MCD – Microbiology 1 – Virus Properties
Anil Chopra
1. Describe the nature of viruses: their small size, mode of replication and diversity.
Viruses
 Viruses are small in size, 20-450 nm
 They are obligate (cannot survive outside host) intracellular parasites.
 Composition: nucleic acid (DNA or RNA, can be single stranded or double
stranded, linear or circular) and protein, (sometimes lipid and carbohydrate too)
 They have a unique mode of replication.
 Diversity: all species are infected by viruses, may cause great plagues or be
asymptomatic
 Classification according to:
o Type of disease
o Mode of transmission
o Structure
o Immunological relatedness
o Nucleic acid sequence
o Mode of replication.
 Viruses are measured in various ways including:
o Observing disease in host
o Plaque assay (infectivity) – A selection of susceptible host cells is infected
with the virus. An area of killed cells large enough to be seen = plaque.
o Electron microscopy
o Polymerase chain reaction
o Immunological evidence of infection
2. Examples of virus replication HIV, polio, influenza, herpes simplex virus
Virus replication occurs in a series of steps. This includes:
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Binding to host cell: often very specific interaction between virus surface proteins
and cell receptors
 HIV gp120 : CD4
 Epstein-Barr virus gp340 : CD21
 Influenza virus haemagglutinin (HA) : sialic acid
Penetration: e.g. fusion of virus and cell membranes
 (A) enveloped viruses: fusion between virus and cell membranes
 HIV and measles virus at cell surface
 Influenza virus with acidified intracellular vesicles
 (B) non-enveloped viruses: disruption of host cell membrane integrity,
genome or core crosses into cytosol
 Polio virus
 T4 in E. coli
The eclipse phase: expression of virus proteins and replication of nucleic acid.
 Virus particles have been disassembled so no infectious particles are present
 Expression of virus proteins
 Highly regulated: temporal and quantitative
 Replication of virus nucleic acid
Assembly: production of new infectious particles
Release: cell lysis or budding
Polio
 The virion is 20nm and is very stable to acid pH and proteases.
 It is a simple mRNA molecule that is used to synthesise giant polyprotiens.
 It uses a complementary mRNA strand intermediate to replicate itself.
Influenza
 Genome transcribed into mRNAs and cRNAs by virus RNA-dependent RNA
polymerase.
 mRNAs translated to virus protein.
 cRNAs are copied into virus RNA (vRNA)
HIV
 A retrovirus, (replicates via reverse transcription)
 Enveloped particle, 110 nm
 ss RNA genome (+ve), 10 kb, diploid
 RNA genome converted into dsDNA via reverse transcription
 DNA integrates into host DNA (= provirus), may remain dormant
 Enables vertical transmission of virus
 Expressed by host RNA pol II, to make HIV mRNAs, regulated by splicing
Her
pes
Sim
plex
Vir
us
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Virion: icosahedral capsid surrounded by lipid envelope, 130 nm
Genome: Linear ds DNA 152 kb, ~ 80 genes
Replication: in nucleus, may replicate via
lytic cycle, or become latent
HSV-1, cold sores; HSV-2, genital herpes
Infection: via skin abrasions
Lytic replication: cascade of gene expression
o IE proteins: regulatory
o DE proteins: replicative, e.g. DNA
polymerase
o Late proteins: structural
o Each class requires prior expression of
proteins of the previous class
3. Define the following terms as used in the description and classification of viruses:
DNA virus, RNA virus, capsid type, envelope non-enveloped
DNA Virus: uses DNA as its genetic material. They may be double or single stranded.
RNA Virus: uses RNA as genetic material.
Capsid Type: the type of outer shell
the virus has. Helical capsids,
Baltimore Classification
icosahedral (isometric) capsids, or
Group
Contains
enveloped.
I
dsDNA viruses
Enveloped: Coating for virus particle.
II
ssDNA viruses
III
dsRNA viruses
IV
(+)ssRNA viruses
V
(-)ssRNA viruses
VI
ssRNA-RT viruses
VII
dsDNA-RT viruses
ss: single-stranded, ds: double stranded
RT: reverse transcribing