Virus Life Cycle

Question 1

How do viruses grow?

Answer 1

• inverted microscopes are used to observe virus and cell cultures
• viruses are NOT visible by light microscopy
• virus infection may cause cytopathic effects that can be used to study virus replication and infectivity (Not all viruses cause CPE)

Question 2

Key concepts about viruses

Answer 2

• obligate intracellular parasites
• no genes for biosynthetic machinery
• assemble from host cell molecules
• evolve rapidly compared to host

Question 3

What does a virus need?

Answer 3

• the right host (tropism)
• cells with the right receptors (susceptible)
• appropriate intracellular environment (permissive)
• biosynthesis machinery
• abundant building blocks- nucleotides (RNA, DNA), amino acids, ATP, lipids, sugars, etc.
• time to finish replication

Question 4

Steps in Virus Replication

Answer 4

1) Recognition of the target cell
2) Attachment
3) Entry: Penetration or 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

Steps 1 and 2: Recognition and Attachment

Answer 5

• recogntion: interactions between virions and tissues
• attachment: binding of a virion surface molecule to its specific cellular receptor
• Entry: Internalization of the virion into the cell. Herpes recognizes the extracellular matrix, then attaches to specific protein receptors

Question 6

Host Range

Answer 6

-the preferred species

Question 7

Tissue tropism

Answer 7

-the preferred cell type

Question 8

Susceptible

Answer 8

-cells that a virus can enter

Question 9

Permissive

Answer 9

-cells that support virus replication and virion synthesis

Question 10

Abortive infection

Answer 10

-replication is incomplete

Question 11

Step 3: Entry

Answer 11

• virions may use multiple routes to enter cells
• varies by cell type
• may have consequences for disease outcome
• penetration: engulfment of entire virion into cell
• receptor-mediated endocytosis
• pinocytosis
• phagocytosis

-fusion: virion envelop fuses with plasma membrane, leaving parts of the virion behind

Question 12

Step 4: Uncoating

Answer 12

• release of the genome into the cell
• for infection to begin, capsids must open to release the genome into the cytoplasm or nucleus
• uncoating marks the beginning of the “eclipse phase”

Mechanisms of entry and uncoating:

• at the plasma membrane
• within endosomes
• at the nuclear pore

Question 13

Step 5: Transcription of mRNA

Answer 13

• all viruses must make mRNA
• viral genome is the template for transcription
• viral and host transcription factors regulate mRNA synthesis- RNA-dependent RNA polymerase (RDRP)
• mRNA is made by viral or host polymerases

Question 14

Step 6: Protein Synthesis

Answer 14

• viral mRNAs are translated into protein by the host machinery- ribosomes, tRNAs, amino acids
• viral proteins are sorted to site of virion assembly
• capsid proteins interact with the newly made genomes
• membrane proteins traffic through the secretory pathway
• cytosolic proteins accumulate next to the membrane

Question 15

Step 7: Replication of the Genome

Answer 15

• viral genomes come in many types
• RNA double or single stranded, + or - sense
• DNA double or single stranded, + or - sense
• Linear, circular, segmented, sealed ends, etc
• polymerases make new genomes using host cell nucleotides
• viral RNA dependent RNA polymerase (RDRP)
• viral DNA polymerase
• host cell DNA polymerase
• host cell RNA Pol II (RNAP II)

Question 16

Viral Replication Enzymes

Answer 16

• DNA viruses use viral or host DNA polymerases

- RNA viruses use viral RNA dependent RNA polymerase to make mRNA and genomes

Question 17

Step 8: Assembly of virions

Answer 17

• marks the end of the “eclipse phase”
• capsid proteins first form an empty shell
• viral DNA is then inserted into the capsids, making them appear dark on this TEM

Question 18

Capsid assembly

Answer 18

• capsid proteins are usually made late during infection
• icosahedral and helical capsids self-assemble
• complex capsids are made of genomes coated with nucleoproteins
• some capsids mature outside of the cell

Question 19

Virion Envelopment

Answer 19

• 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 traffic through the secretory pathway
• cytosolic proteins accumulate next to the membrane

Question 20

Assembly with Envelopment

Answer 20

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

Question 21

Step 9: Egress of virions

Answer 21

• virions are released by: budding from the cell, exocytosis, lysis of the cell, cell-to-cell spread
• virions may transfer to new cells by fusion -> syncytium

Question 22

Routes of virus transfer to new cells: Mechanisms of Egress

Answer 22

• Cell associated virions
• cell-to-cell spread
• cell fusion: syncytium formation

-virions released into extracellular space- lysis, budding, exocytosis

Question 23

How a Virus Kills a cell

Answer 23

1) Virus enters cell (1 virus)
2) Virus in nucleus, duplicates itself (None, eclipse phase)
3) Cell explodes, releasing thousands of brand-new virus particles (>1000, Burst size)

Question 24

Single-step Virus Growth Curve

Answer 24

• 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: the number of infectious viral progeny from a single round of replication

Question 25

Cell lysis produces virus plaques

Answer 25

- virus egress lyses cells - lysed cells appear clear when cell monolayers are stained with dye - infectious virions are measured in PFUs (Plaque Forming Units)

Question 26

Adenovirus Replication Kinetics

Answer 26

- hours 0-12 hours- eclipse period - the intracellular PFU increases from 12 to 28 hours - from 0-16 hours is latent phase - then after 16 extracellular concentration increases