4. 4
• RNA or DNA
• segmented or non-segmented
• linear or circular
• single-stranded or double-stranded
• if single-stranded RNA
– is genome mRNA (+) sense or
complementary to mRNA (-) sense
CLASSIFICATION
NUCLEIC ACID
5. 5
• symmetry
– icosahedral, helical, complex
• number of capsomers if icosahedral
• enveloped or non-enveloped
CLASSIFICATION
CAPSID
6. 6
• viral genome properties:
– Type of nucleic acid (may be DNA or RNA).
– Molecular weight of nucleic acid.
– Number of strands (whether single strand or
double strand)of nucleic acid.
– Nucleic acid form(may be linear or circular ).
– Number and size of nucleic acid segments.
– Manner of replication.
:
Classification and
nomenclature of viruses
7. • Capsid properties including :
– Shape and size of viral particle.
– Number of capsomeres in capsid.
– Type of symmetry of nucleoprotein.
– Presence or absence of envelope.
– Presence or absence of spikes.
7
8. • Biological properties including:
– Susceptibility to physical and chemical
agents , especially ether and detergents ,
and stability to pH and temperature.
– Host specificity (natural host range).
– Mode of viral transmission.
– Organ specificity(tissue tropism) and
pathogenicity .
• 8
9. The nomenclature of viruses
• is not as in other organisms, but
classified into groupings which called
families, the family names have the
suffix-viridae . Each family , subdivided
into genera. The genus names carry
the suffix-virus.
9
10. The nomenclature of viruses
• Names of viruses are derived from:
• The name of disease caused by virus(eg:Influenza
virus, Hepatitis virus).
• The locality where the virus was first isolated (such
as ; West Nile virus).
• The name of scientists responsible for isolating
virus(such as; Epstein-Barr virus).
• Unique epidemiological characteristics of virus (such
as; Arboviruses, these are arthropod-borne viruses).
10
13. 13
BASIC STEPS IN VIRAL LIFE
CYCLE
• ADSORPTION
• PENETRATION
• UNCOATING AND ECLIPSE
• SYNTHESIS OF VIRAL NUCLEIC
ACID AND PROTEIN
• ASSEMBLY (maturation)
• RELEASE
25. 25
UNCOATING
• NEED TO MAKE GENOME
AVAILABLE
• ONCE UNCOATING OCCURS, ENTER
ECLIPSE PHASE
• ECLIPSE PHASE LASTS UNTIL FIRST
NEW VIRUS PARTICLE FORMED
26. 26
SYNTHESIS OF VIRAL
NUCLEIC ACID AND
PROTEIN
• MANY STRATEGIES
• NUCLEIC ACID MAY BE MADE IN
NUCLEUS OR CYTOPLASM
• PROTEIN SYNTHESIS IS ALWAYS IN
THE CYTOPLASM
28. 28
smallpox virus cytoplasmic assembly
and maturation
F. A. Murphy, School of Veterinary Medicine, University of California, Davis.
http://www.vetnet.ucdavis.edu/fam_graphics/download.html
30. 30
HIV budding and maturation
Hsiung, GD et al., Diagnostic Virology 1994 p204 (D. Medina)
31. • When a virus infects a cell, nucleic acid must be uncoated and gain
access to metabolic machinery of cell.
• Virus life cycle is characterized by:
– attachment
– penetration, with entry of nucleic acid into cell
– early expression of virus genes (either directly by translation, if virus
contains "+" RNA, or indirectly after transcription and then translation)
– replication of virus nucleic acid
– synthesis of new virion components
– packaging and assembly of new virions
– exit from cell
Viral Replication
32. • Attachment
– specific binding of a virion protein (the anti-receptor) to a
constituent of the cell surface (the receptor)
• e.g. hemagglutinin of influenza virus
• some complex viruses (HSV) may have more than one
species of anti-receptor molecule
• Penetration
– energy-dependent step
– occurs almost instantaneously after attachment
33. • After the virus attaches to the host cell, it can enter the cell by several
mechanisms:
– Transfer of the entire viral particle across the cell membrane by
endocytosis
– Transfer of only the viral genome through the cell membrane
– Fusion of the viral envelope with the host cell membrane
34. • Uncoating
– at same time as penetration or shortly after
– separation of viral nucleic acid (n.a.) from outer structural components
• Released as:
– free n.a. (picornaviruses)
– as nucleocapsid (reoviruses) = may need acidic pH in endosome
– viruses only infectious agent for which dissolution of infecting agent
obligatory step in replicative pathway
• Expression of viral genome and synthesis of viral components
35. • After the viral nucleic acid is released inside the host cell:
– The transcription and translation processes of the host cell are
redirected for the production of viral proteins and nucleic acids
– The different types of nucleic acid genomes are expressed and
replicated in several ways:
• DNA genomes undergo replication-using processes similar to
cellular replication
• RNA genomes may be +ssRNA; Can be read directly as an
mRNA or reverse transcribed by reverse transcriptase into
DNA
• RNA genomes may also be -ssRNA; The RNA must first be
used as a template to form +mRNAs
36. Assembly and Release
• Components of capsid synthesis directed by late genes
• Assembly of enveloped viruses needs interaction with plasma
membrane which has been modified
• Final stage of infection
• Enveloped viruses released gradually by budding or exocytosis
• Naked viruses accumulate in cytoplasm and released during
lysis
