Viruses Flashcards
what are the general characteristics of viruses?
- obligatory parasites - they need to hijack host machinery to replicate
- small
- made of nucleic acid genome which is surrounded by a protein coat (capsid) and a lipid envelope
- they can infect all living organisms, even other viruses (virophages)
what is the virus nucleic acid genome comprised of?
- DNA = ss or ds
- RNA = ss (+/-), or ds
- usually between 2-20kbp
- minimal genetic info is needed as they hijack host machinery
- ds>ss, DNA genome > RNA genome
- organised usually as one molecule which can be fragmented
what is the composition and structure of the viral capsid?
- made of capsomer proteins
- highly ordered structure which can be self-assembled
isosahedral symmetry - spherical viruses
- shape is dependent on no. of subunits on each fase
helical symmetry - rod viruses
- nucleic acid filments bind to capsomers which hold genome togeter
what is the composition and role of the lipid envelope?
- made of lipid bilayer with glycoproteins from host (viral encoded)
- can take glycoproteins from host cell
role: allows entry into host cells - fusion and endocytosis
what are examples of complex viruses?
Bacteriophages:
- contain a mixture of isosahedral and filamentous structures
- tail can contract and disperse genetic material into the cell
Non-symmetricral viruses e.g. poxyviridae (smallpox)
archaea phages: long rod shaped
What is the overview of the Baltimore classification?
Group I, II and VII: DNA viruses
Group III, IV, V and VI: RNA viruses
takes into account:
- nature of the genome: RNA/DNA
- type of RNA/DNA: ss, ds and polarity
- the genome replication mechanism
what do viruses need to be able to invade the host?
positive mRNA
what is the genome and replication machinery of Class I?
- dsDNA
- classical semiconservative replication
- transcribe minus strand to produce +mRNA
what is the genome and replication machinery of Class II?
- ssDNA (+)
- classical semiconservative, synthesise other strand to produce dsDNA. then transcribe minus strand to produce +mRNA
what is the genome and replication machinery of Class III?
- dsRNA
- transcribe the minus strand to produce +mRNA
what is the genome and replication machinery of Class IV?
- ssRNA (+)
- can be directly translated as +mRNA
what is the genome and replication machinery of Class V?
- ssRNA (-)
- minus strand is transcribed to produce +mRNA
what is the genome and replication machinery of Class VI?
- ssRNA (+) retrovirus
- reverse transcribed to produce dsDNA, and then minus strand is transcribed to produce +mRNA
what is the genome and replication machinery of Class VII?
- dsDNA
- transcription followed by reverse transcription
What kind of virus is HPV16 and what effects does it have on human health?
- dsDNA - Class I
- Papillomavirus - warts, cervical cancer
- no lipid envelope
- genome size = 5.3-8kbp
- 90% of cervical, 85% vulvar and 80% vaginal HPV related cancers in women
- 90% genital warts in men and women
What kind of virus is is adeno-associated virus and what effects does it have on human health?
- ssDNA = class II
- adeno-associated virus
- no lipid envelope
- genome size = 5kbp
- harmless to humans
What kind of virus is rotavirus and what effects does it have on human health?
- dsRNA = class III
- reovirus
- no lipid envelope
- genome size = 18-31kbp
- 2 million hospitalisations world wide 1986-2000
- rotavirus vaccine protects against 90% of the strains in UK
What kind of virus is aphtovirus and what effects does it have on human health?
- (+) ssRNA = class IV
- foot and mouth disease virus
- no lipid envelope
- genome size = 7.5kbp
- over 4 million cattle killed in 2001
What kind of virus is the flu virus and what effects does it have on human health?
- (-) ssRNA = class V
- influenza virus
- has a lipid envelope
- genome size = 12-15kbp
- 1918 pandemic killed 20-100 million
- killed 3-5% of world population 1918-20
What kind of virus is AIDS and what effects does it have on human health?
- reverse (+) ssRNA retrovirus = class VI
- HIV
- has a lipid envelope
- genome size = 9.7kbp
- causes AIDS which is progressive failure of immune system, kill CD4 cells, macrophages and dendritic cells
- 37 million have died from HIV to date
What kind of virus is cirrhosis and what effects does it have on human health?
- reverse dsDNA = class VII
- hepatitis B virus - causes liver cancer
- has a lipid envelope
- genome size = 3.1kbp
what is the life cycle of bacteriophages?
- attachment: tail fibres interact with cell-surface proteins and form irreversible contact
- Genome injection: contractile tail injects genome through hole in envelope into cytoplasm via hydrolysis of peptidoglycan
- production of nucleic acids and proteins via epistome which uses host machinery
- maturation: progeny viruses assemble in cytoplasm
- release: phage proteins destabilise membrane and form endolysin pores in peptidoglycan layer to leave the cell
what are temperate phages?
- they can enter virulent stage or dormant stage
- stage 1 and 2 (attachment and injection) are the same
- genome integrates into host cell and is maintained over generations
- phages can multiply in non-harmful way
- they can revert to virulent phase by stress which causes cell machinery to enter lytic process
how do DNA viruses replicate?
- virus has a capsid (no envelope)
- capsomers interact with cell surface proteins and virus is engulfed by endocytosis
- viruses are uncoated and release genome into nucleus for transcriptioon of +mRNA
- viral mRNA enters cytoplasm for translation to form viral proteins
- virions assemble and are released
how do RNA viruses replicate?
- viral RNA enters cytoplasm and can be transcribed to form correct polarity, or directly used in translation if +
how do retroviruses (class VI) replicate?
- viruses attach to host cell membrane via glycoproteins
- ssRNA genome enters the cell with reverse transcriptase, integrase and protease
- reverse transcriptase copies viral RNA into dsDNA
- integrase transports viral DNA into chromosomes which can then remain dormant
- viral DNA is transcribed to produce viral mRNA which is translated into viral proteins to form more HIVs
how are dsDNA viruses packaged?
- encapsulation is coupled with maturation of the replicative genome
how are ssDNA viruses and retroviruses packaged?
- electrostatic interactions between capsid proteins and RNA genome
- promote cooperative assembly of capsomers
how are fragmented genomes packaged?
- genomic fragments in complex with proteins are in pairs before encapsulation
