Virology - 1 Fundamentals
Skern - Coffee House Notes on Virology
Skern - Coffee House Notes on Virology
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Cartes-fiches
What are viruses?
*obligate intracellular parasites
*made up pf nucleic adic, protein, sometimes lipid
*does not code for ribosomes, mitochondria
*do not reproduce by division, they are assembled from pre-formed complexes
Key steps in viral life-cycles
*Attachment
*Entry
*Uncoating
*Expression of genetic information
*Replication of genetic information
*Assembly
*Maturation
*Release
Uncoating can occur...
*at the plasma membrane
*in the cytosol
*at the nuclear membrane
Expression of genetic information produces proteins for
*virus particle
*nucleic acid synthesis
*virus-host interactions
example for Class I Virus:
ds DNA Virus:
eg Herpes simplex virus type 1
example for Class II Virus:
(-)ssDNA
eg feline parovirus
example for Class III virus:
ds RNA Virus:
eg human rotavirus
example for Class IV Virus:
+ssRNA Virus
eg Hepatitis C Virus
example for Class V Virus:
-ssRNA Virus:
Measles Virus
example for Class VI Virus:
+ssRNA Virus with a DNA intermediate (Retrovirus)
Human immunodeficiency virus type 1 (HIV)
Why do viruses keep their genomes small?
*the larger the genome, the longer the time needed to replicate it
*....the larger the capsid must be
*...the higher the probability of mutations
Four strategies to keep viral genomes small:
*all viruses use cellular machinery to generate energy and synthesise proteins - viral genomes do not need to code for such machinery
*some viruses use the same region of nucleic acid to code for different proteins
----more than 1 ORF in same region of nucleic acid
----using different reading frames on one strand
---- encoding proteins on both strands
*some viruses produce one large RNA (PRIMARY TRANSCRIPT) which is spliced by cellular splicing machinery to generate several mRNAs
*some viruses produce single large protein POLYPROTEIN which is cleaved by viral and cellular proteases to mature proteins
What characteristics drive viral evolution?
*viruses produce a large number of descendants - viral replication generates a large number of mutants
*viruses can evolve by recombination of genomes
*certain viruses can evolve by reassortment of genome segments
large numbers of mutant viruses can be selected for fitness in new environments and hosts - new possibilities often provided by human behaviour
*high density agriculture
*water regulation
*drug abuse
*sexual practices
emerging viruses are generated from existing ones:
*known viruses change to become able to infect an immune population
*known virus is exposed to a naive population
* nown viruses changes its host range to enter a different species
which perameters limit the evolution of viruses?
*icosahedral viral capsids limit size of nucleid acid
*virus is dependent on host systems for decoding genetic information
*mutations may adversly affect activity of viral proteins
*an extremely virulent virus kills its host and therefore itself
*too many mutations lead to a non-viable virus
two types of symmtetry, found in virus particles:
*icosahedral symmetry
*helical symmetry
icosahedral symmetry:
*regular polyhedron with 20 faces
*polyhedron has 3 types of symmetry axes: 12 five fold, 20 three fold, 30 two fold axes
VIRUS PARTICLES MUST HAVE AT LEAST 60 COPIES OF ONE PROTEIN TO OBEY ICOSAHEDRAL SYMMETRY
or multiples of 60
helical symmetry:
*proteins arranged in simple repeating unit around nucleic acid
*each protein occupies the same position, makes identical interactions with its neighbours and the nucleic acid
consequences of icosahedral symmetry:
* volume of icosahedral limits size of nucleic acid
-rules of symmetry must still be obeyed - limits number of proteins to certain multiples of 60 - 180, 240, 420, etc
difference in proteins on surface of icosahedron allows antigenic variation
consequences of helical symmetry:
helical structure formed around nucleic acid - theoretically no limit to lenght
proteins occupying identical position makes antigenic variation difficult
viruses present in blood
Hepatitis B
Hepatitis C
viruses present in semen
HIV1
HTLV1
viruses present in feces:
poliovirus
human rotavirus
viruses present in respiratory droplets:
measles
influenza A
viruses in saliva:
Epstein.Barr virus
herpes simplex 1
viruses which are risk factors for cancer:
Hepatitis B, C
HTLV1
Epstein-Barr
HPV
viruses which can cause long-term infections
Hepatitis B, C
HSV1
HIV1
HTLV1
Epstein-Barr
Which properties should a viral vaccine have to make it sage and efficient?
*should induce protection against a pathogenic virus without itself causing disease
*immune response generated by vaccine should resemble as closely as possible the response generated by pathogenic agent
*vaccines should induce the production of memory B and memory T cell
*vaccines should induce long-term production with just one or few inoculations
*should be cheap
*should be seen as a clear advantage by society
