MCM 2-2 Virus Life Cycle

Question 1

The 5 requirements for virus replication

Answer 1

1. Right host/host range (tropism)
2. Cells with right receptors (susceptible)
3. Appropriate intracellular environment for virus replication/virion synthesis (permissive)
4. available biosynthesis machinery from the host
5. abundant building blocks (RNA, DNA, Amino Acids)

Question 2

General Steps of Viral Replication

Answer 2

1. recognition of target cell
2. attatchment
3. entry (penetration or fusion)
4. uncoating (elipse begins)
5. transcription of mRNA
6. protein synthesis
7. Replication of Genome
8. assembly of virions (eclipse ends)
9. egress (budding, lysis, exocytosis

Question 3

describe the 2 ways of viral entry into host cell

Answer 3

penetration - engulfment of entire virion into cell (receptor-mediated endocytosis, pinocytosis, or phagocytosis)

Fusion - virion envelope fuses with plasma membrane, leaves portion of virion behind

Question 4

describe the uncoating stage

Answer 4

uncoating begins the eclipse phase

release of genome into cell, capsid must open to release genome into cytoplasm or nucleus for infection to begin

Question 5

describe the transcription of viral mRNA

Answer 5

the viral genome is the template

viral and host transcription factors regulate mRNA synthesis

mRNA is made by viral or host polymerases

Question 6

describe viral protein synthesis

Answer 6

viral mRNA’s are translated into protein by the HOST machinery (ribosomes, tRNA’s, amino acids)

Question 7

where are viral proteins sorted?

Answer 7

to the 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

Question 8

Describe the diversity that exists in viral genomes

Answer 8

can either be RNA or DNA that is ds or ss, + or - sense

can be linear, circular, segmented, sealed ends, etc..

Question 9

_________ polymerases make new genomes using _______

Answer 9

Host or Viral Polymerases make new genomes using host cell nucleotides

Question 10

describe the polymerases involved in making new viral genomes

Answer 10

1) Viral RNA-dependent RNA polymerase (RDRP) (used by RNA viruses)
2) Viral DNA polymerase (used by DNA viruses)
3) Host cell DNA polymerase (used by DNA viruses)
4) Host cell RNA Pol II (RNAP II) (used by RNA viruses to make mRNA and genomes)

Question 11

when does the eclipse phase end?

Answer 11

during assembly of virions, when the capsid proteins form an empty shell

Question 12

describe the assembly of virions

Answer 12

this ends the eclipse phase

1. capsid proteins form an empty shell late during infection
2. envelopment

Question 13

describe when and how capsid proteins form an empty shell

Answer 13

Icosahedral and helical capsids self assemble

complex capsids are made from genomes coated with nucleoproteins

some mature outside of the cell

Question 14

describe envelopment

Answer 14

• acquire membrane from cellular source (ER, golgi, PM)
• viral/cellular proteins sorted to site of envelopment
  
  • membrane proteins through secretory pathway
  • cytosolic proteins accumulate at membrane

-concerted assembly with envelopment can also occur (capsid assembly occurs at the same time) - all virion components accumulate at the site of capsid formation, genome incorporation, matrix, glycoproteins, and envelopment
-

Question 15

describe the two types of virion egress

Answer 15

1. for cell-associated virions (cell-cell spread, cell fusion/syncytium formation)
2. for virions released into extracellular space (lysis, budding, exocytosis)

Question 16

The phases of single step virus growth curve

Answer 16

1. eclipse phase - no virus recovered during the replication/assembly phases
2. maturation and release phases - virus particles are made and can infect other cells

Question 17

what is the burst size?

Answer 17

the number of infectious progeny from a single round of replication

Question 18

describe the appearance of cell lysis in viral culture and how this is measured

Answer 18

viral plaques

virus egress lyses cells
lysed cells appear clear when monolayers are stained
infectious virions are measured in PFU’s (plaque forming units)

Question 19

exocytosis of envoloped virions occurs in a burst and lyses the cell (TF)

Answer 19

false