Midterm 3 Flashcards
why do viruses want to go into a host?
it wants to replicate. thats it and in the case of humans the best way it can do that is by being passed on to more hosts.
its mindless
history of virus?
viral diseases have plagued humans since before we knew what they were – evidence for small pox in egypt
1800s we discovered viruses
walter reed
showed in 1901 that a human disease, yellow fever, was caused by a virus transmitted by a mosquito
viruses are defined as ______
intracellular obligate parasites
size of a virus
10 to 100 nm in size
can only see with electron microscope
viral genome has ____ nucleotides
a few thousand to 200,000 nucleotides in length
Sizes of viruses
HIV has 9200 bp for 9 genes
SARs-CoV-2 has 30000 bp which is large for a RNA virus
t or f viruses can be single or double stranded DNA or RNA
true
capsid
protein shell around genome composed of capsomere proteins
nucleocapsid
capsid and genome together
do all viruses have an envelope?
no! if they don’t they’re considered naked. an envelope is basically just a plasma membrane derived structure around capsid
shape of viruses
symmetry. tldr we have helical, polyhedral, and complex
capsids often exhibit either helical or polyhedral (iscosahedral = 20 triangular faces) shapes
can also have complex shapes. for example flu has 8 nucleocapsid structures
complex virus
bacteriophage head contains genome with elaborate tail structure
virion
viral particles that are infectious but they are metabolically inert
is a virus dead or alive?
depends on your definition of life!
replication cycle of a virus
attachment- stick on to host cell
entry (penetration/uncoating) - get into the cell and release genome
biosynthesis- replicate its genome and express its genes to make proteins
assembly- put everything together
exit- get the new virus particles out
coevolution
viruses orginated before the last universal common ancestor (LUCA), during the RNA world period and evolved along with their host cells.
descendents of the original RNA replicators in this view
no RNA viruses are known to infect any member of archaea AND some have proposed retroviruses as a case for transition from RNA world to DNA world BUT they only effect plants and animals so not really good enough
little evidence that this is true
regressive hypothesis
viruses are cells that lost some of the replicative and metabolic traits over time
doesnt adequately explain the origins of RNA viruses.
Giant DNA viruses are often cited as examples to support this hypothesis
example of regression is a tapeworm. used to be a real worm now he a creep
progressive hypothesis
most people prob think this is the way it happens
existing genetic elements gradually gained the ability to move from cell to cell
evidence in the form of transposons (for DNA viruses) and retrotransposons (for retroviruses)
how can we grow and quantify viruses?
bacteriophages –> add agar and then put on a nutrient agar base
animal viruses –> have to work with tissue culture of host cells to do viral cultivation.
what do we use for virus identifcation?
electron microscopy
nucleic acid analysis through pcr and reverse pcr
what is one thing that viruses always need the host cell for?
protein synthesis
bacteriophages
viruses that infect bacteria!
almost all have double stranded dna
almost always have complex morphology
most widely studied are viruses of e coli including t-even phages and phage lambda
bacteriophages have _____
double stranded dna most of the time
lytic cycle
a phage causes the lysis and death of its host bacterium as it replicates
example t even phages
lysogenic cycle
a phage incorporates its nucleic acid into thehost cell chromosme and remains dormant for a period of time
example temperate phages
stages of lytic cycle
attachment- sites on bacterophage tail fibers attach to receptor sites on bacterium
penetration- the tail sheath contracts to force the tail core through the cell wall, and phage DNA enters the cell
biosynthesis- transcription, translaton, and replication of viral DNA occurs
maturation- new phage DNA and capsids are assembled into virions
release - phage lysozyme destroys the cell wall, the cell bursts, and the new virions are released
stages of lysogenic
attachment/penetration - same as lytic cycle
viral dna recombines with the bacterial chromosome to form a prophage
the prophage replicates with the bacterial dna and will continue to do so until something triggers it to excise itself it then continues with the steps of the lytic cycle
t or f retroviruses never do the lysogenic cycle
true
how do viruses recognize appropriate cells?
each virus has a specific range/type of cells that it can infect
the possible range is dictated by the interaction between viral attachment proteins and host cell receptor molecules
host cell receptors are often molecules critical for the cellular function
can viruses get through the plant wall?
not really so plant viruses depend on damage to the plant tissues to find an open spot in the cell wall
damage can include: insects feeding on plants wind damage hail/rain damage fire damage human-induced damage
can cross kingdom viruses happen? like can plant viruses infect a human
NO
receptors arent the same
where are viral receptors in plant cells?
inside the cell
define uncoating
genome release
evens out animal viruses and bacteriophages
how are viruses grouped?
the way they get to the mRNA phase
DNA Viruses
usually carry their genome as dsDNA (exceptions exist)
example herpes virus.
Double stranded DNA viral life cycle
DNA travels to the nucleus
early genes are transcribed and subsequently translated creating viral enzymes necessary for later steps
late genes are trancsribed and viral dna is replicated
late translation synthesizes capsid proteins
capsid proteins migrate into the nucleus where maturation occurs
release of viral particles occurs through the endoplasmic reticulum via budding and does not need to kill the cell
herpesvirus is class _
I
Class I
basically makes mrna the way humans do
dsDNA –> mRNA
poxviruses
atypical dna virus
they do not use the host cell nucelus or the host cells dna or rna polymerase. brings its own rna polymerase to the party. difference between encoding and bringing! early genes has dna polymerase.
ex. smallpox
CHICKEN POX ISNT A POX VIRUS
in dna + strand is _____
coding “sense” strand; shares the same sequence as the mRNA
in dna the - strand is ____
template or “antisense” strand which is complementary to the mRNA
in rna the + sense strand _____
is the mrna or at least is able to function that way
in rna the - sense strand ______
complementary to the + strand ad usually only exists to act as a template to make + strands
rna viruses carry their genome as _______
+ or - RNA
do we need the whole virus to infect the cell?
No!
which “baltimore classes” are picornaviridae ?
Class IV which is single stranded RNA virus + used directly as the genome
which baltimore class are rhabdoviridae
Class V which is single stranded RNA virus - sense. transcribe the minus strand into + mrna
retroviral life cycle
+ single stranded RNA genome
they contain a reverse transcriptase which is RNA-dependent DNA polymerase
single strand + serves as a template to make dsDNA which migrates to the nucleus
The DNA integrates into a host cell chromosome and is now called a provirus
provirus
when viral dna integrates into the host cell chromosome
NEVER LEAVES THE DNA
t or f provirus never leaves the DNA
t
proviruses can be ____
transcribed and the + strands are translated into viral proteins including reverse transcriptase and the + strands also act as the genome to make new virions.
similarities and differences between retroviruses and bacteriophage
lambda leaves when cell dies and progeny go to the surrounding cell
provirus from retrovirus cannot do that^^^
unlike lambda retroviruses have a random insertion point
difference bw bacteria and human being
integrase
integrates dna into host cell
HIV is class ____
VI retrovirus reverse transcriptase takes single stranded positive sense RNA and turns it into double stranded DNA
coronavirus is class ___
IV single stranded positive sense RNA translated straight into protein
egress
exit from cell
mechanisms depends on virus type and host cell type
enveloped viruses plant viral proteins in the host cell plasma membrane, dock to them, and bud
t or f provirus can never leave dna
t
what happens to the provirus?
it can be transcribed and the + strand (mRNA) will be translated into viral proteins including reverse transcriptase and then the + strand can also act as the genome to make new virions so the virus can replicate without the provirus excising itself
T or F HIV is enveloped
t
HIV carries ____ copies of RNA, poliovirus only carries ______
2;1
T or F HIV can pass on its genes through vertical gene transfer
F
herpesvirus and poxvirus are class _
I
picornavirus is class
IV
rhabdovirus is class
V
coronavirus is class
IV
poliovirus is class
IV
viral self assembly
once proteins are translated the viral components translate themselves fast and cheaply
the capsid proteins may interact with packaging sequences on viral genome to coalesce around nucleic acid (tobacco mosaic virus)
empty capsids may partially form and then viral genomes insert into them (poliovirus)
with respect to the spike proteins where do they come from and where do they go?
any protein that is made from virus or you has to be made on rough ER/ribosome and then it follows secretory pathway to the host cell plasma membrane. they dock there and wait for the nucleocapsid to come to the surface and become enveloped
