

Study of viral morphology•

Negative contrast Electron microscopy:

– Study of virus structure

• Thin section Electron microscopy: – Study of virus and host interaction

• Immunoelectron microscopy: – To detect either component

• Electron cryomicroscopy: – To avoid artifact produced in negative contrast EM or Thin section EM

• X ray diffraction:• Production of sufficient amount of pure virus/ viral component

Structure of virus

• Viral protein

• Viral nucleic acid

• Viral lipid envelope

• Viral glycoproteins

Basic virus structure

Viral glycoprotein

Properties of naked capsid viruses

Properties of enveloped viruses

Genome of DNA virus

• All double stranded except Parvovirus• Arrangement:

– linear or circularHepadenaPapova

• Size: • 3.2 kbp to >300 kbp

Genome of RNA virus

• SS or DS :• Reo

• Single or segmented:» Arena, Bunya, Orthomyxo, Reo

• Polarity: Positive ( mRNA) or negative sense

Classification Parameters

• Several Parameters Are Used for Classification– Viral classification study is referred to as

Taxonomy – 63 families exist so far– Type of genomic nucleic acid – Size of virion and genome– Capsid structure– Host– Replication mechanism

Classification of virus

History: • Properties of virus: filtrability• Disease produced: hepatitis, resp etc• Hierarchial system:

– Monothetical: based on any single property

– Polythetical: group of properties

– Systematic approach:

– Non systematic approach:

“A virus is a virus, whether to be regarded as organism is a matter of taste”

Types of symmetry

Icosahedral Helical

Icosahedron:

• 20 faces

(equilateral triangle)

• 30 edges

• 12 corners/vertices

Icosahedral symmetry

5:3:2 rotational symmetry

edges

vertices

faces

edges

The icosahedral shape of a soccer ball.

Penton subunits (black) and Hexon subunits (white)

Complex symmetry

Size of Viruses• Ranges of sizes

– 20 nm to 500 nm (spherical)– 12 nm to 300-2000 nm (rod like)– Easily observed with electron microscope

ICTV Classification

• Polythetic: collection of properties

• Non systematic: no fixed list of properties to be considered for all viruses

• Hierarchial system: • order family subfamily genus species

Nomenclature of viruses

Taxon Suffix Example

Order virales Mononegavirales (Paramyxoviridae, Rhabdoviridae,

Filoviriridae)

Family viridae Herpesviridae

Subfamily virinae Alphaherepesvirinae

Genus virus Herpes simplex virus1

Baltimore classification

All viruses must generate positive strand mRNA from their genome

Basic strategy for viral replication

Viral Replication

Typical infectious cycle

1. Attachment

2. Penetration

3. Uncoating

4. Transcription and/or translation

5. Replication

6. Assembly

7. Release

• Adenovirus• Coxsackievirus• Echovirus• Epstein-Barr Virus• HIV-1• Measles virus• Parvovirus• Poliovirus• Rhinovirus

CARCAR, CD55Integrin VLA-2, CD55CD21CD4, CCR5, CXCR4CD46Erythrocyte P AgPVRICAM-1

Variations on the replication theme

Viral envelope formation