Infectious particles

Question 1

What are viruses?

Answer 1

Viruses are obligate intracellular parasites, they can replicate only inside host cells.

Question 2

Which type f cellular organism can be a host for viruses?

Answer 2

Host cells of viruses include all groups of cellular organisms (bacteria, archaea, eukaryotes).

Question 3

How do viruses exist outside the host?

Answer 3

viruses exist as inert infectious particles called virions.

Question 4

structure of a virus:

Answer 4

• Capsid: protein coat around the nucleic acid.
• Nucleocapsid: nucleic acid + protein coat.
• Capsomeres: protein subunit that makes up the capsid.
• Envelope: lipid-containing layer with embedded proteins; in animal viruses, this envelope originates from the cytoplasmic membrane of the host cells. Proteins in the envelope are virus specific and are encoded on the viral genome. They are usually involved in the attachment of the virus to the next host cell.

Question 5

how many types of nucleic acids are found in the vision of any given virus?

Answer 5

one type

Question 6

proteins in a simple virus ? complex?

Answer 6

Simple viruses: 3-4 proteins.

• Complex viruses: >100 proteins.

Question 7

What structures does the host cell supply? analogy + example

Answer 7

(nucleic acids, amino acids), the machinery (ribosomes) and the energy required to produce new virus particles.

Question 8

3 shapes of virions

Answer 8

helical, polyhedral, complex

Question 9

what determines the shape of the virus?

Answer 9

the nature of the capsomere

Question 10

Helical virus:

• example
• structure

Answer 10

tobacco mosaic virus

virus RNA is protected by capsomeres that coil around the RNA

Question 11

Polyhedral virus:

• most common shape
• why are only some geometries possible?
• example

Answer 11

• Several shapes are possible but the most common is the icosahedron, a regular polyhedron with 20 triangular faces (a, b). Because of the geometry, only some capsomere numbers are possible.
• Other viruses may have more faces. Human papilloma virus (c, d) contains 72 clusters of 5 capsomeres.

Question 12

Complex virus:

• parts
• examples

Answer 12

head, collar, tail, tail pins, endplate, tail fibres

* on top of outer membrane

Question 13

what are bacteriophages?

Answer 13

The viruses that infect bacteria

Question 14

Lytic cycle

Answer 14

it hijacks its host cell and uses the cell’s resources to make lots of new phages, causing the cell to lyse (burst) and die in the process.

Question 15

Lysogenic cycle

Answer 15

allows a phage to reproduce without killing its host.
In the lysogenic cycle, the first two steps (attachment and DNA injection) occur just as they do for the lytic cycle. However, once the phage DNA is inside the cell, it is not immediately copied or expressed to make proteins. Instead, it recombines with a particular region of the bacterial chromosome. This causes the phage DNA to be integrated into the chromosome.
The integrated phage DNA, called a prophage, is not active: its genes aren’t expressed, and it doesn’t drive production of new phages. However, each time a host cell divides, the prophage is copied along with the host DNA, getting a free ride. The lysogenic cycle is less flashy (and less gory) than the lytic cycle, but at the end of the day, it’s just another way for the phage to reproduce.
Under the right conditions, the prophage can become active and come back out of the bacterial chromosome, triggering the remaining steps of the lytic cycle (DNA copying and protein synthesis, phage assembly, and lysis).

Question 16

The stages of the lytic cycle are:

Answer 16

The stages of the lytic cycle are:
Attachment: Proteins in the “tail” of the phage bind to a specific receptor (in this case, a sugar transporter) on the surface of the bacterial cell.
Entry: The phage injects its double-stranded DNA genome into the cytoplasm of the bacterium.
DNA copying and protein synthesis: Phage DNA is copied, and phage genes are expressed to make proteins, such as capsid proteins.
Assembly of new phage: Capsids assemble from the capsid proteins and are stuffed with DNA to make lots of new phage particles.
Lysis: Late in the lytic cycle, the phage expresses genes for proteins that poke holes in the plasma membrane and cell wall. The holes let water flow in, making the cell expand and burst like an overfilled water balloon.
Cell bursting, or lysis, releases hundreds of new phages, which can find and infect other host cells nearby. In this way, a few cycles of lytic infection can let the phage spread like wildfire through a bacterial population.

Question 17

what is a temperate bacteriophage?

Answer 17

In virology, temperate refers to the ability of some bacteriophages (notably coliphage λ) to display a lysogenic life cycle. Many (but not all) temperate phages can integrate their genomes into their host bacterium’s chromosome, together becoming a lysogen as the phage genome becomes a prophage.

Question 18

In the lipogenic pathway, why is the prophage not expressed?

Answer 18

Prophage not expressed due to a phage-encoded repressor protein

Question 19

STEPS: Quantification of bacterial virus by plaque assay

Answer 19

1. The cell-phage mixture is poured onto a solidified nutrient agar plate.
2. the mixture is left to solidify
3. incubation flows for bacterial growth and phage replication

Question 20

What does the mixture contain for the plaque assay?

Answer 20

motion top agar, bacterial cells, and diluted phage suspension

Question 21

What are viroids?

Answer 21

• Viroids are closed circles of single-stranded RNA containing 240-380 nucleotides.

Question 22

what does the viroid du to the host machinery?

Answer 22

• Viroids are closed circles of single-stranded RNA containing 240-380 nucleotides.

Question 23

diseases related to viroids (2)

Answer 23

• Cadang-cadangdiseaseofcoconuts. • Potato spindle tuber.

Question 24

How do viroids move inside plants?

Answer 24

its inside the nucleus and passes through the plasmodesma to bypass the plant cell wall and the plant vascular system

Question 25

What are prions?

Answer 25

• Prions consist solely of one protein. and they cause neurological degenerative
disorders

Question 26

example of neurological degenerative

disorders caused by prions:

Answer 26

• Scrapie (sheep)
• Bovinespongiformencephalopathy (cattle) – BSE – mad cow disease.
• Creutzfeldt-Jakob disease – CJD (human)
• vCJD:transmittedbyingestionof BSE-contaminated product (human)
• Kuru (human)

Question 27

explain what happens to the prion (misfolded protein)?

Answer 27

The misfolded prion proteins (PrPSc) induces misfolding of the PrPC protein. The prion protein does not replicate, it induces accumulation of misfolded PrPSc proteins, which kills the cell. The PrPC protein is primarily found in neurons.