6. Viruses 3

Question 1

What are some examples of (-) strand RNA viruses of animals?

Answer 1

-Include rabies, influenza and Ebola

The (-) strand encodes the genome but it cannot directly act as the mRNA

Question 2

Describe the replication of rabies? (3)

Answer 2

A viral RNA replicase is essential and is carried by the virion
• 2 distinct classes of RNA are transcribed

-Parental RNA gets into host cell cytoplasm, fusing with host cell membrane in vesicle

• Host doesn’t know how to replicate RNA
• Enzyme is used (RNA polymerase)

Packaged into virions and kill cell

Question 3

What is the role of the enzyme RNA polymerase in rabies replication? (2)

Answer 3

Has 2 jobs

1) To copy - stand into + strand to make it a plus strand MRNA,(Makes extra RNA polymerase)
2) Replicates turns + strand back into minus strand, RNA polymerase takes strand into intermediate to turn replicate genomic RNA

Question 4

What is the structure of influenza? (4)

Answer 4

• Virus particle has to carry an RNA polymerase, needs to turn (-) into (+)
• Has envelop around it, when pinched off and released by host cell
• RNA endonuclease is there can cut into RNA
• Two envelope proteins

-Genome is chopped up into 8 different sections, 8 strands of linear RNA

Question 5

Describe influenza replication? (3)

Answer 5

• the viral nucleic acid replicates in the host nucleus
• the overall pattern of viral genomic RNA synthesis resembles that of rabies i.e. (-) copied to (+) used to generate (-) progeny
• transcription results in viral mRNAs with 5’ caps
• primers are cut from the 5’ ends of newly synthesised capped host mRNAs by the viral endonuclease
• polyA tails are added
• the viral mRNA moves to the cytoplasm for translation

Question 6

Why is an annual vaccination required?

How do pandemics occur?

Answer 6

• Highly immunogenic proteins on surface of influenza virus
• These can mutate
• Genome structure also affects this

Question 7

How does Antigenic drift arise?

Answer 7

• antigenic drift arises from mutations in the genes for the surface proteins

Question 8

What are the main immunogenic regions of the virus?

Answer 8

• the surface proteins are the main immunogenic regions of the virus

Question 9

What does Antigenic shift do?

And what does it lead to?

Answer 9

• in antigenic shift portions of the RNA genome from 2 genetically distinct strains, both infecting a cell, are re- assorted
• this leads to a significantly different combination of surface proteins

Question 10

What is the origin of pandemics and epidemics?

Answer 10

Antigenic shift

Question 11

Why does influenza require two different spike proteins?

Answer 11

More combinations of surface proteins

Question 12

What type of virus is HIV?

Answer 12

a retrovirus

Question 13

What is a retrovirus?

Answer 13

any of a group of RNA viruses which insert a DNA copy of their genome into the host cell in order to replicate, e.g. HIV

Question 14

Describe the structure of HIV? (3)

Answer 14

• The genome has 2 strands of ssRNA
• The genome is replicated through a DNA intermediate
• This requires a reverse transcriptase

Question 15

Describe the structure of the HIV genome?

Answer 15

Retroviruses have 2 identical ss (+) RNA strands

Question 16

What is the purpose fo different parts of the HIV genome? (4)
R-terminal repeats
gag region
pol
env

Answer 16

• The R- terminal repeats are essential for replication
• The gag region encodes structural proteins
• pol encodes the reverse transcriptase and an integrase
• env encodes the envelope proteins that sit in the membrane

Question 17

Describe the process fo HIV replication? (6)

Answer 17

• Fuses with vesicle
• Proteins and nuclease acid is released which means RNA released into cytoplasm
• Reverse transcriptase copies RNA to make a double stranded DNA copy
• These repeats are important to allow genomic DNA of virus to integrate into host cell genome (look like piece of DNA in host cell)
• Can be replicated like normal host cell replication(remains latent) or can be transcribed normal to make mRNA or genomic RNA
• Coated nucleic acid escaped and takes membrane viral proteins with it

Question 18

Describe HIV gene expression? (4)

Answer 18

• in the provirus form, the viral genome may be expressed or it may remain latent
• activation of promoters in the LTR region leads to mRNA transcripts that are capped and polyadenylated
• viral mRNAs are either encapsidated or translated
• similarly to polio, polyproteins are synthesised and then processed

Question 19

What do retroviruses require for replication through a DNA intermediate step?

Answer 19

Reverse transriptase

Question 20

Where do DNA viruses of eukaryotes replicate?

Answer 20

all replicate in the host cell nucleus (except pox)

Question 21

What can gem polymer viruses such as SV40 induce?

Answer 21

tumours

Question 22

What does SV40 stand for?

Answer 22

Simian virus 40

Question 23

What is the genome of SV40 like?

Answer 23

one dsDNA circle

Question 24

What is SV40 extensively used as?

Answer 24

a vector for moving genes into eukaryotic cells

has no viral encoded enzymes

Question 25

What does SV40 genome encode?

Answer 25

SV40 genome encodes early and late proteins

Question 26

How does SV40 encode early and late proteins?

Answer 26

* a single RNA (the primary transcript) is made by the cellular RNA polymerase from the early region * this is processed into 2 mRNAs, both of which are capped - We make copies of circular DNA once we make T antigen - T-antigen kick-starts replication by binding to origin - We can get 3 different proteins from single piece of DNA (VP1,VP2,VP3) - This is due to overlapping genes

Question 27

How does the SV40 genome have overlapping genes? (2)

Answer 27

* the introns are excised out | * the expressed T antigen protein binds to the origin of replication to initiate genome synthesis

Question 28

Describe SV40 replication? (2)

Answer 28

* the genome is too small to encode a viral DNA polymerase- it uses that of the host cell * DNA is replicated in a bidirectional fashion using the host cell machinery

Question 29

What are SV40 coats encoded by?

Answer 29

SV40 coat proteins are encoded by the late region

Question 30

How are SV40 coat proteins are encoded by the late region?

Answer 30

* Transcription of late mRNA begins at a promoter near the origin of replication * This late RNA is then processed by splicing, capping, and polyadenylation to yield mRNA corresponding to the three coat proteins: VP1, VP2, and VP3 * The genes for these proteins overlap- a phenomenon seen in several other small viruses

Question 31

Describe the principles of Coronaviruses?

Answer 31

* Single (+) strand RNA viruses * Replication in cytoplasm (like polio) * Respiratory infections in humans * 15% of common colds but also occasionally fatal such as in 2003 SARS, 2012 MERS and COVID-19

Question 32

Describe the structure of coronaviruses?

Answer 32

* Enveloped virions * Glycoprotein spikes give “crown” (corona)-like a appearance * Largest known RNA viruses with ~30 kb

Question 33

Describe the infection cycle in Coronaviruses? (6)

Answer 33

* Genome has 5’ cap and poly(A) tail and so can act directly as mRNA * However only the replicase is translated * This then generates a (-) strand RNA copy * From the (-) strand, several monocistronic mRNAs are transcribed * These are translated into viral proteins * Progeny genomes are also produced

Question 34

What are strategies adopted by viruses to cope with having a small genome?

Answer 34

Use of a host