9. Viruses

Question 1

virus

Answer 1

obligate intracellular parasite
genetic element that cannot replicate independently of a living (host) cell
intracellular form

Question 2

virus particle (virion)

Answer 2

extracellular form of a virus
-exists outside host and facilitate transmission from one host cell to another

Question 3

replication/ reproduction occurs only upon

Answer 3

infection
entry into host cell

Question 4

viruses can infect all cell types

Answer 4

bacterial viruses called bacteriophages (phages)
few archaeal viruses
most are eukaryotic viruses

classified into families based on:
-genome structure (DNA or RNA)
-life cycle
-morphology
-genetic relatedness

Question 5

structure of viruses

Answer 5

all virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)
-some viruses consist only of a nucleocapsid, others have additional components (envelopes)

Question 6

capsids

Answer 6

large macromolecular structures which serve as protein coat of virus

protect viral genetic material and aid in its transfer between host cells

made of protein subunits called protomers

capsids are helical, cosahedral, or complex

Question 7

helical capsids

Answer 7

-shaped like hollow tubes with protein walls
-protomers self assemble
-length of capsid is a function of nucleic acid

*naked is harder to control- no lipid bilayer
*phospholipid bilayer can be targeted by disinfectants (enveloped)

Question 8

icosahedral capsids

Answer 8

an icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices

capsomers
-ring or knob-shaped units made of 5 or 6 protomers
-pentamers - 5 subunit capsomers
-hexamers- 6 subunit capsomers

Question 9

viral envelopes and enzymes

Answer 9

many viruses are bound by an outer, flexible, membranous layer called the envelope

animal virus envelopes (lipids and carbohydrates) usually arise from host cell plasma or nuclear membranes

Question 10

viral envelope proteins

Answer 10

envelope proteins (involved in attachment) which are viral encoded, may project from the envelope surface as spikes (identification) or peplomers
1. involved in viral attachment to host cell
2. used for identification of virus (spike proteins)
3. may have enzymatic or other activity (neuraminidase of influenza virus- enable virus to escape from host cell)
4. may play a role in nucleic acid replication

Question 11

structure of the virion

Answer 11

enzymes inside virions
1. lysozyme
2. neuraminidase
3. nucleic acid polymerases (RNA replicases: RNA-dependent RNA polymerases)

Question 12

lysozyme

Answer 12

-makes hole in cell wall to allow nucleic acid entry
-also lyses bacterial cell to release new virions

*break down peptidoglycan (in bacteriophages)

Question 13

neuraminidases

Answer 13

part of viral envelope
-destroy glycoproteins and glycolipids
-allows liberation of viruses from cell

animal virus –> escape

Question 14

nucleic acid polymerases

Answer 14

RNA replicases: RNA-dependent RNA polymerases
reverse transcriptase: RNA-dependent DNA polymerase in retroviruses (RNA - DNA - RNA)

only found in RNA viruses

Question 15

viral genomes are structurally diverse

Answer 15

-single or double stranded DNA or RNA
-the length of the nucleic acid also varies from virus to virus
-genomes can be linear or circular (some RNA viruses have segmented genomes)

Question 16

phases of viral replication in a permissive (supportive) host

Answer 16

attachment (the most important step)
penetration
synthesis
assembly
release

Question 17

attachment (adsorption)

Answer 17

specific receptor attachment
receptor determines host preference:
-may be specific tissue (tropism)
-may be more than one host
-may be more than one receptor

Question 18

entry into the host

Answer 18

entire genome or nucleocapsid
varies between naked or enveloped virus
three methods used:
1. fusion of the viral envelope with host membrane; nucleocapsid enters (animal virus)
2. endocytosis in vesicle: endosome aids in viral uncoating (animal virus)
3. injection of nucleic acid (phages)

Question 19

bacterial and archaeal viral infections

Answer 19

virulent phage
temperate phage

Question 20

virulent phage

Answer 20

one reproductive choice
-multiplies immediately upon entry
-lyses bacterial host cell

Question 21

temperate phage

Answer 21

two reproductive options
-reproduce lytically as virulent phages do
-remain within host cell without destroying it
-many temperate phages integrate their genome into host genome (becoming a prophage in a lysogenic bacterium) in a relationship called lysogeny

lysogenic only for bacteriophages

Question 22

overview of animal virus infection

Answer 22

-major tenets (capsid and DNA/RNA genome, infection and takeoverof host, assembly and release) univeral
-classified by genomes (RNA or DNA)
-most human viral diseased are caused by RNA viruses

Question 23

two key differences of animal virus infection

Answer 23

1. entire virion enters the animal cell
2. eukaryotic nucleus is the site of replication for many animal viruses

Question 24

animal virus entry mechanisms

Answer 24

1. fusion
2. enveloped endocytosis
3. non-enveloped endocytosis

Question 25

animal virus infection

Answer 25

viral infection of animal cells -uncoating occurs at cytoplasmic membrane or cytoplasm -viral DNA genomes enter nucleus, most viral RNA is converted to DNA within nucleocapsid --bind specific host cell receptors, typically used for cell-cell contact or immune function

Question 26

virion release (animal)

Answer 26

nonenveloped viruses lyse the host cell -viral proteins may attack peptidoglycan or membrane enveloped viruses use budding -viral proteins are incorporated into host membrane -nucleocapsid may bind to viral proteins -envelope derived from host plasma membrane, but may be Golgi, ER, or other -virus may use host actin tails to propel through host membrane

Question 27

animal virus infection- cellular outcomes

Answer 27

1. transformation (tumor cell division) 2. lysis (death of the cell and release of virus) 3. persistent infection (slow release of virus without causing cell death) 4. latent infection (virus present but not replicating)

Question 28

hosts for bacterial and archaeal viruses

Answer 28

usually cultivated in broth or agar cultures of suitable, young, actively growing bacteria broth cultures lose turbidity as viruses reproduce plaques observed on agar cultures

Question 29

hosts for animal viruses

Answer 29

tissue (cell) cultures -cells are infected with virus -bacteriophages- localized area of cellular destruction and lysis that enlarge as the virus replicates cytopathic effects embryonated eggs

Question 30

prions

Answer 30

neurodegenerative diseases in humans and animals

Question 31

virus classification

Answer 31

ds vs ss symmetry presence/ absence envelope dimensions