Chapter 4

Question 1

characteristics of viruses, who they attack

Answer 1

• obligated intracellular parasites, can replicate ONLY inside host cells
• no one is safe: host cells include every group of organism (bacteria, archaea, eukaryotes…)
• are called virions outside the host, exist as inert infectious particles

Question 2

what is a capsid

Answer 2

a protein coat around a nucleic acid (where each type of protein is essentially a piece of nucleic acid enclosed within it)

Question 3

how can we classified viruses (2)

Answer 3

by their cell host (animal, bacteria, plants..) and their type of nucleic acid (DNA or RNA)

Question 4

number of proteins in simple and complex viruses

Answer 4

simple: 3-4 proteins
complex: more than 100 proteins

Question 5

in all cases, the host cell supply what (3)

Answer 5

• the building block (amino acids, nucleic acids)
• the machinery (ribosomes)
• the energy requires to produce new viruses particles

Question 6

what is nucleocapsid

Answer 6

• nucleic acid + protein coat (capsid)

Question 7

what are capsomeres

Answer 7

protein subunit that makes up the capsid

Question 8

what is the enveloppe

Answer 8

lipid-containing layer with embedded proteins

• the proteins of the enveloppe are virus specific and are encoded on the viral genome
• the proteins are involvend in the attachment of the virus to the next host cell

Question 9

in animal viruses, the enveloppe is originated from what

Answer 9

the cytoplasmic membrane of the host cells

Question 10

shapes of virions are determine by what and give 3 examples

Answer 10

by the nature of the capsomere

- helical virus, polyhedral virus, complex virus

Question 11

example of helical virus

Answer 11

tobacco mosaic virus

Question 12

how many capsomere in the helical virus (the most typical virus of many plants)

Answer 12

2130 capsomeres

Question 13

the most common shape of the polyhedral virus

Answer 13

icosahedron, a regular polyhedron with 20 triangular faces and because of the geometry, only some capsomere numbers are possible

Question 14

example of polyhedral virus with more faces

Answer 14

human papilloma virus contains 72 clusters of 5 capsomeres

Question 15

viruses that are the most complixated in term of structure

Answer 15

the viruses of bacteria (bacteriophages)

Question 16

6 parts of the complex virus

Answer 16

head (contain DNA)
tail (tail pins and tail fibers)
collar (between head and tail)
endplate

Question 17

what does lamba, a complex virus, miss

Answer 17

tail fibers

Question 18

why T4 virion only affect Gram-negative cells

Answer 18

because T4 virion recognizes specific receptors (lipo-polysaccharides) on the outer membrane and since Gram-positive doesn’t have lipo-polysaccharides, it doesn’t affect them

Question 19

5 steps of the replication of a bacterial virus

Answer 19

1. attachment (adsorption of phage virion)
2. penetration of viral nucleic acid in the host cell (the protein coat remain outside)
3. synthesis of viral nucleic acid and protein
4. assembly and packaging of new viruses (maturation)
5. cell lysis and release of new virions
   * * the cell is dead

Question 20

two ways the cell can go after the injection of viral DNA

Answer 20

lytic pathway (dangerous)
lysogenic pathway (can lead to lytic pathway if the cell is stress and the prophage goes to the phenomon of induction)

Question 21

3 steps of the lytic pathway (after the injection of the viral DNA)

Answer 21

1. lytic events are initiated
2. phages components are synthesized and virions are assembled
3. lysis of the host cell and release of new phage virions

Question 22

2 steps of lysogenic pathway

Answer 22

1. viral DNA is integrated into the host DNA
2. viral DNA is replicated with host DNA at cell division
   * called prophage (it is going to replicate but it will not bother the cell, not kill it)
   * ** prophage not expressed due to a phage-encoded receptor protein

Question 23

3 steps of the quantification of bacterial virus by plaque assay

Answer 23

** the mixture containing molten top agar, bacterial cell and diluted phage suspension

1. the cell-phage mixtured is poured into a solidified nutirent agar plate
2. the mixture is left to solidify
3. incubation allows for bacteria growth and phage replication

** the clear patch that we see is the cell lysis (bacteria that have been attack by the viruses) so the number of clear patch= the number of viruses at the beginning (approximatively)

Question 24

what are viroids

Answer 24

they are closed circles of single-stranded RNA containing 220-380 nucleotides

Question 25

replication of the viroids depend on what

Answer 25

it is dependent on host machinery, disease is caused by the overtaking of this machinery by the viroid

Question 26

viroids are the cause of what (2)

Answer 26

cadang-cadang disease of coconuts (means dying)

potato spindle tuber

Question 27

how viroids move from cell to cell

Answer 27

by using the plasmodesma

Question 28

what are prions

Answer 28

they consist solely of protein

Question 29

5 neurological degenerative disorders cause by prions

Answer 29

1. scrapie (sheep)
2. bovine spongiform encephalopathy (cattle) - BSE- mad cow disease
3. creutzfeldt- Jakob disease- CJD (human)
4. vCJD: transmitted by ingestion of BSE-contaminated product (human)
5. kuru (human)

Question 30

what do the misfolded prion proteins

Answer 30

they induce misfolding of the PrPc protein.

• • prion protein does not replicate, it induces the accumulation of misfolded PrPc proteins, which kill the cell
• ** PrPc is primally found in neurons

Question 31

exampke of helical virus in news

Answer 31

ebola virus

Question 32

example of icosahedral virus in news

Answer 32

Zika Virus