viral latency

Question 1

what is viral latency

Answer 1

when viruses remain in the host without proliferating and causing damage to the cells
it is present in the body but exists in a resting state

Question 2

what part of the viral life cycle does viral latency lay

Answer 2

the lysogenic part

Question 3

what is the lytic cycle

Answer 3

when circular dsDNA is replicated, transcirbed and translated into proteins to make the components of viral progeny which is then released by lysis

Question 4

what is the lysogenic cycle

Answer 4

when circular dsDNA is integrated into the chromosome forming a prophage which is replicated with the host chromosome and passed to daughter cells until chnages in the environment excise it and the lytic cycle begins

Question 5

what are the types of latency

Answer 5

1) integrated/proviral
2) episomal

Question 6

what is integrated/proviral latency

Answer 6

when the viral genome gets into the nucleus and inserts itself into the host genome

Question 7

what are the advantages and disadvantages of integrated latency

Answer 7

+automatic host cell division results in replication
+ near impossible to remove without killing the cell
-needs to enter the nucleus
-increased difficulty to remain latent

Question 8

what is an episome

Answer 8

an extrachromosomal element which replicates alongside the host genome

Question 9

what occurs in episomal latency

Answer 9

the virus circulates the cell in an episome
latency associated nuclear antigens tether the episome to the cell chromosome
the viral episome replicates once in concert with cell DNA
it is replicated but new viral particles are not produced and the infected cell dies

Question 10

what are the advanatges and disadvantages of episomal latency

Answer 10

+ easier to maintain and reactivate that proviral latency
- more exposure to cell defences leading to possible degredation of virus by cellular enzymes

Question 11

what is an acute infection

Answer 11

when the virus infects the host and takes over the cellular machinery

Question 12

what is latent infection

Answer 12

when the viral genome is replicated with the host genome but the viral gene expression is silenced

Question 13

what is reactivation

Answer 13

acute expression may occur later resulting in symptoms appearing long after the virus is acquired

Question 14

what are the different outcomes of a viral infection

Answer 14

1) persistant infection
2) transformation

Question 15

what is a persistent infection

Answer 15

when the virus replicates at a rate the immune system can tolerate but at some point the immune system wears down allowing increased replication leading to symptoms

Question 16

what is meant by transformation

Answer 16

when a virus carries a gene which promotes cell proliferation such as onconogenes whcih integrate into the host genome and produce tumours

Question 17

why do viruses remain latent

Answer 17

1) avoid killing the cell allowing prolonged survival
2) keep gene expression undercontrol

Question 18

what is apoptosis

Answer 18

programmed cell death

Question 19

outline how the herpes virus invades the mucous membrane

Answer 19

1) comes in contact with the membrane
2) binds to epithelial cells
3) viral proteins change conformation which releases energy driving the fusion of viral and host membrane
4) a fusion pore is generated and the viral capsid slips into the cytoplasm

Question 20

how does the herpes virus generate new virions in the epithelial cells

Answer 20

1) capsid reaches the nucleus
2) capsid docks in a nuclear pore and delivers DNA
3) numerous lytic cycles result in a blister on the skin

Question 21

how does the herpes virus reach the nucleus

Answer 21

1) host dynein motor proteins transport the virus towards the minus end of the microtubule to the microtubule organsising centre
2) once there the kinesin motor proeins move the virus to the plus ends of another microtubule and to the nucleus

Question 22

what are the to transport proteins associated with the entry of the herpes virus to the nucleus

Answer 22

1) dynein
2) kinesin

Question 23

how does the herpes virus deliver DNA

Answer 23

1) DNA circularises
2) recrutes factors which silence viral DNA to prevent inferon cell receptors recognising it and triggering cell mediated immunity

Question 24

how do HSV lesions reappear again and again

Answer 24

1) once epithelial cells are infected HSV travels to the neurons which innervate these mucous membranes
2) here they become latent by recruiting chromatin assembly factors to pack DNA
3) it becomes repressed expect for the gene which codes for latency associated transcript
4) it is reactivated when host cells sense stress which reinitiates viral gene expression and therefore replication

Question 25

what does latency associated transcript do

Answer 25

stimulates heterochromatin packaging and prevents the cell triggering apoptosis

Question 26

what are syncytins

Answer 26

• retroviral envelope proteins which allow cell to cell fusion and adopted by the host for their ability to promote development of the placenta

Question 27

what is an oncogene

Answer 27

a mutated gene which contributes to the development of cancer, its unmutated state is referred to as an proto-oncogene which plays a role in cell division

Question 28

what are the three genes which code for a retro virus

Answer 28

1) gag= codes for structural virion proteins
2) pol= codes for reverse transcriptase
3) env= codes for envelop glycoproteins
4) a long terminal repeat which is a non-coding sequence at the end of the genome

Question 29

what is the LTR needed for in retroviruses

Answer 29

integration into the host chromosome as well as encoding gene expression

Question 30

what is the difference between a normal retrovirus and a cancer causing retrovirus

Answer 30

transforming viruses are vairents where part of the viral sequence has been replaced by an oncogene (the pol and env gene)

Question 31

outline how onocogenes are aquired by retroviruses

Answer 31

1) viruses integrate near a host oncogene
2) it is integrated and packed into the virion
3) virus infects a new host carrying onco gene with it

Question 32

what is a viroid

Answer 32

not a virus
- smaller in size
- not enclosed in a protein shell
- causes disease by inducing RNA silencing of host mRNA alterimg gene expression
- only causes disease in plants

Question 33

what is a prion

Answer 33

not a virus
- proteins whihc undergo conformational transition into an insoluable form
- they accumulate in the brain cells causing brain impairments
- resistant to proteases and high temps