virus replication

Question 1

what are the three phases of viral replication?

Answer 1

initiation of infection, replication + expression, and release of mature virions

Question 2

which two experiments provided evidence for viral replication?

Answer 2

Ellis &; Delbruck 1939 single burst experiment, and Hershey &; Chase 1952

Question 3

how did Ellis & Delbruck provide evidence for viral replication?

Answer 3

1- added bacteriophage particles to a culture of rapidly dividing bacterial cells
2- diluted culture after a few minutes to synchronise the cells
3- took samples and analysed them by plating on agar and bacterial lawn
the results showed stepwise increase in phage concentration

Question 4

how do you calculate the total phage titre including intracellular phages?

Answer 4

disrupt the bacteria with chloroform

Question 5

what is meant by eclipse period?

Answer 5

after dilution of culture, there is a period of 10-15 mins where no phage is detectable

Question 6

what is the latent period?

Answer 6

the time before the first extracellular virus particles appear

Question 7

what is attachment?

Answer 7

specific binding of a virus attachment protein to a cellular receptor molecule

Question 8

what can host cell receptors be?

Answer 8

proteins or carbohydrate side chains on glycoproteins or glycolipids

Question 9

why are protein host cell receptors more specific?

Answer 9

the same arrangement of sugar side chain may be prsesent on multiple types of cell

Question 10

how does penetration differ from attachment?

Answer 10

usually energy dependent

Question 11

by which mechanisms can penetration occur?

Answer 11

translocation of the entire virus across the cytoplasmic membrane - rare
endocytosis of the virus into intracellular vacuoles - common
fusion of the virus envelope with the cell membrane

Question 12

what is meant by uncoating?

Answer 12

virus capsid is completely or partially removed and the virus genome is exposed, usually in the form of a nucleoprotein complex

Question 13

what is the role of virus fusion proteins?

Answer 13

promote fusion between virus envelope and the endosomal envelopes

Question 14

what is assembly?

Answer 14

involves collection of all the components necessary for the formation of the mature virion

Question 15

what are lipid rafts?

Answer 15

membrane microdomains enriched with glycolipids, cholesterol and a specific set of associated proteins

Question 16

what are inclusion bodies?

Answer 16

subcellular components full of newly synthesised structural components

Question 17

what is maturation?

Answer 17

the stage in the replication cycle where the virus becomes infectious. usually involves structural changes in the virus particle that may result from specific cleavages of capsid proteins

Question 18

why is it desirable for a virus to encode its own proteases for maturation?

Answer 18

cell proteases can lack specificity and destroy the whole capsid

Question 19

what is budding?

Answer 19

the process by which enveloped viruses leave the cell, they acquire their envelope from the host cell membrane or an intracellular vesicle

Question 20

how does genome replication occur for class I viruses?

Answer 20

with the exception of poxviridae, replication is exclusively nuclear

Question 21

how does genome replication occur for class II viruses?

Answer 21

replication occurs in the nucleus and involves the formation of a ds intermediate which serves as a template for the synthesis of single stranded progeny

Question 22

how does genome replication occur for class III viruses?

Answer 22

these viruses have segmented genomes, each segment is transcribed separately to produce individual monocistronic mRNAs

Question 23

how does genome replication occur for class IV viruses with polycistronic mRNA?

Answer 23

genomic RNA forms mRNA which is translated into a polyprotein and cleaved

Question 24

how does genome replication occur in class IV viruses with complex transcription?

Answer 24

two rounds of translation of subgenomic RNAs are necessary to produce genomic RNA

Question 25

how does genome replication occur in class V viruses with non-segmented genomes?

Answer 25

the first step is transcription of the - sense RNA genome by the virion dependent RNA polymerase to produce monocistronic mRNAs, these also serve as the template strand for subsequent

Question 26

what occurs during viral assembly?

Answer 26

• involves collection of all the components necessary for the formation of a mature virion
• the basic structure of the virus particle is formed

Question 27

what occurs during viral maturation?

Answer 27

-the virus becomes infectious, this usually involves structural changes in the virus particle which originate from specific cleavages of capsid proteins

Question 28

why is it important for maturation that viruses encode their own proteases?

Answer 28

relying on host proteases could cause the entire capsid to be destroyed

Question 29

what are the two methods in which a virus is released?

Answer 29

lysis - infected cell breaks open

budding - enveloped viruses acquire their lipid membrane as the virus buds out of the cell

Question 30

what occurs during the attachment stage of virus replication?

Answer 30

the virus binds to a host cell receptor