6. Viruses 3 Flashcards
What are some examples of (-) strand RNA viruses of animals?
-Include rabies, influenza and Ebola
The (-) strand encodes the genome but it cannot directly act as the mRNA
Describe the replication of rabies? (3)
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
What is the role of the enzyme RNA polymerase in rabies replication? (2)
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
What is the structure of influenza? (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
Describe influenza replication? (3)
- 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
Why is an annual vaccination required?
How do pandemics occur?
- Highly immunogenic proteins on surface of influenza virus
- These can mutate
- Genome structure also affects this
How does Antigenic drift arise?
• antigenic drift arises from mutations in the genes for the surface proteins
What are the main immunogenic regions of the virus?
• the surface proteins are the main immunogenic regions of the virus
What does Antigenic shift do?
And what does it lead to?
- 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
What is the origin of pandemics and epidemics?
Antigenic shift
Why does influenza require two different spike proteins?
More combinations of surface proteins
What type of virus is HIV?
a retrovirus
What is a retrovirus?
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
Describe the structure of HIV? (3)
- The genome has 2 strands of ssRNA
- The genome is replicated through a DNA intermediate
- This requires a reverse transcriptase
Describe the structure of the HIV genome?
Retroviruses have 2 identical ss (+) RNA strands
What is the purpose fo different parts of the HIV genome? (4) R-terminal repeats gag region pol env
- 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
Describe the process fo HIV replication? (6)
- 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
Describe HIV gene expression? (4)
- 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
What do retroviruses require for replication through a DNA intermediate step?
Reverse transriptase
Where do DNA viruses of eukaryotes replicate?
all replicate in the host cell nucleus (except pox)
What can gem polymer viruses such as SV40 induce?
tumours
What does SV40 stand for?
Simian virus 40
What is the genome of SV40 like?
one dsDNA circle
What is SV40 extensively used as?
a vector for moving genes into eukaryotic cells
has no viral encoded enzymes
What does SV40 genome encode?
SV40 genome encodes early and late proteins
How does SV40 encode early and late proteins?
- 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
How does the SV40 genome have overlapping genes? (2)
- the introns are excised out
* the expressed T antigen protein binds to the origin of replication to initiate genome synthesis
Describe SV40 replication? (2)
- 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
What are SV40 coats encoded by?
SV40 coat proteins are encoded by the late region
How are SV40 coat proteins are encoded by the late region?
- 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
Describe the principles of Coronaviruses?
- 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
Describe the structure of coronaviruses?
- Enveloped virions
- Glycoprotein spikes give “crown” (corona)-like a appearance
- Largest known RNA viruses with ~30 kb
Describe the infection cycle in Coronaviruses? (6)
- 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
What are strategies adopted by viruses to cope with having a small genome?
Use of a host
