A2.3 Viruses Flashcards
Virus
Obligate parasites - they require a host cell to reproduce because they don’t have the ribosomes and metabolic enzymes required for protein synthesis.
Common features
small - 20-400nm in diameter
fixed size - viruses don’t grow so no increase in size
genetic material - DNA or RNA (ss or ds)
capsid - protein coat that encloses the genetic material. Often composed of multiple different proteins, sometimes only one.
no cytoplasm, few or no enzymes - use the metabolism of their host cells and most of their enzymes. Only few enzymes are required for replication of virus’ genetic material.
Diversity of structure
1) Bacteriophage - Head with DNA (nucleocapsid), tail sheath, tail fiber (for attaching to host)
- viruses that infect bacteria
2) Adenovirus - icosahedral capsid (20 sides), has spikes
3) Tobacco mosaic - helical capsid, looks like a rigid rod
4) Influenza - have 8 different RNA molecules, each is wrapped in a helical capsid - capsid and outer envelope with spikes covering (most viruses don’t have)
Genetic material
- DNA or RNA
- single stranded or double stranded
- ss can be +ve or -ve
positive ss - the strand has the same nucleotide as the host on mRNA. Hence can be used as a template to quickly assemble viral proteins during viral replication
negative ss - the strand is an opposite or antisense (complementary) to host mRNA. Hence, has to be converted to template strand for viral protein assembly
Envelope
Non-enveloped:
- affects plants or bacteria
- do not come enclosed in a membrane which typically originates from the host cell plasma membrane
Enveloped:
- animal viruses are mostly enveloped
- become covered in membrane when breaking free from their host cell. Phospholipid membrane surrounding the virus originates from host cell, while the protein comes from the virus itself
- usually lytic cycle viruses
- protects the entire virion
- glycoproteins on envelope help virus enter host cell by binding to receptor molecules on the surface
Lytic cycle
1) Attachment: Proteins in tail of the virus bind to membrane receptors
2) DNA entry: viral DNA enters the host via a pore generated in the membrane of the bacterium
3) DNA replication and transcription: replicates DNA with cell machinery and translates using host cell ribosomes
4) Protein synthesis: more viral proteins are made and the new bacteriophages are self-assembling by packing the viral DNA molecule in a capsid
5) Lysis: viral proteins make holes through the wall and membrane of host cell and spread to infect other host cells
Lytic cycle evaluation
Advantages for host:
- no integrations/only prior to replication in host DNA hence cell function will be retained
- allows for detection of cells by the immune system as they lyse out
Disadvantages for host:
- cell death/loss of cells
Advantages for virus:
- high and rapid reproduction rate
- immediate spread to infect new cells
Disadvantages for virus:
- runs the risk of being detected and destroyed
Lysogenic cycle
1) Infects bacteria cell by injecting its DNA into the cell. Remains “temperate” in lysogenic cycle.
2) viral DNA is incorporated into host genome to form prophage
3) viral DNA replicates with host cell DNA, prophage DNA is passed on to daughter cells
4) Under stressful conditions or recombination event prophage DNA is excised from the bacterial chromosome and enters the lytic cycle.
5) Phage DNA replicates and phage proteins are made
6) new phage particles are assembled
7) the cell lyses, releasing the newly made phages
Lysogenic cycle evaluation
Advantages for host cell:
- it doesn’t immediately kill or harm the cells
Disadvantages for host cell:
- some of the genes/cell function may be lost due to integration (to “rid” the viral DNA, the whole cell would have to be killed)
- more energy is required to replicate the genome along with viral insert
Advantages for virus:
- it may not be easily detected and killed immediately
- it increases the viral load in the host
Disadvantages for virus:
- it can survive only when the conducive/optimal condition arises
- slower process
- prone to environmental mutation the host is undergoing.
Factors affecting evolution rate
1) Generation rate - the number of generations produced in viruses is very fast (20-60 minutes)
2) Mutation rate - the basis of evolution is genetic variation, and the source of this is mutation. Mutation rate is high in viruses. (high MR allows natural selection of the virus in varied conditions)
3) Natural selection - there are many defense mechanisms in place by host cells which increase selective pressure on viruses. Natural selection is powerful and encourages evolution
Influenza
+ uses 8 negative ss RNA molecules as genetic material
+ RNA replicase used to replicate doesn’t have proofreading mechanism which makes it prone to high mutation rates
+ can be transmitted between species, particularly between birds and humans [can evolve and inject multiple hosts]
+ two proteins in the enveloping membrane of the virus (haemoglobin and neuroaminidase) can exchange and end up in new combinations creating a novel virus.
HIV
+ retrovirus using the enzyme reverse transcriptase to convert ss RNA genome into DNA (no proofreading mechanism)
+ host cells also cause mutations by converting cytosine in the RNA into uracil, leading to an even higher mutation rate
+ high mutation rate causes constantly changing glycoproteins which is harmful to the virus —> leads to a new generation of strains so the virus can easily evade the immune system
