2 Viruses (Host Pathogen Interactions)

Question 1

Virus - What are they?

Answer 1

• Obligate parasites: require host cell to replicate
• Contains RNA/DNA and can be single/double stranded
• Capsid (protein coat) can be naked or enveloped (container)
• Uses host cell for translation machinery

Question 2

Viral Structures

Answer 2

• Protomer: single repeating polypeptide
• Capsomer: multiple polypeptides
• Helical (hydrogen bonds used for assembly): Protomers, capsid protein shell
• Icosahedral: 20 equilateral triangles, capsomer or protomers, capsid protein shell
• Enveloped: lipid containing membrane around nucleocapsid (capsid w/genomic material); envelope is host w/viral proteins

Question 3

Viral Replication

Answer 3

• Eclipse period: no longer infectious
• Exponential growth period: virions produced until plateau reached
• Tropism: Viruses exhibit species/organ/cell-type specificity mediated by cell receptors
• Adsorption: proteins on surface of viruses (capsomer, glycoproteins)

Question 4

Viral Penetration

Answer 4

• Receptor mediated endocytosis: naked viral particle binds to receptor causing membrane invagination; endocytic vesicle formed; virion released into cytosol
• Membrane fusion: enveloped virus attaches to receptor and fuses with host membrane; nucleocapsid enters cell

Question 5

DNA Virus Replication

Answer 5

• Early genes: genome replication, regulation of gene expression, modulation of host cell metabolism
• Late genes: structural proteins
• Most DNA viruses replicate in nucleus; most RNA viruses replicate in cytosol

Question 6

Type I: (+) ssRNA Viral Replication

Answer 6

• Functions as mRNA to translate viral proteins, including RNA-dependent RNA polymerase
• Serves as template for complementary (-)ssRNA synthesis w/RNA dependent RNA polymerase
• (-)ssRNA becomes template for (+)ssRNA

Question 7

Type II: (-) ssRNA Viral Replication

Answer 7

• Transcription of (+)ssRNA from (-)ssRNA using RNA-dependent RNA polymerase carried inside the virus particle
• Translation of viral protein from (+)ssRNA and transcription of (-)ssRNA from (+)ssRNA template

Question 8

Type III: dsRNA Viral Replication

Answer 8

• (+)ssRNA is transcribed from the dsRNA by RNA-dependent RNA polymerase (transcriptase)
• (+)ssRNA translated to viral proteins. Template for complementary (-)ssRNA synthesis with RNA-dependent RNA polymerase

Question 9

Type IV: (+) ssRNA Viral Replication (retroviruses)

Answer 9

• (+)ssRNA converted to dsDNA by RNA-dependent DNA polymerase (reverse transcriptase)
• Integrated into genome by integrase

Question 10

Viral Release

Answer 10

• Naked viruses released as a passive event

- Enveloped viruses are released by viral budding

Question 11

Effects of infection

Answer 11

• Abortive
• Productive
• Latency
• Cell death