Lecture Set 2 : Part 6

Question 1

what is a virus?

Answer 1

-acellular infectious particle
-obligate intracellular pathogen
-NOT LIVING

Question 2

why is a virus an obligate intracellular pathogen?

Answer 2

-cannot function as a pathogen unless inside a host cell
-lacks independent metabolism
-can only reproduce inside living cells

Question 3

what are the 2 minimum requirements to classify something as a virus?

Answer 3

-nucleic acid genome (DNA or RNA)
-protein coat (capsid) surrounding the genome
-together these form the nucleocapsid

Question 4

what is a virus that only has the 2 minimum components?

Answer 4

-naked virus (non-enveloped)

Question 5

what additionally may surround the nucleocapsid of some virus’?

Answer 5

-envelope

Question 6

what is a virus genome composed of?

Answer 6

-DNA or RNA (cannot be both)
-both can be single stranded or double stranded
-both can be circular or linear
-can be in several pieces (segmented) or in 1 piece

Question 7

what is the smallest virus genome size?

Answer 7

-3.6 kilo-bases for some ssRNA viruses (3 genes)

Question 8

what is the largest virus genome size

Answer 8

-greater than 150 kilo-base-pairs (double stranded) for some dsDNA viruses (greater than 100 genes)

Question 9

what is the purpose of the virus capsid?

Answer 9

-protection of the genome
-allows transfer of viral genome between host cells

Question 10

what is the virus capsid made of?

Answer 10

-identical polypeptides can protomers
-protomers add up to form capsomeres
-can form different shapes

Question 11

what shapes can the virus capsid form?

Answer 11

-helical
-icosahedral
-binal

Question 12

what is a helical capsid?

Answer 12

-protomers form a spiral cylinder with the genome coiled inside

Question 13

what is an example of a virus with a helical capsid?

Answer 13

-tobacco mosaic virus
-made of approx. 2100 identical protomers

Question 14

what is an icosahedral capsid?

Answer 14

-regular geometric shape with 20 triangular faces that repeat around the nucleic acids (soccerball)
-exhibits great symmetry

Question 15

what is an example of a virus with a icosahedral capsid?

Answer 15

-human papillomaviruses
-capsid is formed from pentamers (clusters of 5)

Question 16

what is a binal capsid?

Answer 16

-2 arrangements
-geometric head with an attached helical tail
-genome is carried within the head and the tail is used to inject DNA into a host cell
-specific to bacteriophages
-not seen in human, animal, or plant viruses

Question 17

what is an example of a virus with a binal capsid?

Answer 17

-T4 bacteriophage of E.coli
-polyhedral head with the genome and a helical tail

Question 18

what are nucleocytoplasmic large DNA viruses?

Answer 18

-viruses with complex multi-layered structures

Question 19

what is an example of a nucleocytoplasmic large DNA virus?

Answer 19

-mimivirus
-infects amoebae
-0.75 micrometeres in diameter
-has 1200 kilo-base-pairs
-larger than some bacteria

Question 20

what is the structure of the envelope surrounding some viruses? how is this acquired?

Answer 20

-lipid bilayer
-acquired from the host membrane
-consists of host lipids and viral proteins (spikes)

Question 21

what is an example of an enveloped virus?

Answer 21

-influenza virus
-flexible helical capsid surrounded by an envelope
-2 major protein spikes

Question 22

what are the 2 major protein spikes of the influenza virus?

Answer 22

-hemagglutanin (H)
-neuraminidase (N)

Question 23

what is the host range of a virus?

Answer 23

-how many hosts a virus can infect
-viruses can infect all domains of life, but some may be more specific to the hosts they can infect
-most viruses are specific to a single host species
-broad host range = many
-narrow host range = few (more specific)

Question 24

what is an example of a virus with a narrow host range?

Answer 24

-bacteriophage
-viruses that only infect bacteria
-ex: T4 only infects E.coli

Question 25

what is an example of an animal virus with a narrow host range?

Answer 25

-human papillomavirus -only infects human epithelial cells

Question 26

what can human papillomavirus cause?

Answer 26

-benign tumors (warts)

Question 27

what makes viruses so specific with what hosts they can infect?

Answer 27

-viruses must attach to specific receptors on the host cell surface -these receptors define the host range (whether or not the receptors are present on multiple different hosts or not)

Question 28

what is an example of a virus that can only infect humans due to its receptor specificity?

Answer 28

-HIV -binds to CD4 -chemoreceptor on the surface of some human immune system cells

Question 29

what is an example of a virus that can infect more than one species due to the broadness of its receptor?

Answer 29

-influenza -attaches to a glycoprotein that is found on the surface of several animal cells -can infect humans, pigs, chickens, seals, etc

Question 30

what are the 5 stages of the virus life cycle?

Answer 30

-adsorption -penetration and uncoating -synthesis of viral nucleic acids and proteins -assembly of new virions -release of new virions

Question 31

what occurs in the adsorption stage of the virus life cycle?

Answer 31

-attachment to the host cell -virus attaches to specific receptors on the host cell surface

Question 32

what are examples of possible receptors for viruses?

Answer 32

-LPS (lipopolysaccharide) -outer membrane proteins -glycoproteins -anything outfacing + accessible -these receptors have other purposes as well (not just for virus identification)

Question 33

what occurs in the penetration and uncoating stage of the virus life cycle?

Answer 33

-entry into the host cell -can be done by fusion (enveloped viruses) or endocytosis (both enveloped and naked viruses)

Question 34

what typical happens when a bacteriophage enters into the host cell?

Answer 34

-inject their nucleic acid into the cell -capsid is left outside the cell as a "ghost"

Question 35

what is the process of fusion?

Answer 35

-virus spikes bind to the membrane receptors -lipid bilayer of the viral envelope fuses with the host cell membrane (match, seen as self almost) -nucleocapsid is then released into the cytoplasm

Question 36

what is the process of endocytosis for an enveloped virus?

Answer 36

-virus spikes bind to membrane receptors -this triggers normal endocytic activity (virus is surrounded by a coated vesicle/endosome) -virus envelope fuses with the endosome due to a low pH and allows the nucleocapsid to be released to the cytoplasm

Question 37

what is the process of endocytosis for a naked virus?

Answer 37

-virus capsid proteins bind to membrane receptors -this triggers normal endocytic activity (virus is surrounded by a coated vesicle/endosome) -nucleic acid leaves the endosome and enters the cytoplasm

Question 38

what happens in either case of endocytosis?

Answer 38

-capsid is removed and the viral genome is released into the cell

Question 39

what occurs in the synthesis of viral nucleic acids and proteins stage of the virus life cycle?

Answer 39

-viral genes are expressed and viral proteins are synthesized by the host's ribosomes -viral genome is replicated by the host's replication machinery

Question 40

what occurs in the assembly of new virions stage of the virus life cycle?

Answer 40

-viral proteins are assembled into capsids and then genomes are packaged into nucleocapsids

Question 41

what is a common misconception of the way viruses reproduce within the host cell?

Answer 41

-that they reproduce by division -THEY DO NOT REPRODUCE BY DIVISION

Question 42

what is the ratio of virus entry to virion exit?

Answer 42

-1 virus enters and many virions exit -not a 1:1 ratio

Question 43

what occurs in the release of new virions stage of the virus life cycle for naked viruses?

Answer 43

-typically naked viruses will accumulate and eventually lyse the host cell to release progeny (new virions) -a lytic infection (viral infection that results in the lysis of the host cell)

Question 44

what occurs in the release of new virions stage of the virus life cycle for enveloped viruses?

Answer 44

-typically enveloped viruses are released by budding -virions push through the cytoplasmic membrane without killing the host cell -persistent infection (infection can keep going due to the host cell still being alive)

Question 45

what is the process of budding?

Answer 45

-viral proteins (spikes) are inserted into the host cell membrane (produced inside the host cell and then replace typical membrane proteins) -the nucleocapsid will associate with these spikes and then buds through the membrane to form the envelope

Question 46

what is the purpose of the neuraminidase spike on the influenza virus?

Answer 46

-allows new virions to exit the host cell

Question 47

what is the purpose of the hemagglutanin spike on the influenza virus?

Answer 47

-allows viruses to adsorb to the next host