Virology lecture 1

Question 1

structure of simplest virion and more complex structures

Answer 1

nucleic acid (genome) surrounded by a protein shell (capsid) of repeating subunits - capsomers. more complex viruses surrounded by phospholipid membrane (the envelope) acquired from the host. membrane embedded with viral proteins which may be glycosylated.

Question 2

non structural viral proteins

Answer 2

virus encoded proteins expressed from within the infected host cell

Question 3

Tobacco mosaic virus

Capsid and envelope

Answer 3

1 - helical capsid, no envelope

Question 4

Poliovirus

Capsid, genome

Answer 4

1 - icosadhedral capsid, no envelope
2 - +ve ssRNA, monopartite
3 - 7.5kb

Question 5

Influenza viruses

Capsid, genome

Answer 5

1 - helical capsid plus envelope

2 - segmented -ve strand RNA

Question 6

Herpes simplex virus

Capsid genome

Answer 6

1 - icosahedral capsid plus lipid envelope

2 - dsDNA

Question 7

virus genome structure types

Answer 7

1 - DNA or RNA
2 - linear or circular
3 - monopartite or segmented
4 - double stranded or single stranded

Question 8

viral RNA subclasses

Answer 8

1 - sense mRNA can be translated into viral protein and is called +ve strand RNA
2 - if complementary then the viral genome is transcribed to produce the mRNA - it’s instead called -ve sense
3 - most are monopartite, few are segmented (ie influenza virus)

Question 9

Measles

Genome

Answer 9

1 - -ve ssRNA
2 - 15.9kb
3 - monopartite

Question 10

Rotaviruses

Genome

Answer 10

1 - segmented dsRNA

Question 11

mechs of increasing coding capacity.

smallest and largest genomes

Answer 11

1 - densely packed genes
2 - small intergenic spaces
3 - few non-coding spaces
4 - overlapping reading frames, one gene for up to 3 different proteins
5 - splicing
6 - smallest = 3kb, largest = 1200kb. v roughly 1kb codes 1 average sized polypeptide

Question 12

limits on RNA genome size

Answer 12

1 - largest around 30kb (ie coronaviruses), most less than half
2 - limited by error prone nature of RNA polymerases as no proofreading. too large and too many mutations occur which are lethal.

Question 13

Influenza A

Capsid genome

Answer 13

1 - helical capsid plus envelope
2 - segmented -ve strand RNA
3 - 8 segments
4 - 13.6kb
5 -

Question 14

HIV

Genome

Answer 14

1 - 10kb monopartite but diploid. a retrovirus

2 - +ve ssRNA

Question 15

virus classification tree

5 stages

Answer 15

families (viridae), subfamilies (virinae), genera, species and strains

Question 16

chickenpox

Family

Answer 16

1 - a herpesvirus

Question 17

what are arboviruses?

Answer 17

1 - a large group of viruses transmitted by biting insects (arthropod bourne viruses)

Question 18

Describe the Papovaviridae family of viruses

Answer 18

1 - icosahedral capsid, no envelope.
2 - 5-8kbp, dsDNA
3 - includes papillomavirus species which gives rise to warts and cervical carcinoma

Question 19

Describe the adenoviridae family of viruses

Answer 19

1 - icosahedral capsid, no envelope, 35kbp dsDNA

2 - inc adenovirus species, giving rise to acute respiratory disease

Question 20

Describe the herpesviridae family of viruses

Answer 20

1 - icosahedral caspid, envelope, around 150kbp (hsv). dsDNA
2 - herpes simplex virus - type 1 gives cold sores, type 2 gives genital herpes.
3 - varicella-zoster virus (VZV) - chicken pox and shingles
4 - epstein-barr virus (EBV) glandular fever and burkitt lymphoma
5 - Human cytomegalovirus (HCMV) - cytomegalic inclusion disease in neonates
6 - Human herpesvirus 8 - Karposi’s sarcoma
7 - Marek’s disease virus - lymphomas in chickens
8 - pseudorabies virus - aujesky’s disease of pigs

Question 21

Describe the poxviridae family of viruses

Answer 21

1 - complex caspid, envelope, around 200kbp, virion transcriptase. dsDNA
2 - includes species: Variola virus - smallpox
3 - Vaccinia virus - vaccine against smallpox
4 - Myxoma virus - myxomatosis in rabbits
5 - Orf virus - contagious pustular dermatitis in sheep and goats.

Question 22

Describe the parvovirus family of viruses

Answer 22

1 - icosahedral virion, no envelope, 5kbp, ssDNA.
2 - inc species: canine parvovirus
3 - feline panleucopenia virus - causes cat fever and enteritis
4 - human parvovirus

Question 23

Describe the Reoviridae family of viruses

Answer 23

1 - icosahedral virion, no envelope, 10-11 fragments, around 20kbp, virion transcriptase. dsRNA.
2 - blue tounge virus - catarrhal fever of sheep
3 - human and animal rotaviruses - acute infantile gastroenteritis.

Question 24

Describe the picornaviridae family of viruses

Answer 24

1 - icosahedral capsid, no envelope, no envelope, around 7.5kbp. +ve ssRNA
2 - poliovirus - poliomyelitis
3 - hepatitis A virus - acute hepatitis
4 - rhinovirus - common cold
5 - foot and mouth disease virus (FMDV) - foot and mouth disease

Question 25

Describe the caliciviridae family of viruses

Answer 25

1 - icosahedral caspid, no envelope, 8kb.+ve ssRNA
2 - norovirus - winter vomiting disease
3 - feline calicivirus - respiratory disease

Question 26

Describe the togaviridae and flaviviridae families of viruses

Answer 26

1 - icosahedral capsid, envelope, 11kbp. +ve ssRNA
2 - rubella virus - congenital abnormality (german measles)-toga
3 - Hepatitis C virus - acute and chronic hepatitis- flavi
4 - yellow fever virus - yellow fever -flavi
5 - classical swine fever virus - swine fever -flavi
6 - west nile virus - encephalitis -flavi

Question 27

Describe the coronaviridae family of viruses

Answer 27

1 - helical nucleocapsid and envelope. approx 30kbp. +ve ssRNA
2 - infectious bronchitis virus - avian bronchitis
3 - SARS-coronavirus - severe acute respiratory syndrome (SARS)

Question 28

Describe the orthomyxoviridae family of viruses

Answer 28

1 - helical capsid and envelope, 8 segments, 12kb, virion transcriptase. -ve ssRNA
2 - influenza A virus - influenza epidemics and pandemics, avian and equine influenza

Question 29

Describe the paramyxoviridae family of viruses

Answer 29

1 - helical capsid and envelope, pleomorphic, 15kb, virion transcriptase. -ve ssRNA
2 - measles virus - measles
3 - distemper virus (CDV) - canine distemper
4 - mumps virus - mumps
5 - respiratory syncytial vius - bronchitis
6 - newcastle disease virus (NDV) - fowl pest

Question 30

Describe the rhabdoviridae family of viruses

Answer 30

1 - helical capsid and envelope, bullet shape, 10kb, virion transcriptase, -ve ssRNA
2 - vesicular stomatitis virus (VSV) - visicular stomatitis in cattle and horses
3 - rabies virus - rabies in mammals

Question 31

Describe the retroviridae family of viruses

Answer 31

1 - capsid with envelope, 8-10kb, virion transcriptase, +ve ssRNA with DNA intermediate.
2 - rous sarcoma virus (RSV) - sarcomas in fowl
3 - feline leukaemia virus - leukaemia in cats
4 - human immuno-deficiency virus (HIV) - acquired immuno-deficiency syndrome (AIDS)

Question 32

Describe the hepadnaviridae family of viruses

Answer 32

1 - icosahedral capsid and envelope, 3.2kbp, dsDNA with RNA intermediate
2 - hepatitis B virus - acute and chronic hepatitis, primary liver cancer.

Question 33

methods of measuring viruses?

Answer 33

1 - electron microscopy - mix viral solution with small beads of known conc, measure relative conc and deduce viral. gives total no of virions, not no of infectious.
2 - polymerase chain reaction (PCR) - genome sequences of many viruses known so PCR with specific primers to identify and quantify. v. sensitive but like em doesnt give infectious level.
3 - immunological evidence of infection - detect presence/increase in antibodies/T cells for pathogen antigens. slow but good for retrospective epidemiology.
4 - plaque assay - preferred method to deduce infectivity. add series of dilutions to lawns of susceptible cells, cells destroyed and plaques form. each plaque derives from one initial infectious virus particle, so the conc of infectious virions can be calculated. expressed as plaque forming units (PFU) per ml. (em value vs PA value = particle/pfu ratio and can be over 1000 to 1.

Question 34

virus life cycle phases?

Answer 34

1 - adsorption and penetration
2 - eclipse phase
3 - assembly and release
shortly after infection the cell viral titre drops close to zero, rising again at the end of the eclipse phase. final titre being much higher than the input titre due to replication.

Question 35

viral latent period?

Answer 35

the time taken to start producing new particles - this varies greatly

Question 36

mean burst size?

Answer 36

average yield of viral particles per cell. varies greatly with specific virus-host combination and host metabolic activity.

Question 37

describe virus binding (HIV,influenza, EBV)

Answer 37

1 - virus binds specific cell surface molecules, the receptor may determine tissue trophism. HIV uses its gp120 to bind CD4 and a chemokine co-receptor. CD4 limited to T cells and macrophages/dendritic cells
2 - influenza uses its haemagglutinin (HA) to bind sialic acid (terminal sugar on most glycoproteins). as it binds essentially all cell types this doesnt determine its tissue trophism. (will agglutinate RBCs in vitro)
3 - EBV uses its glycoprotein 340 (GP340) to bind CD21
neutralising antibodies can be used to block this process

Question 38

describe virus penetration

Answer 38

1 - viral envelope fises with surface membrane or endosome after endocytosis. non enveloped virus process is porrly understood - may disrupt host cell membrane integrity.
many will membrane fuse via a receptor-target conformational change (ie HIV), but influenza is endocytosed, the vesicle acidified and this causes the conformational change in the HA.

Question 39

why quantative and temporal regulation of viral synthesis?

Answer 39

temporal - some proteins needed early in assembly ie nucleic acid polymerases and as enzymes, in low amounts. vs capsid proteins needed late and in higher amounts to build the virions.

Question 40

how does each class of virus replicate fundamentally?

Answer 40

all need to prduce mRNA to translate via host ribosomes. dsRNA uses its own polymerase to make mRNA.
2 - +ve ssRNA has a 5’cap and 3’ poly A tail so looks like our own mRNA.
3 - -ve ssRNA uses its own RNA viral replicase to make the mRNA
4 - dsDNA uses host or own enzymes to make the mRNA
5 - +ve ssRNA, and both +ve and -ve ssDNAuse their own enzymes to convert their genome to dsDNA then follow path 4.
The Baltimore classification - by their replicative strategy.