The characteristics of virus

1. GENERAL
VIROLOGY
CHARACTERISTICS OF VIRUSES

2. PROPERTIES
Growth on
artificial
media
Division by
binary
fission
Have both
DNA &
RNA
Have
ribosomes
Have
muramic
acid
Sensitivity
to
antibiotics
Bacteria
Yes Yes Yes Yes Yes Yes
Mycoplasma
Yes Yes Yes Yes No Yes
Rickettsia
No Yes Yes Yes Yes Yes
Chlamydia
No Yes Yes Yes No Yes
Viruses
No No No No No No

3. VIRUSES
 Virus structure & replication
fundamentally different from cellular
organisms
 Viruses infect all major groups of
organisms
 Some viruses have broader host range
than others
 None can cross eukaryotic/prokaryotic
boundary

4. VIRUSES
 NOT CELLS
 NOT MICROORGANISMS
 NO FUNCTIONAL ORGANELLES
 DEPENDENT ON HOST MACHINERY
 CONTAIN EITHER DNA OR RNA
 TWO CLEARLY DEFINED PHASES:
 METABOLICALLY INERT:-TRANSMISSION
 METABOLICALLY ACTIVE:- REPLICATION

5. VIRUS STRUCTURE
 Range in size: less than 100nm diameter
to several hundred nanometers in length
 All viruses contain nucleic acid genome
(RNA or DNA) & protective protein coat
(capsid)
 Nucleic acid genome + capsid =
nucleocapsid
 Nucleocapsid have icosahedral, helical or
complex symmetry

6. VIRAL GENOME
 DNA or RNA
 Double-stranded or single-stranded
 Monopartite (all viral genes contained in
single molecule) or multipartite
(segmented: viral genes distributed in
segments)
 All are haploid – contain only one copy
of each gene; except retrovirus (diploid)

7. VIRAL PROTEINS
 1 (simplest virus) to > 100 (complex)
 Structural:- used to construct capsid &
other components of virion.
 Non-structural:- not part of virion -
involved in viral replication processes or
in virion assembly e.g. enzymes
 Proteins are virus coded including those
associated with envelope.

8. VIRAL PROTEINS
 GLYCOPROTEINS
 FUSION PROTEINS:-
 ASSOCIATED WITH PEPLOMERS
 INVOLVED IN VIRAL ENTRY & RELEASE
 MATRIX PROTEINS:-
 FOUND AS LAYER ON INSIDE OF
ENVELOPE
 PROVIDE RIGIDITY TO VIRION

9. VIRAL GLYCOPROTEINS
 Most occur as membrane-anchored
peplomers (spikes) extending outward
from envelope of enveloped viruses
 Sugar component corresponds to that of
host cell membrane glycoproteins

10. Viral Envelope
 Structurally similar to cell membrane
 Lipid bilayer with transmembrane viral
glycoproteins
 Destroyed by ether or detergent
rendering enveloped viruses non-
infectious

11. Viral Envelope
 Inner layer of membrane protein e.g.
matrix for myxoviruses – anchors
glycoprotein
 Glycoproteins arranged into groups of 2-
4 known as spikes -> observed under
electron microscope.
 Envelopes are more pleomorphic than
nucleocapsids

13. Viral Envelope
 The envelope is obtained as the
nucleocapsid buds through cell
membrane
 All animal viruses with helical
nucleocapsid contain RNA & enveloped
 Majority of animal virus families with
icosahedral symmetry are unenveloped &
those with envelopes contain DNA

15. VIRUS STRUCTURE
 Viruses may or may not contain envelope
 Enveloped viruses obtain envelope by
budding through host cell membrane e.g.
plasma membrane, Golgi body,
endoplasmic reticulum or nucleus
 VIRION – complete virus particle

16. VIRUS STRUCTURE
 Enveloped viruses do not necessarily kill
cell in order to be released – bud out of
cell => persistent infections
 Enveloped viruses are infectious only if
envelope is intact (viral attachment
proteins)
 Agents which damage envelope e.g.
alcohols & detergents destroy infectivity

17. Virion Nucleocapsid
Structures
 Icosahedral symmetry
 Helical symmetry
 Complex symmetry

18. ICOSAHEDRAL
SYMMETRY
 Solid with twenty triangular faces & 5:3:2
rotational symmetry
 Twelve corners or vertices & 5-fold
symmetry around vertices
 Capsid shell is made of repeating
subunits of viral protein
 All faces of icosahedron are identical
 Nucleic acid is packaged inside capsid
shell & protected from environment

22. ICOSAHEDRAL
SYMMETRY
 Proteins associate into structural units
(observed in electron microscope) known
as capsomers
 Capsomers may contain one or several
kinds of polypeptide chain
 Capsomers at the 12 corners have 5-fold
symmetry & interact with 5 neighbouring
capsomers, known as pentons or
pentamers

24. ICOSAHEDRAL
SYMMETRY
 Larger viruses contain more capsomers
 Extra capsomers are arranged in a
regular array on the faces of the
icosahedrons
 They have six neighbours, called hexons
or hexamers
 The size of an icosahedron depends on
the size & number of capsomers: there
will always be 12 pentons but the number
of hexons increases with size

29. HELICAL SYMMETRY
 Protein subunits interact with each other
& with nucleic acid to form coiled ribbon-
like structure
 Best studied virus is non-enveloped plant
virus tobacco mosaic virus
 Enveloped helically symmetrical viruses
e.g. influenza viruses, rabies virus

31. COMPLEX SYMMETRY
 Regular structures
 Examples include the poxviruses

33. FIVE BASIC STRUCTURAL
FORMS OF VIRUSES
 Naked icosahedral e.g. adenovirus
 Naked helical e.g. tobacco mosaic virus
 Enveloped icosahedral e.g. herpes
virus
 Enveloped helical e.g. rabies virus,
influenza virus, parainfluenza virus
 Complex e.g. poxvirus

41. Are viruses living or dead?
 In some ways fulfils criteria use to define
life; in other ways, doesn’t.
 Refer to number of infectious particles
rather than number of living particles

42. UNCONVENTIONAL
AGENTS
 ‘Unconventional viruses’ or ‘atypical
viruses’
 Viroids and prions

43. VIROIDS
 Contain RNA only
 Small (less than 400 nucleotides), single
stranded, circular RNAs
 The RNA do not appear to code for any
proteins
 Have only been associated with plant
disease

44. PRIONS
 Contain protein only (controversial)
 Small proteinaceous particles
 Examples of prion-caused animal
disease is scrapie in sheep and “mad
cow disease” in cattle.