Main classes of virus & mechanism of Viruses

Question 1

What are the stages of virus infecting host cells?

Answer 1

7 stages:
- Attachment
- Penetration
- Uncoating
- Replication
- Assembly
- Maturation
- Release

Question 2

What is attachment?

Answer 2

Specific receptors on host bind the cell w/ viral capsid.
- Reason why viruses usually on infect 1 type of host.

E.g.
- HIV= CD4
- Measles= CD46

Question 3

What is penetration?

Answer 3

• Refers to crossing of the plasma membrane by the virus.

Three main methods:
1. Fusion–attachment to the receptor causes a change in the viral envelope, allowing the membranes to fuse. NOTE: only viruses w/ envelopes can use this method.
4. Endocytosis - receptor-mediated & caveolin-mediated.
3. Phagocytosis

Question 4

What is uncoating?

Answer 4

Enzymes from the virus or host degrade the capsid & release genomic material into host cell cytoplasm.

How does this occur?
- Some viruses expand to form pores in the endosome (they enter in an endosome after endocytosis).
- Other viruses induce fusion of the virion envelope w/ the endosome’s membrane.
- Some do not enter the cell- binding of the capsid to the cell surface receptor induces conformational change that creates a pore in the plasma membrane

Question 5

What is replication?

Answer 5

NOTE: each virus in Baltimore classification has different method of replication.

Based on double stranded (ds) DNA viruses:

1. Transcription of viral DNA into pre mRNA - transcription factors direct the transcription of the genes.
   - Requires RNA polymerase
   - occurs in nucleus in most viruses.
2. Processing of pre mRNA into mRNA - some viruses undergo splicing & others just have a 5’-cap & 3’-poly(A) tail added.
3. Translation occurs

Question 6

What is assembly?

Answer 6

Some of the mRNA produced through replication are translated on host ribosomes into viral proteins.

Proteins are then modified.

Question 7

What is maturation?

Answer 7

• Often involves structural capsid changes.
• Some viruses need to cleave proteins to become infectious e.g. influenza.

Question 8

What is release?

Answer 8

• Lysis
• Bud off – viruses that have envelopes do this

NOTE: view diagram on stages on Notes!

Question 9

How are viruses classified?

Answer 9

Classified into families based on:
- by host
- according to disease or target organ. classification
- according to vector
- by genetic sequence- type of nucleic acid in the genome
- biophysical structure- number of nucleic acid strands & size, structure & symmetry of virus particle.

Question 10

Taxonomy of human viruses?

Answer 10

• Virus Order
• Virus Family
• Subfamily
• Type Species
• Morphology
• Genetic material (DNA viruses or RNA viruses?)
• Envelope
• Mode of replication

Question 11

What is the Baltimore Classification of viruses?

Answer 11

dsDNA- double stranded DNA virsues- small pox & adenovirus

ssDNA- single stranded DNA- hep b

dsRNA- double stranded RNA- rotavirus

ssRNA (+)- positive-sense single stranded RNA viruses

ssRNA (-)- negative-sense single stranded RNA virus

ssRNA- RT- reverse transcribing- singel stranded RNA virus

dsRNA- RT- reverse transcribing- double stranded RNA virus

Question 12

General structure of viruses?

Answer 12

Do not have cell walls, ribosomes or specialised lipids

Classified by genome:
- Single- stranded RNA or double s-stranded RNA
- Single- stranded DNA or double s-stranded DNA

Linear or circular RNA or DNA

Non-enveloped (e.g. bacteriophage)
- Head- contains genome (DNA)
- Neck
- Tail sheath
- Tail fibre
NOTE: more resistant to environment e.g. bile & stomach acid

Enveloped (e.g. Influenza & chicken pox)
- Capsid- contains genome (RNA)
- Phospholipid envelope
- Spikes

NOTE- view images on notes!