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Genetics L1 > Lecture 9 - Viral genetics 3 > Flashcards

Lecture 9 - Viral genetics 3 Flashcards

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1
Q

What mechanism is used in the replication of lambda phage in the lytic pathway?

A

The rolling circle mechanism

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2
Q

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

A
  • includes rabies, influenza and Ebola
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3
Q

Describe what the (-) strand does

A

the (-) strand encodes the genome but it can’t directly act as the mRNA

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4
Q

Describe the properties rabies

A
  • a virus RNA replicas is essential and is carried by the virion
  • 2 distinct classes of RNA are transcribed
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5
Q

How is influenza different from polio?

A

Influenza - (-) strand
Polio - (+) strand

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6
Q

How is influenza different from rabies?

A

influenza is segmented
the (-) strand RNA genome is segmented

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7
Q

How many linear ssRNA molecules does influenza?

A

8

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8
Q

How does the RNA in influenza replicate?

A

RNA us replicated from (-) to (+) to (-) in the host nucleus

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9
Q

How does influenza replicate?

A
  • 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
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10
Q

What are the 2 consequences of changes in the genome of influenza?

A
  • antigenic drift
  • antigenic shift
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11
Q

What is antigenic drift?

A
  • 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
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12
Q

What is antigenic shift?

A
  • 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
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13
Q

What is a feature of influenza that arises because of the segmented genome?

A

antigenic shift

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14
Q

What is HIV?

A

a retrovirus, with a genome that has 2 strands of ssRNA (+). The genome is replicated through a DNA intermediate

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15
Q

What does HIV require?

A

reverse transcriptase - the virus tries to convert its RNA into DNA

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16
Q

What is essential for replication in HIV?

A

R-terminal repeats are essential for replication

17
Q

What are the 3 regions of the HIV genome?

A
  • gag region
  • pol
  • env
18
Q

What does the gag region do?

A

encodes structural proteins

19
Q

What does the pol region do?

A

encodes the reverse transcriptase and an integrase

20
Q

What does the env region do?

A

encodes the envelope proteins that sit in the membrane

21
Q

Describe HIV replication

A
  1. Virus enters cell
  2. 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)
  3. The DNA then goes into the nucleus and is integrates into the host cell
  4. This is called a provirus
  5. it can then be transcribed into mRNA
  6. the mRNA can then be translated to make viral proteins or can form copies of the genome, become packaged to form nucleocapsid
  7. It is then released from the cell by stealing some of the cell membrane
  8. The viral membrane spike proteins are encoded and inserted in the membrane before encapsulation and release occurs.
  9. This means that it can remain as a provirus (non active & dormant) or make virions
22
Q

Describe HIV gene expression

A
  • 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
23
Q

What do retroviruses require?

A

a reverse transcriptase for replication through a DNA intermediate step.

24
Q

What is Simian virus 40 an example of?

A

a virus with overlapping genes

25
Q

Describe the genome of SV40

A

one dsDNA circle
- with host cell histones
- it has no viral-encoded enzymes

26
Q

What is the primary transcript made into?

A
  • a single RNA (the primary transcript) is made by the cellular RNA polymerase from the early regions
  • it is then processed into 2 mRNAs, both of which are capped
  • the introns are excised out
  • the expressed T antigen protein binds to the origin of replication to initiate GENOME SYNTHESIS
27
Q

How is the genome then encoded?

A
  • 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
28
Q

Describe the transcription of late mRNA

A
  • 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 3 cost proteins: VP1, VP2 and VP3
  • the genes for thee proteins OVERLAP - phenomenon seen in several other small viruses
29
Q

What are coronaviruses?

A
  • 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
30
Q

Describe the structure of coronaviruses

A
  • enveloped virions
  • glycoproteins spikes give ‘give’ (corona)-like appearance
  • largest known RNA viruses with approximately 30kb
31
Q

Describe the infection cycle of coronavirus

A
  • genome has 5’ and poly(A) tail and so can act directly mRNA
  • however only the replicate is translated
  • this then generates a (-) strand RNA copy
  • from the (-) strand, several MONOCISTRONIC mRNAs are transcribed
  • these translated into viral proteins
  • progeny genomes are also produced
32
Q

What is rabies?

A

a virus RNA replicas is essential and is carried by the virion

Genetics L1 (19 decks)

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