Introduction to Viruses Flashcards
What are viruses
- Obligate intracellular parasites
- Do not carry biosynthetic apparatus for own reproduction
- Require a live host cell to subvert
- Generally host-specific; can be tissue specific
Self-replication
requires Ribosomes
Viruses cause many human diseases
Animal virus diseases
Viruses are responsible for important diseases of domestic animals causing serious economic losses
Foot and mouth disease: 2001 UK
10 million animals culled; All farm animals within 3 km
radius of infected farms slaughtered (£8bn loss)
Plant viruses
Viruses are responsible for economically important losses
of crop plants (second only to fungi)
Viruses are very small
viruses: eg. smallpox 200 nm
rabies 100-150 nm
polio 28 nm
bacteria (approx): ~1000 x 2-3000 nm
animal cell nucleus: ~2800 nm
Most viruses are smaller than most
bacteria
Biggest viruses infect amoeba
Giant double-stranded DNA
virus with mature particles of
400 nm in diameter
~ 1,200,000 bp
Mimivirus – Biggest…….?
Mimivirus (dsDNA virus of amoebas) can be 400 nm diameter.
1.2 megabase pair genome coding for 979 proteins
Pithovirus sibericum - Even bigger!
-Giant virus 1.5 μm (bigger
than E. coli)
- Infects amoebae
- Frozen in ice for 30,000
years in Siberian permafrost
- Still infectious
-Pithovirus sibericum =
‘pithos’ (large container used
by ancient Greeks to store
wine and food)
Smallest viruses infect bacteria
Bacteriophages
(phages)
20-200 nm
How do we classify viruses?
> 350 families
- MORPHOLOGY
* Virion shape, symmetry, size
* Enveloped / non-enveloped - GENOME
* Mechanism of replication
* Genome sequence - ACTION
* Specificity (host / tissue)
* Nature of infection - MORPHOLOGY
- GENOME
- ACTION
Basic Architecture
Capsidecomposed of protein sunits called capsomers
Hollow core containing highly folded nucleic acid
Helical 2 Fold Symmetry
Icosahederal 2-3-5 Fold Symmetry
12 VERTICES
20 SIDES
30 EDGES
Assembly of Capsid Subunits
- Protomers Capsomeres Capsid
Hexons (6 protomers) form 20 triangular faces
Pentons (5 protomers) form 12 corners
NB. Protomers forming hexons and pentons can be
the same or different proteins depending on virus
Some Viruses have Envelopes
Tailed viruses
Some bacteriophage (and
some algal viruses) have a tail
structure which can either be
rigid or flexible.
It consists of a tail-plate with
or without tail fibres which is
attached to a contractile
sheath surrounding a hollow
core.
The tail aids in penetration of
the cell wall.
SARS-CoV-2 the cause of COVID-19
Coronaviruses ( + sense RNA)
ICTV Taxonomy: Rules on virus names
– Order (-virales) - Herpesvirales
– Family (-viridae) - Herpesviridae
– Subfamily (-virinae) Alphaherpesvirinae
– Genus (-virus) - Simplexvirus
– Species - Herpes simplex virus
- (HSV-1)
Skin (e.g. Warts)
200 HPV types
- Infect human skin cells (Keratinocytes)
- Usually asymptomatic
- Can cause warts / advance to cancer
(cervix, vulva, vagina, penis)
Transmission: Contact / STD (genital warts / cervical cancer)
Respiratory Viruses
Transmission: Aerosol / contact
Gastrointestinal viruses
Rotavirus (RNA) Astrovirus (RNA)
Adenovirus (DNA) Norovirus (RNA)
Transmission: Faecal/ oral
Bloodborne (e.g. HIV)
Replication: T lymphocytes (white blood cells)
Transmission: Sex / IV drug use / verticle / blood
Satellites
- Satellite viruses (e.g.,
hepatitis D virus, HDV)
– Require a helper virus for
their replication
– Carry their own capsid
protein gene
Viroids (Satellite RNA)
- Consist only of naked RNA
- are extremely small (less than 400 nucleotides)
- Have a high degree of internal complementarity
- Are resistant to ribonucleases
- So far, only observed to cause disease in plants
Prions
Proteinaceous Infectious Particles
Protein Infection
– No nucleic acid, no
genes… just protein
– Cause transmissible
spongiform
encephalopathies (TSEs)
Animal Prion Diseases
- Scrapie.
- Bovine spongiform encephalopathy.
- Transmissible mink encephalopathy.
- Chronic wasting disease (mule deer, elk).
- Feline spongiform encephalopathy.
- Zoological spongiform encephalopathy (eland, nyala,
oryx, kudu, gemsbok, cheetah, puma, ocelot, ostrich).
Scrapie
- Sheep rub against
fences; restlessness;
tremor; weight loss - Transmissible
- Fatal
- Endemic in many
parts of the world, (not
Australia and NZ)
Transmission of Scrapie
Persistence in the environment: In Iceland infected sheep were slaughtered
and non-infected sheep imported from Australia and NZ
* Sheep grazed on land. 3 years later contracted the disease.
In 1943 a population of Scottish sheep
was inoculated against a common virus
with a formalin extract of lymphoid tissue
unknowingly derived from an animal with
scrapie.
After 2 yrs, nearly 10% of the flock
developed scrapie.
Prion associated lesion
- The prion agent that
causes scrapie can
be transmitted to
mice.
Bovine Spongiform Encephalitis
(BSE) – Mad cow disease
- First recognised in 1985.
- By 1992, 3 cows in every 1,000
had BSE in UK - 850 cases per week in 1994
- Incubation period up to 5 yrs
- 55% of milking herds in UK
contain cases of BSE
Spread: consumption of bonemeal? - Symptoms:
– Nervous, aggressive
temperament; Abnormal
posture; weight loss;
Ataxia
Prion diseases in Humans
- Kuru (endemic in Papua New Guinea)
- Creutzfeld Jacob disease (CJD)
- v(variant) CJD
- Gerstmann-Straussler-Scheinker disease
- Fatal familial insomnia
- Alpers disease
- Alzheimer’s disease
Creutzfeld Jacob disease (CJD)
Classic CJD
is not related to BSE
“mad cow” disease.
Classic CJD also is
distinct from “variant
CJD”, another prion
disease
that is related to
BSE
Prion replication?
– Replication method still unclear
– Thought to revolve around conversion of protein
conformations from normal to abnormal form over time
Changing shape
PrPC & PrPSC
Prions are…
heat resistant
Prion Summary
- Self-propagating, infectious agents with no nucleic acid
- Resistant to heat and disinfectants
- 3D conformational change triggers protein pathogenicity
- Cause transmissible spongiform encephalopathy
(characteristic “holes” in the brain – like a sponge) = Rapid
progressing neurodegenerative disease - Transmission associated with consumption of neuronal
material and can take a long time to develop
