Infectious Agents and the Immune System

Question 1

definition of a virus

Answer 1

packaged set of genes that is inert outside a living cell

capable of invading cells

replicates in cells by disassembly, synthesis of component parts, and reassembly

needs cell for protein and nucleic acid synthesis and energy

Question 2

virion

Answer 2

the extracellular form of a virus

is disassembled to release the viral genome

Question 3

properties of virion

Answer 3

size

shape and symmetry

numbers and sizes of capsomers

enveloped or not

Question 4

properties of the viral genome

Answer 4

DNA or RNA

ds or ss (+ or - sense)

linear or circular

segmented or not

size

Question 5

icosahedral symmetry

Answer 5

roughly spherical with the genome tightly packed within

subunits of the capsid are located around the vertices or face of an icosahedron

20 triangular faces, 30 edges, and 12 verticies

2-, 3-, and 5-fold axes of symmetry

Question 6

helical symmetry

Answer 6

genome is coated with proteins that assume a helical lattice

the sizes of the proteins and subunit interactions will determine the pitch and diameter of the helix as well as the rigidity of the helix

the size of the RNA will in part determine the length of the helix

Question 7

What do virus-cell interactions determine?

Answer 7

pathology

host response

viral evasion

targets for therapy

Question 8

resistant vs. susceptible

Answer 8

resistant cells lack the receptors for viral entry

susceptible cells allow virus to enter and either express genes or establish their genomes inside the cell

Question 9

productive infection

Answer 9

a full viral replicative cycle occurs and viral progeny are produced

necessary to generate enough virus to cause disease

usually associated with cell death- important aspect of pathogenesis

Question 10

abortive infection

Answer 10

viral genes are expressed but infectious progeny do not result, the cell probably dies

Question 11

latent infection

Answer 11

the viral genome is established inside the cell, some genes may be expressed but viral replication does not occur and the cell remains viable despite altered function

Question 12

stages in productive infection

Answer 12

attachment to cell

penetration of cell

disassembly, release of genome

expression of viral genes

replication of genome

assembly of progeny virions

release from cell

Question 13

attachment (adsorption)

Answer 13

birus binds to a cell surgace through interaction of virion proteins with specific cell surface “receptors”

some viruses recognize more than one receptor, permitting use of different receptors on different cell types or alternate routes of entry into a single cell type

Question 14

penetration

Answer 14

energy dependent and requires:

translocation of virus or viral genome across the plasma membrane

fusion of the virion envelope with the plasma membrane

endocytosis of the virus particle followed by translocation across the endosome membrane or fusion with the endosome membrane

Question 15

disassembly, release of genome

Answer 15

the virus particle is further disassembled so that the geonome becomes accessible for translation or transcription and later for genome replication

the viral genome will be delivered to the cell cytoplasm for most RNA viruses and to the cell nucleus for most DNA viruses

Question 16

eclipse or replication

Answer 16

the periods during which viral nucleic acids and proteins are being syntehsized but infectious virus cannot be detected in the infected cell

transcription of viral mRNA

syntehsis of proteins

replication of viral genome

Question 17

assembly

Answer 17

when progeny viral genomes and viral structural proteins have accumulated to sufficient levels, assembly of progeny virions can take place

this occurs in the cell compartment where genome replication occurred

Question 18

release from cell

Answer 18

it is thought that most non-enveloped viruses are released from infected cells only when the cell lyses though the use of cell excretory pathways for release has not been ruled out

release of many enveloped viruses occurs coincident with acquiring the envelope by budding through the plasma membrane

other enveloped viruses acquire an envelope by budding through an internal cell membrane into a secretory compartment and are then secreted from the cell

Question 19

eclipse period

Answer 19

the period between active viral gene expression and genome replication

infectious units do not reappear until the end of the eclipse period when there is sufficient viral genomes and structural proteins for assembly of new virions

Question 20

types of nucleic acids and replication strategies

Answer 20

double stranded DNA

single stranded DNA

double stranded RNA

single stranded (+) RNA

single stranded (-) RNA

single stranded RNA + DNA

double stranded DNA + RNA

Question 21

double stranded DNA virus replication

Answer 21

ex. adenoviruses and herpesviruses

Question 22

same sense mRNA replication

Answer 22

ex. polioviruses

Question 23

single strand negative sense RNA virus replication

Answer 23

need to make + strand template to replicate and make mRNA

ex. orthomyoxoviruses (influenza)

Question 24

RNA retrovirus replication

Answer 24

RNA genome in virons but goes through a double strand DNA intermediate in cells

ex. retroviruses, HIV

Question 25

consequences of productive infection to the cell

Answer 25

production is usually incompatible with cell survival because of cytotoxic effects of viral effects including the ones that give the virus a competitive advantage for cellular biosynthetic machinery

induction of innate responses can result in death of cells

in some inefections such as hepatitis B, the cell doesn’t die but continually releases virus

Question 26

poliovirus protease

Answer 26

cleaves cell cap-binding protein and inhibiting translation of capped mRNAs (poliovirus enters through internal ribosome entry sites)

Question 27

herpesvirus protein

Answer 27

shuts down splicing of mRNAs (most herpesvirus late mRNAs are unspliced)

Question 28

latent infections

Answer 28

comm for many DNA viruses

no viral genome replication except in concert with cell division

few if any viral proteins expressed, those expressed can be non-immunogenic

latent state is compatible with cell survival and normal cell functions though some functions may be altered

latent virus is a reservoir for reactivation of infectious virus