Virus Flashcards
What is a virus
A simple, miniscule, infectious, obligate intracellular parasite comprising of genetic material (RNA or DNA) surrounded by a protein coat (capsid) and/or an envelope derived from a host cell membrane
Genetic material of virus
Virus contain either DNA or RNA genomes, (double stranded or single stranded)
Intracellular parasite definition
Virus have to have a living host to replicate/reproduce and require cellular machinery to do this
Relative size of virus
Usually they’re 20-300nm, they’re very small! And can only be seen through an electron microscope
However there are giant virus, which can be seen under a light microscope
Giant virus example
Pandoravirus
> 1000nm length. Biggest virus known so far, 2.9 MB genome in size. Enclosed 2556 gene
Virus exist in what 2 states?
Extracellular Virion (outside the host cell) - for transmission
Intracellular Virus (after infecting the host cell) - for replication
Tools that can be used to view vital structure
Electron microscope
X-ray crystallography
Cryo-electron microscopy
Nuclear magnetic resonance spectroscopy (NMR)
Structure of virus (The nucleocapsid)
Outside of cell: virus particles = virion
Capsid (protein coat/shell) (made up of many protein subunits = capsomers)
Nucleic acid (DNA or RNA)
Additional: Capsomers can either be identical or (more often) the capsid will be composed of a mixture of structurally distinct capsomers
Shapes of virus (capris symmetry)
Helical - capsomers bond together in a spiral fashion
Polyhedral - capsid is roughly spherical
Binal (complex) - neither helical nor polyhedral forms, have irregular shapes, or have complex structures. May consist of both helical and isometric symmetries
Functions of the capsid
Protects nucleic acid from digestion by enzymes
Enables penetration of the host cell membrane to inject the infectious nucleic acid into the cells cytoplasm
Allow the virion to attach to a host cell
Naked/non-envelope vs enveloped virion
Enveloped has an envelope layer, (most infect humans) enables an extra layer of protection so they’re usually acquired by the host
What groups of organisms can be infected by virus
All groups of cellular organisms are infected by viruses:
Bacteria
Protozoa
Algae
Fungi
Plants
Animals
Virophage
Each virus only infects a specific host or group of hosts, not all!
Viruses of bacteria: Bacteriophages
More than 10^30 bacteriophage particles in the world waters
They weigh about a femtogram (10^-15grams)
The length of head-to-tail joining of 10^30 phases = 1.25x10^21 km = 100 million light years
Bacteriophages: genomes
Only a few with envelopes
Most had dsDNA
Many are complex
Replication cycle of bacteriophage step 1
Adsorption: attachment of specific receptors
Replication cycle of bacteriophage step 2
Penetration: entry of DNA
Replication cycle of bacteriophage step 3
Replication: synthesis of viral nucleic acids and protein
Replication cycle of bacteriophage step 4
Assembly and packing of new virions
Replication cycle of bacteriophage step 5
Release: release of new virion (lysis)
Replication cycle of bacteriophage step 6
Re infection: infect another cell
Lytic and temperate bacteriophages
Bacteriophage T4 is virulent and kills the host: it’s a lytic phage
Many bacteriophages have a alternative life cycle where they don’t kill the host: temperate phages or lysogenic phages
What happens after infection of a temperate phage
The genome can intergrate into the host’s chromosome; it is then termed a “prophage”
These produce a depressor protein
Repressive blocks lytic genes
The prophage replicates with the host chromosome: lysogeny or lysogenic cycle
The bacterial hosts that harbour a prophage are termed lysogens, (they are immune against infection with the same phage because of the repressor
Stresses initiate induction to resume lytic cycle
Why do we care about virus,
They’re a “planetary force” (10million viruses in one drop of seawater)
They keep control of “bacteria”
They’re primarily known for impact on health
Define virus
Intracellular replicative form of particle