5. Lecture

Question 1

describe the multiplication cycle of viruses

Answer 1

• adsorption (attachment to the host cell)
• penetration (entry into the host cell)
• decapsidation (nucleic acid release)
• eclipse (expression of the genetic information)
• transcription, translation and nucleic acid replication
• maturation (assembly of progeny virions)
• release (evacuation from the host cell)

Question 2

how many virions/cell can be produced

Answer 2

102 – 106 virion/cell

• > new cells, new cycles
• > log phase

Question 3

cell surface receptor

Answer 3

10⁴⁻⁵ /cell (stable)
genetically coded (ie. H19 chromosome  poliovirus
H3 chromosome  HSV)
•usually already expressed in the embryo, some after birth
•necessary for cellular functions
(ie. CD4 – HIV, acetylcholine receptor – rabies virus)

Question 4

virus surface antireceptor

Answer 4

more variable than the cell surface receptor as they adapt them during their evolution

related viruses usually target the same cell surface receptor

sometimes non-related viruses have the same receptor

Question 5

how is the connection btw. receptor and antireceptor made

Answer 5

both negatively charged -> repulsion, collision by chance

reversible connection -> cations neutralize the electrostatic power -> irreversible connection -> chemical bonds between theproteins

Question 6

penetration: energy use

Answer 6

energy-dependent: 4 × activation energy than adsorption
only possible in living cells
only possible over +4°C

Question 7

types of penetration

Answer 7

translocation
endocytosis
membrane fusion

Question 8

translocation

Answer 8

„trapdoor” mechanism, amorphous RNA-protein complex

Question 9

endocytosis

Answer 9

(most non-enveloped viruses + herpes, pox)

cell nutrition endosome -> phagolysosome -> decapsidation

Question 10

membrane fusion

Answer 10

(only enveloped viruses)
• virual fusion (F) protein induced
• the viral envelope merges into the cytoplasmic membrane of
the host cell; only the nucleocapsid gets into the cytosol

Question 11

alternative forms of penetration

Answer 11

• injection
• sexfimbria
• passive penetration

Question 12

injections

Answer 12

(tailed bacteriophages):

• lysozyme enzyme + contractile proteins

Question 13

sexfimbria

Answer 13

(ribophages):

•lack of cellular wall  „bacterial sexual disease”

Question 14

passive penetration

Answer 14

(plant viruses):

•cellular wall injuries, arthropod bites

Question 15

decapsidation

Answer 15

uncoating:
• Release of the nucleic acid from the capsid
• Dangerous for the virus, but necessary for the transcription (capsid = protection) so NA is reachable

Question 16

strategies of decapsidisation

Answer 16

•Use of cellular proteases (in lysosomes)
->Viral uncoating proteins (ie. poxviruses)

•Partial decapsidation
->hiding the nucleic acid until the early virus protein production (two capsid layer – only outer is decomposed, inner, holes are made to reach double stranded RNA )
(ie. DNA viruses in the nucleus, reo, orthomyxo,
paramyxoviruses)
•Sometimes simultaneous penetration and decapsidation (ie. by translocation, injection)

Question 17

eclipse

Answer 17

The expression and copy of the viral genetic information
•transcription, translation, nucleic acid replication of NA
•different strategies according to the type of the nucleic acid
– the Baltimore-system

Question 18

the baltimore system

Answer 18

virus classification system that groups viruses into families, depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds), etc..) and their method of replication.

Question 19

maturation

Answer 19

Polypeptide → protein
• Glycosylation, dimer formation, antigen development
• at the ER surface or Golgi

Question 20

Virus assembly

Answer 20

• Usually at the site of replication for protection
– RNA viruses + Pox, Asfarviridae in cytoplasm
– other DNA viruses in nucleus
• protein transport – may evolve inclusion bodies for tp.
• nucleic acid signal connects to the capsomers

envelope and matrix proteins are acquired from cellular membranes

Question 21

mistake frequency in virus assembly

Answer 21

frequent

→the production of incomplete virions usually about 70%

Question 22

methods of virus assembly

Answer 22

The nucleic acid enters into the capsid: icosahedral

The capsomers surround the nucleic acid: helical

Question 23

regulation of virus assembly

Answer 23

scaffolding proteins regulate the assembly

Question 24

virus release of non-enveloped viruses

Answer 24

cytolysis

rapid (Picornav.) / slow (Parvov.) / apoptosis (Adenoviridae)

Question 25

virus release of enveloped viruses

Answer 25

Budding
• rapid (Togav., Paramyxov., Rhabdov.) / slow (Arenav., Retrov.)
– Budding site is virus-specific:
• Herpesv.: nuclear membrane
• Flaviv.: ER
• Coronav., Bunyav.: Golgi
• Asfarv., Togav., Orthomyxov.,Paramyxov., Rhabdov.:
cytoplasmic membrane

Question 26

Cell-associated viruses (Gammaherpesvirinae)

Answer 26

virus release at cell death or injuries

virus cannot survive outside cells?

Question 27

virus release by cell fusion

Answer 27

Cell fusion (syncytium formation) -> membrane tunnels
• safe way of spreading (Herpesv., Paramyxov.)

Question 28

alternative forms of virus release

Answer 28

lysosome disintegrations (Herpesviridae)

T bacteriophages: lysozyme enzyme digests the cellular wall -> virus release

• plant cells are firmly connected to each other

• > viruses spread from cell to cell
• > mosaic pattern

Question 29

virus release general

Answer 29

passive or active release