Lecture 13 - how viruses break the rules Flashcards
what was the first virus discovered and how was it discovered?
Tobacco mosaic virus.
it was discovered by the following steps:
1) sap from tabacco plant with tobacco mosaic disease was isolated.
20 sap filtered to remove bacteria
3) filtered sap rubbed on leaf of healthy plant
4) healthy plant became infected.
what is the size of a viral genome compared to humans?`
genome size = 4000- 400,000 nt
while humans are ~ 3 billion nt.
what are the components of a virsus?
> know the diagram.
viruses are very small infectious particles consisting of a nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope (animals not plant viruses).
a capsid is the protein shell that encloses the viral genome.
an envelope is a membrane that is derived from the host cell.
a virus particle can have various structures from simple to complex.
what are the different morpologies of viruses?
KNOW DIAGRAMS FOR EACH >Spherical -envelope -RNA -capsid -glycoprotein spikes >Icosahedral -glycoprotein spikes -capsid -DNA > Helical - RNA -Capsid > Complex - Head - DNA - Tail - Sheath - Tail protein fibers
what is the function of the virus capsid?
TO PROTECT AGAINST:
>physical damage - shearing by mechanical forces
> Chemical damage - UV irradiation (from sunlight) leading to chemical modification
> Enzymatic damage - Nucleases derived from dead or leaky cells or deliberately secreted by vertebrates as defence against infection
> responsible for recognition of the host cell. Takes the form of binding of a specific virus-attachment protein to a cellular receptor molecule.
what are the points in favour of a virus being a living organism?
> replicate through progeny
evolve through mutation and natural selection
each contain only one kind of nucleic acid - can be DNA or RNA
what are the points against viruses being classified as living organisms?
> have only limited genetic material 4 to ~ 300 genes
can only replicate in living cells
employ host cell’s replicative and metabolic pathways
do not have their own ribosome or the ability to produce energy
outside cells they are essentially inert macromolecules.
What are the stages of a virus lifecycle?
1) attachment
2) penetration
3) uncoating
4) transcription/translation
5) genome replication
6) assembly
7) release
describe what happens at each stage and now the diagram
1) attachment
> The capsid or envelope of the virus attaches to the surface of the host cell
2) penetration
> the virus moves into the host cell
3) uncoating
> the capsid breaks down into proteins and its nucleic acid
4) transcription/translation
> the nucleic acid is either translated if it is RNA or transcribed if it is DNA. This produces more proteins to be made into a new capsid
5) genome replication
> the genome of the cell is also replicated to make new copies
6) assembly
> the new copies of the viral genome are placed inside the newly translated proteins
7) release
> these new viruses are released from the host cell into the environment.
what are retroviruses?
they integrate a DNA copy of their RNA genome into the host DNA (HIV)
What is polio virus?
> polio virus causes poliomyelitis, an infection of the motor neurons causing paralysis or, in extreme cases, death.
disease caused by this virus was common until the advent of vaccines in the 1960s - Salk (killed vaccine) and Sabin (live, attenuated vaccine)
using this vaccine the WHO planned to eliminate all naturally-occuring poliovirus infections from the earth by 2000
polio virus is a single stranded +RNA virus.
describe the replication of poliovirus
1) receptor attachment
2) RNA genome release
3) Translation of incoming viral genome
4) polyprotein processing: both capsid proteins and RdRP
5) transcription of viral RNA in viral induced vesicles
6) assembly and release of mature virus particles
give a summary of polio replication
> polio virus has a genome of single stranded +RNA
In order to replicate this virus utilises a virally encoded RNA dependent RNA polymerase (RdRP)
this enzyme copies the single stranded + RNA to produce a - RNA copy.
the -RNA copy acts as a template for the production of more +RNA for the production of new genomes and viral specific proteins which are packaged to form new virus particles.
What is HIV?
human immunodeficiency Virus
> HIV is the virus that causes AIDS
> HIV infects several kinds of cells in the body the most important of which is a type of white blood cell called the CD4 lymphocyte (or “t-cells”). the CD4 cell is a major component of the human immune system.
> by infecting and destroying T-cells, HIV can effectively disable the body’s immune system.
> there is currently no vaccine to prevent HIV. Treatment with combinations of drugs can prolong life.
> HIV is a relatively new virus - discovered in 1983. it it a retrovirus with a single stranded +RNA genome.
HIV replication - summarise
> like polio virus, HIV is a single stranded + RNA virus and must use a novel enzyme activity to replicate its genome (RNA dep. DNA pol.)
this is achieved through using reverse transcriptase. this enzyme uses the +RNA template to make DNA
this DNA is copied to make double stranded DNA which is incorporated into the genome of the host cell forming a “provirus”. Viruses using this mode of replication are called “retroviruses”
the provirus produces mRNA (or + RNA) which directs the synthesis of more viral proteins and is also packaged to form new virus particles.
