Viruses

Question 1

arguments for viruses being abiotic?

Answer 1

no metabolic activity
non cellular
cant replicate outside host cell

Question 2

what are virions

Answer 2

singular complete infectious particle

Question 3

basic requirements of virion

Answer 3

capsid and genomic material

Question 4

different capsid structures?

Answer 4

icosahedral, helical, complex

can be single or double

Question 5

how are capsids formed?

Answer 5

single pp’s (protomers) assemble into oligemers (capsomeres) which self assemble into capsids

Question 6

different topology of genomic material?

Answer 6

linear, circular, single, segmented

Question 7

can virions have an envelope

Answer 7

yes (from host cell membrane during budding)

Question 8

what can viruses infect

Answer 8

any cell type and other viruses

Question 9

what does host range depend on

Answer 9

attachment receptors and replication factors

Question 10

4 structural classes of viruses

Answer 10

• Helical capsid structures
• Isoahedral capsid structure
• Enveloped viruses
• Complex viruses

Question 11

4 main types of viral genomes?

Answer 11

o Double stranded DNA
o Single stranded DNA
o Ds RNA
o ssRNA

Question 12

differentiate between + and - ssRNA

Answer 12

 (+)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)

Question 13

describes general steps in viral replication

Answer 13

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

Question 14

describe additional steps for retroviruses

Answer 14

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

Question 15

describe additional steps for ssRNA viruses

Answer 15

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

Question 16

describe additional steps for temperate bacteriophages

Answer 16

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

Question 17

treatments for viral diseases

Answer 17

• protease inhibitors
• DNA polymerase inhibitors
• cell surface inhibitors

Question 18

limitation of viral disease treatments

Answer 18

• not many targets

- viral replicases v error prone - can produce mutant RNA which can lead to stuff that meds cant work on

Question 19

what are prions

Answer 19

misfolded proteins that can misfold other proteins

Question 20

examples of prion diseases

Answer 20

o Bovine spongiform encephalopathy (Mad Cow disease)
o Creutzfeldt-Jakob disease, Kuru (humans)
o Scrapie (sheep)

Question 21

what are viroids

Answer 21

small, naked circular ssRNA molecules

-dont encode proteins, but has ribozyme activity and is RNA polymerase II substrate

Question 22

how do viroids work

Answer 22

silence RNA in plants