Viruses Flashcards
arguments for viruses being abiotic?
no metabolic activity
non cellular
cant replicate outside host cell
what are virions
singular complete infectious particle
basic requirements of virion
capsid and genomic material
different capsid structures?
icosahedral, helical, complex
can be single or double
how are capsids formed?
single pp’s (protomers) assemble into oligemers (capsomeres) which self assemble into capsids
different topology of genomic material?
linear, circular, single, segmented
can virions have an envelope
yes (from host cell membrane during budding)
what can viruses infect
any cell type and other viruses
what does host range depend on
attachment receptors and replication factors
4 structural classes of viruses
- Helical capsid structures
- Isoahedral capsid structure
- Enveloped viruses
- Complex viruses
4 main types of viral genomes?
o Double stranded DNA
o Single stranded DNA
o Ds RNA
o ssRNA
differentiate between + and - ssRNA
(+)ssRNA (genome sequence the same as mRNA – hence can be directly translated)
(-)ssRNA (genome sequence complementary to mRNA – hence needs to be transcribed first, then translated)
describes general steps in viral replication
o Attachment of virus ligands to cell receptors
o Entry virus
o Capsule dissolved, genetic material released
o Synthesis of new viral components (via transcription and translation)
o Assembly of new viral particles – proteins and genetic material etc to form a virion
o Release of virions (and potential coating)
Host cell may lyse
describe additional steps for retroviruses
o Requries reverse transcriptase, which facilitates reverse transcription from RNA to DNA without multiplying the genome number.
o Then viral derived DNA integrates into cellular genome to be expressed (integrated DNA = provirus)
o Proviral DNA transcribed; provides mRNA (encodes capsid) and genomic RNA for encapsidation
describe additional steps for ssRNA viruses
o Virus coat has ligands for cell receptors
o Entry can involve membrane fusion
o ssRNA genome needs to be replicated
For -ssRNA, capsid contains RNA dependent RNA polymerase to enable transcription: makes mRNA (encodes capsid) and genomic RNA for encapsidation
o Coat proteins picked up from cell membrane
describe additional steps for temperate bacteriophages
o Binds to receptor
o Injects DNA into bacteria
o Phage DNA circularises
Different factors determine whetehr lytic or lysogenic cycle is induced
Lytic
• New phage DNA and proteins are synthesised, and self assemble into phages
• Cell lyses, so phages are released
• Phages inject into bacteria – cycle restarts
Lysogenic
• Phage DNA integrates into bacterial genome (phage DNA now called prophage)
• Bacteria reproduces; copy of prophage is transmitted to daughter cells
treatments for viral diseases
- protease inhibitors
- DNA polymerase inhibitors
- cell surface inhibitors
limitation of viral disease treatments
- not many targets
- viral replicases v error prone - can produce mutant RNA which can lead to stuff that meds cant work on
what are prions
misfolded proteins that can misfold other proteins
examples of prion diseases
o Bovine spongiform encephalopathy (Mad Cow disease)
o Creutzfeldt-Jakob disease, Kuru (humans)
o Scrapie (sheep)
what are viroids
small, naked circular ssRNA molecules
-dont encode proteins, but has ribozyme activity and is RNA polymerase II substrate
how do viroids work
silence RNA in plants
