what was viruses initially discovered as? and by whom?
Adolf mayer and Dmitri ivanowsky in 1892 defined it as filterable agents that required living cells for propogation and had distinct characteristics from bacteria, coined ‘virus’ in 1898 by martinus beijerinck
what is the definition of a virus and the name of its feild of study
a genetic element that cannot replicate independently of a living host cell.
study of viruses is called ‘virology’
what is a virion, its purpose, and its makeup
a complete virus particle that exists outside host cell
facilitates transmission from one host cell to another
contains nucleic acid genome surrounded by protein coat (and sometimes an envelope)
what is tropism in releation to viruses
the ability of a specific virus to infect and replicate within particular cell types, tissues or host species
what are the culturing differences between bacteriophages, animal viruses, and plant viruses?
bacterial viruses are the easiest to grow and are model systems for virology
animal viruses can be cultivated in tissue, cell cultures, eggs or animal models
plant viruses are typically most difficult to study due to requiring growth of whole plant (some are as easy to culture as animal viruses but mostly its harder)
what are the 6 modern methods for studying and identifying viruses
-Electron microscopy (advanced imaging)
-molecular methods:
*PCR
*Genome sequncing/metagenomic sequncing
*comparative genomics
-antigenic and serological tests:
*antigen detection
*serological tests
why are viruses absent from the tree of life?
the tree of life is based on relatedness of ribosomal RNA (rRNA) sequnces, viruses lack ribosomes. they rely on a host cell for their life cycle and all metabolic intermedietes and protein synthesis
what are the structural components of a virus (and the virion outside a host cell)
a complete viral particle is known as a virion, able to protect the viral genome outside a host cell aswell as contributing to cell attachment.
-all virions have a nucleocapsid consisting of capsid, a protein shell around the viral genome made of CAPSOMERES and the genome, viral nucleic acids
-some virions contain virus specific enzymes with roles in viral infections and genome replication
-virions can be naked or enveloped with the evelope laye derived from host cell membrane, viral glycoprotiens embedded in envelope can act as receptor binding molecules for infection of host cells
what is the size range of mammalian viruses
~0.02-1 micrometer
what do the ‘giant’ viruses (1micrometer) infect
infect amoebae and other protozoans
has genome larger then some bacteria
ebola is a big fella being more worm lookin then virus looking
what are the physical properties of virions
highly symmetrical structures:
-helical symmetry = rod shaped viruses
-icosahedral symmetry = spherical viruses
-complex symmetry = mixed structure
helical symmetry has the virion length determined by nucleic acid length and its width determined by capsomere size and packing
icosahedral symmetry is a highly efficent packaging structure with capsomeres forming triangular faces, symmetrically arranged with 20 faces and 12 vertices
complex symmetry has components with both icosahedral and helical symmetry, with tailed bacteriophages being the most structurally complex virions (the classic D20 on a stick with legs virus)
what are VALs
virion associated lysins (VALs) occur in bacteriophages
depolymerases and lytic enzymes degrade bacterial cell structures such as polysaccharide capsids and the peptidoglycan layer
what are nucleic acid polymerases
occur in RNA viruses
polymerase is a structural component of virion
required for infection and replication in host cells
what are NAs?
Neuraminidases (NAs) occur in influenza viruses
NAs are enzymes that cleave glycosidic bonds targeting glycoprotiens and glycolipids (connective tissue)
required for liberation of new virion particles from host cells (breaks open cell to spread virions)
what are viral genomes made of
very varied, can be double stranded, single stranded.
can be DNA or RNA.
what is the baltimore classification of viruses
genomic classification devised by american viroligist david baltimore based on the relationship between the viral genome and its mRNA, splitting viruses into 7 classes:
-class 1 and 7: dsDNA + or -
-class 2: ssDNA + charge
-class 3: dsRNA + or -
-class 4: ssRNA+ charge
-class 5: ssRNA - charge
-class 6: ssRNA + retrovirus
single stranded genomes have polarity
what is the disease based classification of viruses and the 5 types it speaks on
another form of classification based on route of harm. sometimes similar diseases can result from completely un-genetically-related viruses though
-respiratory viruses: often aquired by inhalation of droplets and replicated in repiratory tract e.g influenza
-enteric viruses: replicate in gut and cause gastric infections, aqured by ingestion of faecal-contaminated material e.g poliovirus
-arboviruses: infect insects that ingest vertebrate blood, replicate in tissue of insect and transmit to vertebrate host, eg flaviviruses
-hepatitis viruses: all viruses that cause liver disease eg hepadnaviruses
-sexually transmitted viruses eg herpes simplex (HSV), HIV
what is viral tropism and its 3 subtypes?
the range of hosts, tissues or cells that a virus can infect and replicate within
-cellular tropism: the ability of a virus to replicate in a particular cell type
-tissue tropism: the ability of a virus to replicate within a particular tissue
-host tropism: ability of a virus to replicate in particular host species
how is viral tropism determined
by a combination of host cell susceptibility and permissiveness
-succeptibility is the ability to express the correct receptors to enable viral entry
-permissiveness is the ability to support a complete viral multiplication cycle
similar viruses can have different tropisms tho, like HSV-1 and HSV-2 with ine spread by oral contact causing mouth sores and the other spread b sexual contact causing genital herpes
what are the 6 major stages of replication for most viruses?
1- recogniton and attachment
2-penetration
3-uncoating
4-synthesis
5-assembly
6-release
what are the key differences in replication cycles of viruses that infect eukaryotic cells vs those infecting prokaryotic cells
1-viral components entering cell: in prokaryotes only nucleic acid enters the host cell, in eukaryotes the entire virion enters host cell
2-site of replication: prokaryotes lack nuclei so all viral replication occurs in cytoplasm, in eukaryotes most viruses replicate in cell nucleus
3-site of assembly: in prokaryotes the cytoplasm is the site of assembly, while in eukaryotes some viroplasms (viral membrane bound factories) form to increase efficency of virion assembly
what is the 1st step of viral replication in depth?
to initiate infection, virions must bind to specific host cell receptors, these receptors often have vital functions unrelated to viral infection, presence/absence of receptors determines tropism.
viral attatchment proteins include the capsid, proteins extending out the capsid, and glycoproteins
what is steps 2 and 3 of viral replication in depth
once attached virion particle can be taken up into cell via 2 methods depending on if virus is enveloped or naked
-fusion with cytoplasmic membrane (eneloped virus, entire viral capsid may enter or just nucleic acid, eg HIV)
-endocytosis (enveloped and naked viruses)
viral nucleic acid then must be released into cell cytoplasm to be translated and replicated (uncoating) through 2 methods:
-via a pore in cell or endosomal membrane (only in viropexis used by viruses such as influenza A and poliovirus)
-via endosomal fusion of virion and endosomal membrane
describe step 4 of viral replication in depth along with the 4 nucleic acid polymerases associated with genome replication
viral genome replication
viruses use host components for their reproduction, however many encode their own nucleic acid polymerases:
-DNA dependant RNA polymerase (DdRp) copies DNA making RNA, present in all organisms
-DNA dependent DNA polymerase (DdDp) copies DNA to make DNA, present in all organisms
-RNA dependent RNA polymerase (RdRp) copies RNA to make RNA, helps in RNA virus replication, olny present in viruses
-RNA dependent DNA polymerase (RdDp) copies RNA to make DNA serving as reverse transcriptase present only in viruses
translation of viral mRNA depends on host cell functions with eukaryotic ribosomes only recogising plus sense mRNA, menaing all viruses must translate their genome into plus sense mRNA for translation of viral proteins (purpose of RNA dependent RNA polymerase)
viruses prefer preferential translation of mRNA, many viruses are post-translationaly modified such as hepatitis C and ebola virus
