Introduction to Virology

Mtebe Majigo
 Virology
The study of viruses focusing on:
• Structure
• classification and evolution
• Ways to infect and exploit host cells for
reproduction
• Interaction with host organism, physiology and
immunity
• Diseases they cause
• Techniques to isolate and culture
• Use in research and therapy
 Virus
A virus is a small infectious agent that can
replicate only inside the living cells of an
organism. Viruses can infect all types of
organisms
 General characteristics of Viruses
• Smallest infectious particles (18-300nm)
• Consist of DNA or RNA not both
• The nucleic acid encased in protein shell
• Lack an independent metabolic system
• Require host cells for replication
– Obligate intracellular parasites
• Consist of an intracellular reproductive cycle and
an extra cellular transmissive cycle
• Do not replicate by binary fission or division
 Comparison of viruses and cells
Properties Viruses Cells

Nucleic acid DNA or RNA DNA and RNA

Protein Few Many

Lipoprotein membrane Envelope in some Cell membrane

viruses
Ribosomes Absent* Present

Mitochondria Absent Present in Eukaryotic

cells
Binary fusion or mitosis No Yes

Enzymes None or few Many

*Arenaviruse have a few non-functional ribosomes

 Definition of Terms
• Virion – the complete virus particle
• Envelope - A lipid-containing membrane, surrounds the
nucleocapsid, It is acquired during viral maturation by a
budding process through a cellular membrane
• Capsid – the protein coat surrounding the nucleic acid
genome
• Capsomers – the repeating protein subunits that make up
the capsid
• Protomers – the polypeptide chains which make up the
capsomers
• Nucleocapsid = capsid + nucleic acid
• Defective virus: A virus particle that is functionally deficient
in some aspect of replication
 Viral Structure
• Display a wide diversity of shapes and sizes
• shape
– Spherical, rods, filamentous
• Size
– diameter between 20 and 300 nanometers
• A complete virus particle consists
– nucleic acid
– capsid
– A lipid "envelope" in some viruses
• The capsid shape serves as the basis for
morphological distinction
Viral Structure
 Measuring the Sizes of Viruses
• Direct Observation in the Electron Microscope
• Sedimentation in the Ultracentrifuge
• Comparative Measurements
 Viral Symmetry
 The arrangement of capsid around its viral
genome is unique for each type of virus.
 The general properties arrangement define the
shape and symmetry.
 The structures that can be built are:
 Helical symmetry
 Cubical symmetry - Icosahedral Patten
 Complex structure
 Helical symmetry
– composed of a single type of capsomer stacked
around a central axis to form a helical structure:
may have a central cavity, or hollow tube
– Results in rod-shaped or filamentous virions
– Genetic material is bound into the protein helix by
interactions between the negatively charged
nucleic acid and positive charges on the protein
Helical symmetry
 Icosahedral Patten
• Polyhedron with 20 identical equilateral triangular
faces, 30 edges and 12 vertices
• Minimum of 12 identical capsomers
• Capsomer at the apex surrounded by five other
capsomers and are called pentons.
• Capsomers on the triangular faces are surrounded by
six others and are called hexons
Icosahedral Pattern
 4.
1. Mature
Protomers Capsid

2. 3.
Capsomers Pro-
Capsid
 Complex
• Viruses possess a capsid
that is neither purely
helical nor purely
icosahedral
• May possess extra
structures such as
protein tails or a
complex outer wall.
 Chemical Composition of Viruses
• Viral protein
• Viral nucleic acid
• Viral lipid envelope
• Viral glycoprotein
 Viral Protein
• Structural protein
– Protect the viral genome against inactivation
– Participate in attachment to target cells
– Provide the structural symmetry of the virion
– Determine the antigenic characteristics

• Non-structural protein
– Enzymes – essential for viral replicative cycle
 Viral Nucleic Acid
• Contain single kind of nucleic acid (DNA, or RNA)
• Encodes the genetic information necessary for
replication
• The size of viral nucleic acid
– DNA viral genome: 3.2 kbp (hepadnaviruses) - 375
kbp (poxviruses)
– RNA viral genome: 7kb (picornaviruses) - 30 kb
(coronaviruses)
• The viral genomes are diverse
 Genomic diversity
Properties Parameters
Nucleic acid • DNA
• RNA
• Both DNA and RNA (at different stages in
the life cycle)

Shape • Linear
• Circular
• Segmented
Strandedness • Single-stranded
• Double-stranded
• Double-stranded with regions of single-
strandedness
Sense/polarity • Positive sense (+)
•
• Ambisense (+/−)
 Viral Lipid Envelope
• Acquired when nucleocapsid buds through a
cell membrane
• Specific phospholipids composition determine
where budding occurred
• Acquisitions is the integral step in virion
morphogenesis of some viruses
 Viral Glycoproteins
• Contained into the envelopes
• Virus encoded
• The added sugar reflect the cell in which the virus
is grown
Function
• Attachment to target cells
• Involved in membrane fusion step of infection
• Important viral antigens – interact with
neutralizing antibody
Icosahedral Helical
Capsid Capsid
 Naked viruses
• Component: contain only nucleocapsid protein
• Environmentally stable, more resistant to physical, chemical
and environmental forces
• Cause Lytic infection- release virus particles cell lysis –
Associated with acute diseases
• Cause year-round, non-seasonal diseases

Characteristics
• Easy to spread by dust, small droplets
• Can dry out and still retain infectivity
• Can survive harsh conditions of the GI tract
• Humoral (antibody) immunity may be sufficient for
protection
 Enveloped viruses
• Components: membrane, lipids, protein, glycoproteins
• Determine tissue tropism, host specificity
• Budding through cell membrane, more often associated with
chronic and persistent infections
• Environmentally labile (survive for only a short period of time)
Susceptible to physical and chemical forces (acids, detergents,
drying, heat)
• Cause seasonal diseases, cold winter favors survival

• Characteristics
– Must stay wet
– Cannot survive GI
– Do not need to kill cells to spread
– Spread in large droplets, secretions
– Need both humoral and CMI for protection
Reaction to Physical & Chemical
Agents
• Heat and cold
– Icosahedral viruses stable, losing little infectivity after
several hours at 37 °C.
– Enveloped viruses more heat-labile, rapidly dropping in
titer at 37 °C
– infectivity is generally destroyed by heating at 50–60 °C for
30 minutes
– Preserved by storage at subfreezing temperatures
– Enveloped viruses tend to lose infectivity after prolonged
storage
• pH
– usually stable between pH values of 5.0 and 9.0.
– enteroviruses are resistant to acidic conditions.
– All viruses are destroyed by alkaline conditions.
Reaction to Physical & Chemical
Agents
• Stabilization of Viruses by Salts
– Can be stabilized by salts in concentrations of 1 mol/L;
• Not inactivated by heating at 50 °C for 1 hour
– The stability of viruses is important in the preparation
of vaccines.
• Radiation
– Ultraviolet, x-ray, and high-energy particles inactivate
viruses.
– The dose varies for different viruses
– Infectivity is the most radiosensitive property
Reaction to Physical & Chemical
Agents
• Ether Susceptibility
– Can be used to distinguish enveloped from naked
• Detergents
– Nonionic detergents solubilize lipid constituents of viral
membranes.
– The viral proteins in the envelope are released (undenatured).
– Anionic detergents, solubilize viral envelopes and disrupt capsids
into separated polypeptides.

• Formaldehyde
– Destroys viral infectivity by reacting with nucleic acid.
– Single-stranded genomes are inactivated much more readily
– Formaldehyde has minimal adverse effects on the antigenicity of
proteins
– Used in the production of inactivated viral vaccines
 Evolutionary Origin of Virus
• Origin not known
• Profound differences between RNA, DNA viruses
Two theories
• Derived from DNA or RNA component of host cell
– Resemble genes that have capacity to exist
independently
– Some viral sequences are related to portions of
cellular genes encoding protein
• Degenerate forms of intracellular parasites
– There is no evidence that viruses evolved from
bacteria