Microbiology Flashcards
virus
tiny infectious agents
obligate intracellular parasite
able (obligated) to reproduce within (intra) cells. an example is a virus
are viruses cells or living organisms?
NO
what can a viral genome consist of?
DNA or RNA, single or double stranded, circular or linear
for any type of virus, how many nucleic acids can it’s genome carry?
only one type present in mature genomes, and only one type of nucleic acid as its genome
bacteriophage
a virus that infects bacteria
what is the limiting factor of all viral genomes, regardless of the form of nucleic acid used as a genome?
size
why is size the limiting factor for viruses?
the exterior protein shell is typically a rigid and fixed structure that cannot expand
what is an adaptation for the small size of viral genomes?
each genome can encode more than one protein in a given length by utilizing multiple reading frames
capsid
protein coat surrounding the viral nucleic acid gnome
helical capsids
rod shaped
polyhedral capsids
multiple-sided geometric figure
envelope
membrane on the exterior of the virus that surrounds the capsid, is derived from the membrane of the host cell
budding
the process through which viruses can acquire an envelope through the host cell membrane
naked viruses
viruses which do not have envelopes
which viruses are always naked?
all phages and plant viruses
attachment/adsorption
first step: virus binds to exterior of a bacterial cell
penetration/eclipse
second step: injection of the viral genome into the host cell, capsid remains on the outer surface while genome disappears into the cell, removing infectious virus from the media
what happens to the phage once the viral genome is injected into the host?
it can either enter the lytic or lysogenic cycle
hydrolase
a hydrolytic enzyme that degrades the entire host genome, an early gene
early gene
one of a group of genes that are expressed immediately after infection and which includes any special enzymes required to express viral genes
lysozyme
destroys the bacterial cell wall, causing cell lysis, a late gene
what is the main difference between lytic and lysogenic cycle?
host cells are not destroyed in the lysogenic cycle
prophage
when the phage genome is incorporated into the bacterial genome
lysogen
refers to the host after the phage genome has been incorporated
how are infected cells able to remain dormant in the lysogenic cycle?
transcription of phage genes are blocked by a phage-encoded repressor protein that binds to specific DNA phage promoters and operators
excision
the process whereby the prophage becomes activated and removes itself from the host genome, entering the lytic cell
transduction
the process whereby a small section of host genome is replicated and packaged with the viral genome, and may be coded to create mutated viruses
how do animal viruses enter cells?
by endocytosis (host cell engulfs the virus and internalizes it), the viral genome is released from the capsid (uncoated)
what possible cycles can an animal virus enter?
lytic cycle, lytic-like cycle (productive cycle) or lysogenic cycle
productive cycle
similar to lytic cycle but does not destroy the host cell because it exits the membrane by budding, becoming coated with membrane in the process
provirus
refers to the dormant form of the viral genome in the animal virus lysogenic cycle
+ RNA viruses
must encode RNA-dependent RNA pol (and do not have to carry it: a ssRNA genome, viral genome acts directly as mRNA and is translated by host ribosomes to create viral proteins
infective
means injecting an isolated genome into the host cell will result in virus production
- RNA virus
must carry RNA-dependent RNA pol, - RNA is complementary to the piece of RNA that encodes viral proteins, so -RNA is the template for viral mRNA production
how does the -RNA virus propagate the viral life cycle?
the enzyme will create a + strand from the - genome, this is what encodes viral proteins
retroviruses
must encode reverse transcriptase, usually + RNA viruses that undergo lysogeny
if an RNA virus undergoes lysogeny, what does this imply about its genome?
to be able to integrate into our double-stranded DNA, the viral genome must also be composed of double-stranded DNA (i.e. reverse transcription)
reverse transcription
to make DNA from an RNA template
RNA-dependent DNA polymerase (reverse transcriptase)
enzyme that produces DNA from an RNA template
what are the three main retroviral genes?
gag (codes for viral capsid proteins), pol (polymerase codes for reverse transcriptase) and env (envelope codes for viral envelope proteins)
double-stranded DNA viruses
often encode enzymes required for dNTP (nucleotide) synthesis and DNA replication: these viruses often have large genomes, this is required because dNTP’s aren’t always available but NTP’s are
subviral particles
infectious agents that are even smaller and simpler than viruses, ex. prions and viroids
prion
a subviral particle, a version of a protein that can self-replicate without transcription or translation
helper virus
one virus which complements another
transmissible spongiform encephalopathies (TSEs)
caused by prions, a class of diseases in mammals that causes degeneration in the nervous system, especially the brain where characteristic holes develop, and are always fatal
what is a characteristic of prion diseases?
long incubation periods
viroid
a subviral particle that consists of a short piece of circular, single-stranded RNA with extensive self-complementarity that allows it to base-pair with itself to create regions of double strandedness
do viroids code for proteins?
no, they also lack capsids
what is the effect of viroids?
some are catalytic ribozymes that can silence normal gene expression, or a viroid RNA-dependent RNA polymerase can synthesize a (-) strand that could be used as a template to produce a (+) strand and thus propagate the viroid sequence
