chapter 13: Characterizing Viruses

Question 1

What are some characteristics of viruses

Answer 1

1. They cannot metabolize or reproduce independently, so they have to hijack the cells metabolic pathway to increase their numbers
2. They neither grow nor respond to environment like a cell does
3. they do not have cytoplasmic membranes, cytosol or their own organelles (acellular)
4. they cause many of the infection of humans, animals, plants, and bacteria and are the causes for most diseases

Question 2

What is a virus

Answer 2

miniscule (24-300 nm or 3um), acellular, infectious agents, have either DNA or RNA for genetic material, are they cells
-all viruses are dependent on the host organelles and enzymes to produce new virions

Question 3

Genetic material of viruses

Answer 3

-viruses will show greater variety in their genomes than cells do and scientists classify viruses by their genetic makeup
-viruses can be either dsDNA, ssDNA, dsRNA, ssRNA
-they have smaller genomes than any cell type

Question 4

E.coli size

Answer 4

4.6-5.4 *10^6 bp

Question 5

Phage size

Answer 5

4.85+10^4 bp

Question 6

The extracellular state

Answer 6

is a virion which includes a capsid and some animal viruses have a phospholipid envelope but this is not a membrane (acquired from a host cell membrane)

Question 7

capsid

Answer 7

a protein coat which surrounds the virus and protects the viral nucleic acid

Question 8

The intracellular state

Answer 8

has the capsid removed and the virus exists as it nucleic acid

Question 9

capsid morphology

Answer 9

-the capsid is made of proteinaceous subunits called capsomeres

Question 10

capsid function

Answer 10

a protein coat that provides protection for viral nucleic acid when outside of its host and a means of attachment to the host cell when infecting it
-depending on the animal virus type, the entire capsid with the nucleic acid within it enters the cell via membrane fusion or endocytosis
-while for other viruses the capsid remains extracellular during direct penetration and only genome enters

Question 11

Three mechanisms of cell entry for animal viruses

Answer 11

1. direction penetration
2. membrane fusion
3. endocytosis

Question 12

direct penetration

Answer 12

a process in which the viral capsid attaches and sinks into the cytoplasmic membrane, creating a pore through which the genome alone enters the cell, leaving the empty capsid on the surface
ex: poliovirus

Question 13

membrane fusion

Answer 13

the viral envelope and the host’s cytoplasmic membrane fuse, releasing the capsid into the cell’s cytoplasm while leaving the envelope glycoproteins as part of the cell membrane
ex: measles virus

Question 14

endocytosis

Answer 14

-Attachment of the virus to receptor molecules on the cell’s surface stimulates the cell to endocytose the entire virus
-For a virus that penetrates a host cell with its capsid intact, the capsid must be removed to release the genome before replication of the virus can continue
ex: adenovirus (naked)

Question 15

Viral host specificity

Answer 15

-viruses can be specific or generalists

Question 16

Specific viruses

Answer 16

most viruses infect only a particular specific host cell due to the affinity of the viral surface protein- ligands- for complementary proteins on the host cell surface
-some are so specific they can only infect one particular kind of cell in a particular host (HIV) specific for human cells (T helper)

Question 17

Generalists virsues

Answer 17

-they infect many kinds of cells in many different host

Question 18

Where does an envelope come from?

Answer 18

-composed of a phospholipid bilayer with embedded proteins (host cell membrane)
-some proteins are virally coded glycoproteins (spikes) like covid or as in HIV viruses gp41 and gp120 proteins

Question 19

viral envelope characteristics

Answer 19

-some animal viruses have an envelope surrounding the capsid
-if present recognition/ complementarity of the envelope’s protein and glycoprotein are required for host cell entry

Question 20

Budding in an enveloped virus to release viruses as virions

Answer 20

-in virology the extrusion of enveloped virions through the host cell membrane

Question 21

enveloped animal virus contains

Answer 21

1. glycoproteins
2. helical capsid
3. matrix protein
4. envelope

Question 22

Lytic replication

Answer 22

this replication cycle results in lysis and death of the host cell as new virions are produced then released. Sometimes in less than 30 minutes for some

Question 23

Stages of a lytic replication cycle

Answer 23

1. attachment of the virus to the host cell
2. entry into the host cell
3. host cell chromosome degradation
4. synthesis of new genome and new virion parts
5. assembly of virion parts and genome into new virions
6. release of virions from cell via lysis

Question 24

lysogeny

Answer 24

a modified replication cycle in phage (lambda)

Question 25

temperate phages

Answer 25

can enter a lysogenic state and then return to the lytic state when induced

Question 26

lysogenic in bacteriophages

Answer 26

-infected host cell grows and reproduce normally with the viruses as a prophage for generations before they eventually lyse the cell

Question 27

prophage

Answer 27

inactive phage as a part of the host genome

Question 28

lysogenic conversion

Answer 28

-results when phages carry genes that alter the phenotype of a bacterium so no other viruses can infect the cell while the viruses remain dormant
-when induced the prophage is released and the virus returns to complete the lytic cycle

Question 29

lysogenic replication cycle

Answer 29

1.attachment
2.entry
3. prophage in chromosome (replication)
4. replication of chromosomes and viruses (cell division)
5. induction (degrades host chromosomes)
6. synthesis
7. assembly
8. release

Question 30

attachment of animal viruses

Answer 30

a biochemical attraction with glycoprotein spikes or other attachments molecules that mediate attachment
-animal viruses so not have tails or tail fibers

Question 31

viral replication in animal cells

Answer 31

-some use basic replication pathways like bacteriophages and lyse the cell but differences result from either the presence of an envelope around some viruses, the eukaryotic nature of animal cells and animal cells lack a cell wall

Question 32

animal virus latency

Answer 32

-similar to lysogeny
-some animal viruses remain dormant in host cells even for years with no viral activity and some latent viruses become incorporated into host chromosomes
-incorporation of the provirus into the host cell DNA is permanent as in HIV infections

Question 33

What 3 ways are viruses cultured

Answer 33

1. culturing viruses in mature organisms (within bacteria cells or plants/animals)
2. culturing viruses in embryonated chicken eggs (ideal cheap environment-can pose a problem for some patients)
3. culturing viruses in cell/tissue culture