Virus Life Cycle

Question 1

infected cells

Answer 1

• cytopathic effects
• viruses not visible by light microscopy
• infection may cause CPE that can be used to study virus replication and infectivity
• not all viruses cause this

Question 2

key concepts about viruses

Answer 2

• obligate intracellular
• no genes for biosynthetic pathway-protein/lipid synthesis
• assemble from host cell molecules
• evolve rapidly compared to host

Question 3

overview of replication

Answer 3

virus needs:

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

Question 4

steps in virus replication

Answer 4

1. recognition of 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

Answer 5

• recognition and attachment
• recognition is interaction between virions and tissues-sensing the environment
• attachment is binding of virion surface to its specific cellular receptor

Question 6

host range

Answer 6

-the preferred species

Question 7

tissue tropism

Answer 7

-preferred cell type

Question 8

susceptible

Answer 8

• cells that a virus can enter

- have specific receptors

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

Answer 11

entry

• virions may use multiple routes to enter cells
• varies by cell type
• may have consequences for disease outcome
• penetration
• fusion

Question 12

penetration

Answer 12

• engulfment of entire virion into cell
• receptor mediated endo
• pinocytosis
• phagocytosis

Question 13

fusion

Answer 13

-virion envelope fuses with PM, leaving parts of virion behind

Question 14

step 4

Answer 14

uncoating

• release of genome into cell
• for infection to begin, capsids must open to release genome into the cytoplasm or nucleus
• marks the beginning of the eclipse phase
• direct penetration or membrane fusion
• can uncoat at PM or within endosomes or at nuclear membrane

Question 15

step 5

Answer 15

transcription of mRNA

• all viruses must make mRNA
• viral genome is template for transcription
• viral and host transcription factors regulate mRNA synthesis
• mRNA is made by viral or host polymerases

Question 16

DNA vs RNA viruses

Answer 16

• DNA can go to transcription then translation

- RNA viruses need RDRP to make copies of RNA for transcription

Question 17

step 6

Answer 17

protein synthesis

• 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 newly made genomes
• membrane proteins traffic through the secretory pathway
• cytosolic proteins accumulate next to the membrane
• then the virion is assembled

Question 18

step 7

Answer 18

replication of the genome

• viral genomes come in many types-ss/ds RNA/DNA, + or - sense, linear, circular, segmented, sealed ends
• polymerases make new genomes using host cell nts
• RDRP, viral DNA pol, host DNA pol, Host RNA pol II

Question 19

DNA viruses

Answer 19

• use viral or host DNA pol (for replication)

- use host RNA pol II (for transcription)

Question 20

RNA viruses

Answer 20

-use RDRP to make mRNA and genomes (for replication and transcription)

Question 21

step 8

Answer 21

virions assemble

• capsid proteins usually made late in infection
• icosahedral and helical capsids self assemble
• complex capsids are made of genomes coated with nucleoproteins
• some capsids mature outside the cell
• envelopment
• end of eclipse phase

Question 22

virion envelopment

Answer 22

• enveloped viruses acquire a membrane from a cellular source-ER, Golgi ,PM
• viral and cellular proteins are sorted to site of envelopment-membrane through secretory and cytosolic next to membrane

Question 23

assembly with envelopment

Answer 23

• 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 envelope

Question 24

step 9

Answer 24

egress of virions

• budding
• exocytosis
• lysis
• cell to cell spread
• may transfer to new cells by fusion–> syncytium (multinucleated cell)

Question 25

single step virus growth curve

Answer 25

• 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- number of infectious viral progeny from a single round of replication
• see graph

Question 26

cell lysis

Answer 26

• produces virus plaques
• lysed cells appear clean when cell monolayers are stained with dye
• infectious virions are measured in PFUs- plaque forming units