2-2 Virus Life Cycle

Question 1

How are viruses visible, and what might one see?

Answer 1

Viruses are visible using inverted microscopes; they are NOT visible by light microscopy.

A viral infection may cause cytopathic effects (CPE) that can be used to study virus replication and infectivity (though not all viruses cause CPE).

Question 2

What are the levels of recognition and specificity for viruses with regard to their hosts?

Answer 2

• Host range: preferred species
• Tissue tropism: preferred cell type
• Susceptibility: cells that encode the receptor that recognizes a certain virus
• Permissiveness: cells w/ conditions that support virus replication and virion synthesis

An abortive infection occurs when replication is incomplete, possibly due to a lack of matching recognition/specificity between a virus and its host.

Question 3

What do viruses need to grow?

Answer 3

• The right host (tropism)
• Cells with the right receptors (susceptible)
• Appropriate intracellular environment (permissive)
• Biosynthesis machinery
• Abundant building blocks: nucleotides (RNA, DNA), AAs, ATP, lipids, sugars, etc.
• Time to finish replication

Question 4

What are the steps of virus replication?

Answer 4

1. Recognition of the target cell
2. Attachment
3. Entry (penetration/fusion)
4. Uncoating
5. Transcription of mRNA
6. Protein synthesis
7. Replication of the genome
8. Assembly of virions
9. Egress (lysis, budding, exocytosis)

Question 5

Explain this picture.

Answer 5

The picture represents the steps of viral replication:

1. Recognition of the target cell
2. Attachment
3. Entry (penetration/fusion)
4. Uncoating
5. Transcription of mRNA
6. Protein synthesis
7. Replication of the genome
8. Assembly of virions
9. Egress (lysis, budding, exocytosis)

Question 6

How do viruses recognize and attach to tissues?

Answer 6

Recognition: interactions between virions and tissues

Attachment: a virion surface molecule binds to its specific cellular receptor.

ex) Herpesvirus recognizes the extracellular matrix, then attaches to specific protein receptors (below)

Question 7

How do viruses enter cells?

Answer 7

Virions may use multiple routes to enter cells, depending on cell type. The form of entry may have consequences for disease outcome. Two methods are:

1. Penetration: engulfment of entire virion into cell via receptor-mediated endocytosis (below: a), pinocytosis, or phagocytosis
2. Fusion: virion envelope fuses w/ plasma membrane, leaving parts of the virion behind (below: b)

Question 8

How do viruses release their genomes into the cell?

Answer 8

Via uncoating, which marks the beginning of the “eclipse phase.” For infection to begin, capsids must open to release the genome into the cytoplasm or nucleus. Along with entry, uncoating can occur:

• At the plasma membrane
• Within endosomes
• At the nuclear pore

Question 9

What is involved in viral transcription?

Answer 9

• All viruses must make mRNA
• Viral genome is the template for transcription
• Viral and host transcription factors regulate mRNA synthesis
• mRNA is made by viral or host polymerases
• Lots of opportunity for splicing and RNA silencing

Question 10

How do viruses synthesize protein? What kinds of protein are made, and what happens to them?

Answer 10

Viral mRNAs are translated into protein by the host machinery: ribosomes, tRNAs, amino acids.

Viral proteins are then sorted to site of virion assembly, using the high level of matching recognition/specificity between the host and viral genes:

• Capsid proteins interact with the newly made genomes
• Membrane proteins traffic through the secretory pathway (rough ER, Golgi, transport vesicle)
• Cytosolic proteins accumulate next to the membrane

Question 11

How do viruses replicate their genomes?

Answer 11

Viral genomes come in many types:

• RNA/DNA
• Double-/single-stranded
• (+)/(-) sense
• Linear/circular, segmented, sealed ends, etc.

Polymerases make new genomes using host cell nucleotides:

• DNA viruses use viral or host DNA Polymerase → host cell RNA Pol II (RNAP II)
• RNA viruses use viral RNA-dependent RNA Polymerase (RDRP) to make mRNA and genomes

Question 12

How do viruses assemble virions?

Answer 12

Capsid assembly:

• Capsid proteins are usu. made late during infection
• Icosahedral and helical capsids self-assemble
• Complex capsids: genomes coated w/ nucleoproteins

Virion envelopment:

• Enveloped viruses acquire a membrane from a cellular source (ER, Golgi, plasma membrane)
• Viral and cellular proteins are sorted to site of envelopment: membrane proteins → secretory pathway, cytosolic proteins accumulate next to the membrane

Assembly with envelopment

• Capsid assembly can occur at the same time as envelopment
• All virion components accumulate at the site of capsid formation, genome incorporation, matrix, glycoproteins, and envelopment

Marks the end of the “eclipse phase.”

Question 13

How do viruses release virions?

Answer 13

Question 14

What is the single-step virus growth curve?

Answer 14

• Eclipse: no virus is recovered during the replication and assembly phases
• Maturation and release: virus particles are made and can infect other cells
• Burst size: # of infectious viral progeny from a single round of replication