Chapter 8- Viruses and Virology Flashcards
Virus
genetic element that cannot replicate independently of a living cell (host)
Virology
the study of viruses
Virus particle (virion)
extracellular form of a virus
- exists outside host and facilitates transmission from one host cell to another
- contains nucleic acid genome surrounded by a protein coat and in some cases other layers of material
Classes of Viruses
based on the hosts that they infect - Bacteria (bacteriophage) - Archaeal viruses - Animal viruses - Plant viruses -
size of viruses
usually smaller than prokaryotic cells; 0.02 to 0.3 µm
Capsid
the protein shell that surrounds the genome of a virus particle
- composed of a number of protein molecules arranged in a precise and highly repetitive pattern around the nucleic acid
Capsomere
Subunit of the capsid
- smallest morphological unit visible with an electron microscope
Nucleocapsid
complete complex of nucleic acid and protein packaged in a virion
Enveloped virus
virus that contains a membrane around the nucleocapsid
- envelope makes initial contact with host cell
(‘naked’ doesn’t have a membrane- most bacteriophage are naked)
Helical symmetry
rod-shaped viruses
- length of virus determines by length of nucleic acid
- width of virus determined by size and packaging of protein subunit
Icosahedral symmetry
spherical viruses
- most efficient arrangement of subunits in a closed shell
Influenze virus
the envelop contains rigid spikes of haemagglutinin and neuraminidase which form a characteristic halo of projections around negatively stained virus particles
Complex virus
Virions composed of several parts, each with separate shapes and symmetries
- Bacterial viruses (Icosahedral heads and helical tails)
Some virions contains enzymes critical to infection:
- lysozyme- makes hole in cell wall, lyses bacterial cell
- nucleic acid polymerases
- neuraminidases- enzymes the cleave glycosidic bonds; allows liberation of viruses from cell
Titer
number of infectious units per volume of fluid
Plaque Assay
analogous to the bacterial colony; one way to measure virus infectivity
placques
are clear zones that develop on lawns of host cells
lawn can be bacterial or tissue culture, each plaque results from infection by single virus particle
Efficiency of planting
is used in quantitative virology
- # of plaque forming units is almost always lower than direct counts by electron microscopy due to: inactive virions, conditions not appropriate for infectivity
Which viruses are easiest to grow
bacterial viruses
which viruses are the most difficult to study
plant viruses- study often requires growth of whole plant
5 Phases of Viral Replication
- Attachment.. of a virus to a susceptible host cell
- Entry.. of the virion or its nucleic acid
- Synthesis.. of virus nucleic acid and protein by cell metabolism as redirected by virus
- Assembly.. of capsids and packaging of viral genomes into new virions (maturation)
- Release.. of mature virions from host cell
One step growth curve:
latent period vs burst size
latent- eclipse and maturation
burst size- number of virions released
Permissive Cell
host cell that allows the complete replication cycle of a virus
T4
one of the most complex penetration mechanisms
- T4 attaches to cells via tail fibres that interact with polysaccharides on the E.coli envelope
- Tail fibres react and tail core males contact with E.coli cell wall
- Attachment of virus to its host cell results in changes to both virus and cell surface that facilitate penetration
- Lysozyme-like enzyme forms mall pore in peptidoglycan
- Tail sheath contracts and viral DNA passes into cytoplasm
Mechanisms to Diminish Viral Infections
- Immune defense mechanisms, RNA interference
- Restriction modification systems (only effective against double stranded DNA viruses, modification of hosts own DNA at RE recognition sites prevents cleavage of its DNA)
Viruses can evade bacterial restriction system by:
- chemical modification of viral DNA
- production of proteins that inhibit host cell RE system
Permissive cell
host cell that allows the complete replication cycle of a virus to occur
T4 genome
contains the modified base 5-hydroxymethylcytosine DNA that is resistant to virtually all known RE
- has a dsDNA genome that is circularly permuted and terminally redundant (which both affect genome packaging)
Circularly permuted
circular permutation is a relationship between proteins whereby the proteins have a changed order of amino acids in their peptide sequence. The result is a protein structure with different connectivity, but overall similar three-dimensional (3D) shape
Terminally redundant
DNA that contains repeated sequences at each end called terminal repeats)
Genome packaging
a fundamental process in a viral life cycle. Many viruses assemble preformed capsids into which the genomic material is subsequently packaged. These viruses use a packaging motor protein that is driven by the hydrolysis of ATP to condense the nucleic acids into a confined space
Three parts of T4 genome
- Early- enzymes needed for DNA replication and transcription
- Middle- same as early
- Late- head and tail proteins and enzymes required to liberate mature phage particles
Packaging of T4 genome
- precursors of bacteriophage head os assembled
- packaging motor os assembled
- dsDNA is pumped into head under pressure using ATP
- After head is filled with DNA, T4 tail, tail fibres and other components are added
Virulent mode
viruses lyse host cell after infection
Temperate mode
viruses replicate their genomes in tandem with host genome and without killing host
(virus can also be lytic)
Temperate viruses
can undergo a stable genetic relationship within the host (but can also kill cells through lytic cycle)
Lysogeny
state where most virus genes are not expressed and virus genome (prophage) is replicated in synchrony with host chomosome
Lysogen
a bacterium containing prophage
Bacterial Viral Diversity
- diverse
- most have dsDNA
- most are naked
- ## can be structurally very complex
Animal Virus Diversity
- entire virion enters cell
- nucleus is the site of replication
- contain all knwon modes of viral replication
- many more kind of enveloped animal viruses
Animal DNA vs RNA viruses
DNA- ds, ss, non-enveloped, enveloped, different shapes and sized
RNA- all enveloped viruses are ss, nonenveloped can be ss or ds, different shapes and sizes
Persistant infections
- release of virions from host cell does not result in cell lysis
- infected cell remain alive and continues to produce virus
Latent infections
delay between infection by the virus and lytic events
transformation
conversion of normal cell into tumor cell
cell fusion
two or more cells become one cell with many nuclei
