a minuscule, acellular, infectious, agent having one or several peices of nucleic acid- either DNA or RNA.
-no cytoplasmic membrane but possess a membrane-like envelope
-lack cytosol and organelles.
-not capable of metabolic activity on their own. take control of cell's metabolic activity.
virus that is outside of the cell in the extracellular state.
-consists of a protein coat called a capsid, surrounding a nucleic acid core.
the outermost layer of a virion
(envelope or capsid) provides the virus both protection and recognition sites tha bind to complementary chemicals on the surfaces of their specific host rells. once inside the cell, capsid is removed
hosts of viruses
a viruses specificity is due to to the precise affinity of viral surface proteins or glycoproteins for complementary proteins or glycoproteins on the surface of the host cell.
introduced into plant cells either through abrasions of the cell wall or by plant parasiteswuch as nematodes or aphids
exist only with in the cell. They cna not penetrate a thick fungal cell wall.
viral infections can be propogated by the fusion of an infected fungal cell with an uninfected one
proteinaceous subunits that make up the capsid of a virus.
-some are composed of only a single type of protein, where as others are composed of several different kinds of protein
three basic shapes of viruses
-helical- the capsid is composed of capsomeres that bond together in a spiral fashion to form a tube around nucleic acid
-polyhedral- capisid is rougly spherical
some viruses have an envelope similar in compositionto a cell membrane surrounding their capsids.
-a virus aquires the envelop from the host during viral replication or release.
-composed of a phospholipid bilayer and proteins
-some of the proteins are virally coded glycoproteins
-an envelopes proteins play a role in the recognition of host cells
classification of viruses
classified by: type of nucleic acid, presence of envelope, shape and size.
5 stages of lytic replication
-attachment of the virion to the host cell.
-entry of the virion or its genome into the host cell.
-synthesis of new nucleic acids and viral proteins by the host cell's enzymes and ribosomes
-assembly of new virions with in the host cell
-release of the new virions from the host cell
-contact with bacterium is random since phages are nonmotile.
-its tail fibers attach it to the host bacterium.
-attachment depends on the chem attraction and precise fit between attachment proteins on tail fibers and the complimentary receptor proteins.
upon contact with E.coli, T4 releases lysosyme, which weakens the peptidoglycan of the cell wall.
-phage's tail sheath then contracts, which forces an internal hollow tube with the tail through the cell wall and membrane.
-phage injects the genome through the tube
-viral enzymes degrade the bacterial DNA into its constituent nucleotides.
-bacterium stops synthesizing its own molecules and begins synthesizing new viruses under control of the viral genome.
-translation by the host cells ribosomes result in viral proteins.
- capsid assembly is spontaneous, requiring no enzymatic activity.
-it has recently been discovered that for some viruses, enzymes pump the genome into the assembled capsid/
Lysogenic replication cycle
process of viral replication in which a bacteriophage enters a bacterial cell, inserts into the DNA of the host and remains inactive. The phage is then replicated every time the host replicates its chromosomes. Later the phage may leave the chromosome
steps of lysogenic replication cycle
1)attachment via the tail
2)entry-host's cell's DNA are not destroyed and phages genome does not take control
3)prophage in chromosomes- remains inactive. the prophage becomes a physical part of the bacterial chromosome.
4)the cell replicated as well as the prophage. daughter cells are infected.
5) induction- the prophage is excised from the chromosome
phage can change the phenotype of the bacterium for example a harmless form into a pathogen
attachment of animal viruses
-dependent on the chemical attraction and exact fit between proteins on the virion and complementary proteinson the animal cell's cytoplasmic membrane.
-animal viruses lack both tails and tail fibers.
instead they typically have spikes
entry and uncoating of animal viruses
3 different mechanisms:
-direct penetration:the viral capsid attaches and sinks into the cytoplasmic membrane, leaving a pore in which the genome enters alone
-membrane fusion: the viral envelope and the host cell membrane fusem releasing the capsid into the cell's cytoplasm and leaving the envelope glycoproteins as part of the cell's membrane.
-endocytosis:attachment of the virus to receptor molecules stimulates the cell to endocytize the entire virus.
synthesis of dsDNA viruses
similar to the normal replication of cellular DNA and translation of proteins.
-genomes enter the nucleus of the cell.
synthesis od ssDNA viruses ( parovirus)
host enzymes produce a new strand of DNA complementary to the viral genome. This complementary strand binds to ssDNA of the virus to form dsDNA.
four types of RNA viruses
positive sense, single stranded RNA
negative sense, single stranded RNA
double stranded RNA
positive sense ssRNA
single stranded viral RNA that can act directly as mRNA
- a complimentary negative strand RNA is transcribed from the +ssRNA genome by viral RNA polymerase.
+ssRNA that do not use their genome as mRNA
-instead they use a DNA intermediary that is transcribed from +RNA by reverse transcriptase carried within the capsid.
negative sense ssRNA
carries within its capsid an enzyme, RNA-dependent RNA transcriptase which transcrives + RNA from the viruses -RNA genome