Lecture 9 - Viral genetics 3 Flashcards
What mechanism is used in the replication of lambda phage in the lytic pathway?
The rolling circle mechanism
What are examples of (-) strand RNA viruses of animals?
- includes rabies, influenza and Ebola
Describe what the (-) strand does
the (-) strand encodes the genome but it can’t directly act as the mRNA
Describe the properties rabies
- a virus RNA replicas is essential and is carried by the virion
- 2 distinct classes of RNA are transcribed
How is influenza different from polio?
Influenza - (-) strand
Polio - (+) strand
How is influenza different from rabies?
influenza is segmented
the (-) strand RNA genome is segmented
How many linear ssRNA molecules does influenza?
8
How does the RNA in influenza replicate?
RNA us replicated from (-) to (+) to (-) in the host nucleus
How does influenza replicate?
- the viral nucleic acid replicates in the host nucleus
- the overall pattern of viral genomic RNA synthesis resembles that of rabies
- transcription results in viral mRNAs with 5’ caps
- primers are cut from the 5’ ends of newly synthesised host mRNAs by the viral endonuclease
- polyA tails are added
- the viral mRNA moves to the cytoplasm for translation
What are the 2 consequences of changes in the genome of influenza?
- antigenic drift
- antigenic shift
What is antigenic drift?
- the surface proteins are the main immunogenic regions of the virus
- antigenic drift arises from mutations in the genes for surface proteins
- an annual vaccination, mimicking the surface protein epitopes, is required
What is antigenic shift?
- in antigenic drift portions of the RNA genome from 2 genetically distinct stains, both infecting cells are re-assorted
- this leads to a significantly different combination of surface proteins
- antigenic shift is the origin of pandemics & epidemics
What is a feature of influenza that arises because of the segmented genome?
antigenic shift
What is HIV?
a retrovirus, with a genome that has 2 strands of ssRNA (+). The genome is replicated through a DNA intermediate
What does HIV require?
reverse transcriptase - the virus tries to convert its RNA into DNA
What is essential for replication in HIV?
R-terminal repeats are essential for replication
What are the 3 regions of the HIV genome?
- gag region
- pol
- env
What does the gag region do?
encodes structural proteins
What does the pol region do?
encodes the reverse transcriptase and an integrase
What does the env region do?
encodes the envelope proteins that sit in the membrane
Describe HIV replication
- Virus enters cell
- genomic RNA enters the host cell. The reverse transcriptase then copies the RNA into double stranded DNA (host cell would never do it if it wasn’t infected by HIV)
- The DNA then goes into the nucleus and is integrates into the host cell
- This is called a provirus
- it can then be transcribed into mRNA
- the mRNA can then be translated to make viral proteins or can form copies of the genome, become packaged to form nucleocapsid
- It is then released from the cell by stealing some of the cell membrane
- The viral membrane spike proteins are encoded and inserted in the membrane before encapsulation and release occurs.
- This means that it can remain as a provirus (non active & dormant) or make virions
Describe HIV gene expression
- 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 called and polyadenylated
- viral mRNAs are either encapsulated or translated
- similarly to polio, polyproteins are synthesised and then processed
What do retroviruses require?
a reverse transcriptase for replication through a DNA intermediate step.
What is Simian virus 40 an example of?
a virus with overlapping genes
