9. Viruses Flashcards
virus
obligate intracellular parasite
genetic element that cannot replicate independently of a living (host) cell
intracellular form
virus particle (virion)
extracellular form of a virus
-exists outside host and facilitate transmission from one host cell to another
replication/ reproduction occurs only upon
infection
entry into host cell
viruses can infect all cell types
bacterial viruses called bacteriophages (phages)
few archaeal viruses
most are eukaryotic viruses
classified into families based on:
-genome structure (DNA or RNA)
-life cycle
-morphology
-genetic relatedness
structure of viruses
all virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)
-some viruses consist only of a nucleocapsid, others have additional components (envelopes)
capsids
large macromolecular structures which serve as protein coat of virus
protect viral genetic material and aid in its transfer between host cells
made of protein subunits called protomers
capsids are helical, cosahedral, or complex
helical capsids
-shaped like hollow tubes with protein walls
-protomers self assemble
-length of capsid is a function of nucleic acid
*naked is harder to control- no lipid bilayer
*phospholipid bilayer can be targeted by disinfectants (enveloped)
icosahedral capsids
an icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices
capsomers
-ring or knob-shaped units made of 5 or 6 protomers
-pentamers - 5 subunit capsomers
-hexamers- 6 subunit capsomers
viral envelopes and enzymes
many viruses are bound by an outer, flexible, membranous layer called the envelope
animal virus envelopes (lipids and carbohydrates) usually arise from host cell plasma or nuclear membranes
viral envelope proteins
envelope proteins (involved in attachment) which are viral encoded, may project from the envelope surface as spikes (identification) or peplomers
1. involved in viral attachment to host cell
2. used for identification of virus (spike proteins)
3. may have enzymatic or other activity (neuraminidase of influenza virus- enable virus to escape from host cell)
4. may play a role in nucleic acid replication
structure of the virion
enzymes inside virions
1. lysozyme
2. neuraminidase
3. nucleic acid polymerases (RNA replicases: RNA-dependent RNA polymerases)
lysozyme
-makes hole in cell wall to allow nucleic acid entry
-also lyses bacterial cell to release new virions
*break down peptidoglycan (in bacteriophages)
neuraminidases
part of viral envelope
-destroy glycoproteins and glycolipids
-allows liberation of viruses from cell
animal virus –> escape
nucleic acid polymerases
RNA replicases: RNA-dependent RNA polymerases
reverse transcriptase: RNA-dependent DNA polymerase in retroviruses (RNA - DNA - RNA)
only found in RNA viruses
viral genomes are structurally diverse
-single or double stranded DNA or RNA
-the length of the nucleic acid also varies from virus to virus
-genomes can be linear or circular (some RNA viruses have segmented genomes)
phases of viral replication in a permissive (supportive) host
attachment (the most important step)
penetration
synthesis
assembly
release
attachment (adsorption)
specific receptor attachment
receptor determines host preference:
-may be specific tissue (tropism)
-may be more than one host
-may be more than one receptor
entry into the host
entire genome or nucleocapsid
varies between naked or enveloped virus
three methods used:
1. fusion of the viral envelope with host membrane; nucleocapsid enters (animal virus)
2. endocytosis in vesicle: endosome aids in viral uncoating (animal virus)
3. injection of nucleic acid (phages)
bacterial and archaeal viral infections
virulent phage
temperate phage
virulent phage
one reproductive choice
-multiplies immediately upon entry
-lyses bacterial host cell
temperate phage
two reproductive options
-reproduce lytically as virulent phages do
-remain within host cell without destroying it
-many temperate phages integrate their genome into host genome (becoming a prophage in a lysogenic bacterium) in a relationship called lysogeny
lysogenic only for bacteriophages
overview of animal virus infection
-major tenets (capsid and DNA/RNA genome, infection and takeoverof host, assembly and release) univeral
-classified by genomes (RNA or DNA)
-most human viral diseased are caused by RNA viruses
two key differences of animal virus infection
- entire virion enters the animal cell
- eukaryotic nucleus is the site of replication for many animal viruses
animal virus entry mechanisms
- fusion
- enveloped endocytosis
- non-enveloped endocytosis
animal virus infection
viral infection of animal cells
-uncoating occurs at cytoplasmic membrane or cytoplasm
-viral DNA genomes enter nucleus, most viral RNA is converted to DNA within nucleocapsid
–bind specific host cell receptors, typically used for cell-cell contact or immune function
virion release (animal)
nonenveloped viruses lyse the host cell
-viral proteins may attack peptidoglycan or membrane
enveloped viruses use budding
-viral proteins are incorporated into host membrane
-nucleocapsid may bind to viral proteins
-envelope derived from host plasma membrane, but may be Golgi, ER, or other
-virus may use host actin tails to propel through host membrane
animal virus infection- cellular outcomes
- transformation (tumor cell division)
- lysis (death of the cell and release of virus)
- persistent infection (slow release of virus without causing cell death)
- latent infection (virus present but not replicating)
hosts for bacterial and archaeal viruses
usually cultivated in broth or agar cultures of suitable, young, actively growing bacteria
broth cultures lose turbidity as viruses reproduce
plaques observed on agar cultures
hosts for animal viruses
tissue (cell) cultures
-cells are infected with virus
-bacteriophages- localized area of cellular destruction and lysis that enlarge as the virus replicates
cytopathic effects
embryonated eggs
prions
neurodegenerative diseases in humans and animals
virus classification
ds vs ss
symmetry
presence/ absence envelope
dimensions
