Virus Background

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acellular agents:

cannot carry out any metabolic pathways

cannot grow or respond to environment

cannot reproduce independently

virus:

minuscule

acellular

infectious agent

one or several pieces of nucleic acid - either DNA or RNA

no cytoplasmic membrane

capsid - protein coat surrounding nucleic acid

some have phospholipid membrane envelope

specificity of host cell because of precise affinity of viral surface proteins for complementary proteins on host surface

generalist: infect many kinds of cells in many different hosts

capsomere: proteinaceous subunits that makeup the capsid

viral shapes: helical, polyhedral, complex

helical:

capsid composed of capsomeres bonded in spiral fashion to form tube around nucleic acid

polyhedral:

capsid roughly spherical

icosahedron capsid has 20 sides and is most common

complex:

capsid of many different shapes

bacteriophage

enveloped virus acquires its envelope from its host cell during replication or release

phospholipid bilayer and proteins

lytic replication cycle

attachment:

depends on chemical attraction and precise fit between attachment proteins on phage's tail fibers and receptor proteins on host cell wall

contact with bacterium is random because viruses are nonmotile

entry:

lysozyme: protein enzyme that weakens peptidoglycan of cell wall

phage's tail sheath contracts, forcing internal hollow tube within tail through cell wall and membrane

phage injects genome through tube into bacterium

synthesis:

viral enzymes degrade bacterial DNA into nucleotides

bacterium starts synthesizing new viruses

assembly:

capsomeres accumulate within cell and spontaneously attach to one another

if a capsid assembles around a leftover piece of host DNA, it can infect new bacteria cells with first host's DNA, transduction

release:

bacterium disintegrates

burst time: period of time required to complete the entire process

burst size: number of new virions released from each lysed bacteria cell

lysogenic replication cycle:

attachment

entry

host cell's DNA is not destroyed

virus remains inactive (prophage)

prophage is inserted into DNA of bacterium

bacterium replicates

induction: prophage is excised from host chromosome

synthesis

assembly

release

attachment of animal viruses:

dependent on chemical attraction and exact fit between proteins/glycoproteins on virion and complementary receptor on animal cell's cytoplasmic membrane

no tails or tail fibers

have glycoprotein spikes on capsid or envelope

entry and uncoating of animal viruses:

direct penetration: viral capsid attaches and sinks into cytoplasmic membrane, creating pore through which genome alone enters the cell

membrane fusion: viral envelope and host cell membrane fuse, releasing capsid into cell's cytoplasm, leaving envelope glycoproteins as part of cell membrane

endocytosis: stimulated by attachment of virus to receptor molecules on cell's surface

uncoating of viral capsid can happen within vesicles by cellular enzymes or by enzymes within cytosol

assembly of animal viruses:

most DNA viruses assemble in nucleus and are released into cytosol

most DNA viruses develop in cytoplasm

release of animal viruses:

enveloped animal viruses released by budding

budding: virions extrude through one of the cell's membranes, acquiring a portion of membrane, which becomes viral envelope

budding allows infected cell to remain alive for some time

naked viruses released either by exocytosis or cause lysis and death of cell

synthesis of animal viruses:

synthesis of double stranded DNA viruses:

similar to normal replication of cellular DNA

genome of most dsDNA viruses enter the nucleus of the cell

mRNA transcribed in nucleus

capsomere proteins made in cytoplasm by host ribosomes

capsomeres enter nucleus, where new virions spontaneously assemble

synthesis of single stranded DNA viruses:

host enzymes produce new strand of DNA complementary to viral genome

complementary strand binds to ssDNA of virus to form dsDNA

transcription of mRNA, replication of new ssDNA, viral assembly

types of RNA viruses:

positive sense single stranded RNA

retrovirus

negative sense single stranded RNA

double stranded RNA

positive sense single stranded RNA:

single stranded viral RNA can act directly as mRNA

complementary negative strand RNA transcribed from +ssRNA genome by viral RNA polymerase

-RNA serves as template for transcription of multiple +ssRNA genomes

retroviruses:

a kind of +ssRNA

genome not used as mRNA

reverse transcriptase transcribes DNA from +RNA

DNA serves as template for synthesis of additional +RNA molecules, which both act as mRNA

negative sense single stranded RNA:

RNA dependent RNA transcriptase transcribes +RNA molecules from viruses -RNA genome

+RNA serves as template for transcription of additional copies of -RNA

double stranded RNA:

positive strand serves as mRNA, which transcribes proteins and RNA polymerase

each strand acts as a template for transcription of its opposite

viral diseases are difficult to treat because disrupting viral replication may disrupt normal cellular processes as well since viral replication uses cellular structures and pathways

when a provirus is incorporated into its host DNA, the condition is permanent

viroids:

extremely small

circular pieces of RNA that are infectious and pathogenic in plants

viroidlike agents:

infectious, pathogenic RNA particles that lack capsids

do not infect plants

affect some fungi

prions:

proteinaceous infective particle

lack any nucleic acid

prion diseases: spongiform encephalopathies - clumps of prions propagate throughout the brain, neurons stop working, die, leaving holes and spongy appearance