Viruses Flashcards
How many viruses are there on the planet?
10^31
What’s an obligate parasite?
Needs a host to replicate and survive
What are the most abundant viruses on the planet?
Bacteriophages - viruses that specifically infect bacteria
What are bacteriophages role in the ocean?
It is estimated that bacteriophages kill and lyse between 15% and 40% of the ocean’s bacteria every single day
What is a viral shunt?
Bacteriophages recycle energy by lysing bacteria
Energy is not passed up the food chain but instead moved back down in the form of dissolved organic material when infected cells are lysed
What is viral transduction? How is it beneficial? (Phage therapy)
Transfer of genes (allows beneficial genes to be shared)
8% of human DNA come from viruses our ancestors were infected with
Phage therapy – infecting bacteria with viruses (could be useful for antibiotic resistance)
How big are viruses in comparison to bacteria?
0.02-0.3 um or 20-300 nm
1-2 um
What components make up a virus?
Nucleic acid.
Protein coat (capsid) with surface receptors.
Lipid membranes (some).
Enzymes (some).
What don’t viruses have?
Viruses don’t generate ATP
Don’t possess machinery for translation
Don’t have ribosomes and can’t independently form proteins from mRNA.
Steal lipid membrane from host cell when they leave the cell
What is the capsid made up of?
Repeating subunits called capsomeres - viruses have symmetry because of how they’re assembled.
When new parts added they fold which protect nucleic acid inside.
What are enveloped viruses?
Enveloped viruses take the lipid membrane from their host as they exit the cell.
Relatively few enveloped plant or bacterial viruses because of cell walls surrounding cell membrane.
Most enveloped viruses are animal viruses.
Envelope proteins attach to and infect animal host cell.
What are surface proteins?
Attach to specific receptors on host cells.
Tricks cell into taking in the virus.
Corona virus S-Protein – ACE receptor
Influenza for example has Hemagglutinin (H) and Neuraminidase. The H binds to sialic acid on the host cell.
What are the enzymes and their roles?
Neuraminidases - Cleaves glycoproteins and allows exit of virus from cell.
Lysozyme - Makes hole in cell wall to allow nucleic acid entry and exit.
Nucleic acid polymerases:
RNA-dependent RNA polymerases.
Reverse transcriptase (RNA-dependent DNA polymerase) in retroviruses - (converts RNA to DNA so it can be used for cellular machinery. )
Viral replication - attachment and penetration
Find host – viruses specific in particular hosts and cell types determined by surface proteins
Surface proteins attach receptors to cell surface and trick cells into thinking virus is useful
Cell lets virus in – either genetic material moves in or whole virus.
Nucleic acid replicated by enzymes in the host cell
Viral replication - synthesis
Synthesis of viral nucleic and protein
Viral replication depends on nucleic acid
Production of mRNA depends on starting point of virus – some viruses can produce it quicker
Aim to create rRNA to make viral proteins in host cell
Viral replication - assembly and release
Assembly and packaging of new viruses
Cells lysis and release of new virions
What infection particles are smaller than viruses?
Prion – proteins no genetic material, don’t reproduce just infold
Viroids – smaller then viruses
What are viroids?
(RNA) 10-20 nm, smallest known just 220 bases
(Virus 20-300nm)
A few hundred nucleotides of RNA.
Does not code for any genes.
No protein coat.
Folded shape gives stability.
Can be replicated.
Mostly found in plants.
Causes stunted growth.
DNA/RNA formation of virus
Virus = Small DNA/RNA molecule surrounded by a
protein coat. Codes for genes. Can be replicated. Can
be transferred from one organism to another.
RNA of viroid
Viroid = Circle of RNA. A few hundred nucleotides,
Does not code for any genes. Can be replicated.
DNA of plasmid
Plasmid = Small circular DNA molecule. Separate from
main genome. Codes for genes. Can be replicated. Can
be transferred from one organism to another.
DNA of transposon
Transposon = DNA segments that can move from one
location to another in a cells genome.
48% of our DNA are jumping genes – transposon, can move to other cells
What are the problems when investigating the origin of viruses?
No fossil record – too small
No ‘LUCA’ equivalent – No same gene present in all viruses.
High mutation rates -
Genome size vs mutation rate.
Gene transfer between host and virus.
What are the main theories for the origin of viruses?
Virus first - RNA-world viruses were first lifeform, later left vents to infect cells.
Reduction – A virus was a small cell, that became an intracellular parasite, and then lost all other cellular components.
Escape hypothesis – Transposons or other ‘jumping genes’ ‘escaped’ from a cell and later developed into the viruses we know today.
Co-evolution – like virus first theory above, except some viruses became cells with the addition of membranes while others stayed as viruses (nutrients concentrated in vesicles as you move away from vent).
Chimeric-origins hypothesis – A mixture of virus-first and escape hypotheses, this theory says that the original virus pick up genes from hosts.
