Infectious particles Flashcards
(Brock 15th ed. Chapter 8 +viroids and prions pp 300-302)
What are viruses?
Viruses are obligate intracellular parasites, they can replicate only inside host cells.
Which type f cellular organism can be a host for viruses?
Host cells of viruses include all groups of cellular organisms (bacteria, archaea, eukaryotes).
How do viruses exist outside the host?
viruses exist as inert infectious particles called virions.
structure of a virus:
- 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.
how many types of nucleic acids are found in the vision of any given virus?
one type
proteins in a simple virus ? complex?
Simple viruses: 3-4 proteins.
• Complex viruses: >100 proteins.
What structures does the host cell supply? analogy + example
(nucleic acids, amino acids), the machinery (ribosomes) and the energy required to produce new virus particles.
3 shapes of virions
helical, polyhedral, complex
what determines the shape of the virus?
the nature of the capsomere
Helical virus:
- example
- structure
tobacco mosaic virus
virus RNA is protected by capsomeres that coil around the RNA
Polyhedral virus:
- most common shape
- why are only some geometries possible?
- example
- 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.
Complex virus:
- parts
- examples
head, collar, tail, tail pins, endplate, tail fibres
* on top of outer membrane
what are bacteriophages?
The viruses that infect bacteria
Lytic cycle
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.
Lysogenic cycle
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).
The stages of the lytic cycle are:
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.
what is a temperate bacteriophage?
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.
In the lipogenic pathway, why is the prophage not expressed?
Prophage not expressed due to a phage-encoded repressor protein
STEPS: Quantification of bacterial virus by plaque assay
- The cell-phage mixture is poured onto a solidified nutrient agar plate.
- the mixture is left to solidify
- incubation flows for bacterial growth and phage replication
What does the mixture contain for the plaque assay?
motion top agar, bacterial cells, and diluted phage suspension
What are viroids?
• Viroids are closed circles of single-stranded RNA containing 240-380 nucleotides.
what does the viroid du to the host machinery?
• Viroids are closed circles of single-stranded RNA containing 240-380 nucleotides.
diseases related to viroids (2)
• Cadang-cadangdiseaseofcoconuts. • Potato spindle tuber.
How do viroids move inside plants?
its inside the nucleus and passes through the plasmodesma to bypass the plant cell wall and the plant vascular system
What are prions?
• Prions consist solely of one protein. and they cause neurological degenerative
disorders
example of neurological degenerative
disorders caused by prions:
- Scrapie (sheep)
- Bovinespongiformencephalopathy (cattle) – BSE – mad cow disease.
- Creutzfeldt-Jakob disease – CJD (human)
- vCJD:transmittedbyingestionof BSE-contaminated product (human)
- Kuru (human)
explain what happens to the prion (misfolded protein)?
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.
