viruses Flashcards
virus
a small infectious particle that consists of nucleic acids ( either ssRNA or dsDNA) packaged in a capsid protein (protein coat)
capsids
composed of one or more protein subunits called capsomeres
what do viruses have that help them bind to a host cell
protein fibers with a knob or a spike glycoprotein
viral envelope
viruses take some of the plasma membrane from the host to make up a lipid bilayer
bacteriophages
phages
viruses that infect bacteria
host cell
cell infected by virus
host range
number of species and cell types that viruses can infect
phage t4
host = ecoli
causes lysis
tobacco mosaic virus
host = many plants
causes necrosis of leaves
life cycle of a virus
- virus attaches to host cell plasma membrane via spikes/knobs
- virus enters cell ; viral envelope fuses with plasma membrane and releases the genetic material (RNA and capsid protein)
- viral genome replicates inside the cell; host enzyme replicates
- transcription occurs; host enzymes transcribe viral genome to make mRNA and this is translated to make capsid proteins
- viral stealth assembly; viral genomes and capsid proteins self assemble to make more viruses
6; exit/release
life cycle of an animal virus: HIV
leads to AIDS
1.attachment; spike glycoproteins binds to specific host cell receptors on host cells plasma membrane
2. entry; the viral envelope fuses with the host cell membrane —> some capsid proteins are removed by host cellular enzymes
3. integration; replication and transcription and translation occurs; integrase
4. pro-virus; directs synthesis of more viruses —> self-assembly process
integrase
integrating viral double stranded DNA that is made from reverse transcriptase into the host chromosome
virus is now called pro-virus
reverse transcriptase
how an rna virus integrates its viral genomes into the host cell’s DNA
viral enzyme carried within capsi protein and released into the host cell along with genetic information (ssRNA)
able to use this ssRNA to make a complementary copy of DNA that is used as a template to make double stranded DNA
life cycle of a virus = host = bacteria/e.coli
attachment of phage ; phage dna is binding to cell membrane of bacteria
entry : injecting its DNA into the cytoplasm
integration: either integrate into bacterial chromosome —> prophage (virus + bacterial chromosome) or go through process of lysis and spill virus into environment
lytic cycle
lysis
breaking virus out of host cell
after gene expression processes the phage DNA make mRNA and then translates to make proteins
this makes new phages through viral assembly
then lysis can occur where the cell is broken up via lysosome enzymes
death of host cell
two alternative versions of replication for phages
lysogenic cycle
lytic cycle
virulent phage
virus that only replicates via lytic cycle
not capable of integrating viral genome into bacterial host chromosome
lysogenic cycle
viral genome becomes integrated into bacterial host chromosome
doesn’t kill host cell nor produce new phage particles
temperate phages
viruses that can replicate through either cycle
why do viruses need protein receptors on cell’s surface to attach to and infect host cell?
lipid bilayer of the plasma membrane of the eukaryotic host cell is not permeable to all molecules, especially hydrophobic proteins, so viruses can’t just fuse through
host cells have protein receptors for normal cell functions, like responding to chemical stimuli or cell signaling
viruses take advantage of this system by having glycoproteins that recognize those receptors, then attach to the host cell
bacteriophages also recognize and bind to specific molecules on the surface of the host cell
how
bacteriophages have more complex protein coats with accessory structures used for anchoring the virus to the host cell and injecting nucleic acid into it
what happens to the capsid proteins after entry into host cell?
entry: some capsids are removed by host cellular enzymes (un coating)
synthesis of viral components: more capsids are made
viral assembly: capsids combine with RNA, reverse transcriptase, integrate , etc
where in host DNA genome does viral genome integration occur?
any gene region, but there are viral integration sites specific to various viruses
what is major reason why bacteria have been spared from extinction
lysogenic cycle
virus is able to propagate without killing host cell
bacterial defense system against phages
CRISPR - Cas system
provides bacteria with defense against phages
CRISPR gene
chromosome is made up of CRISPR gene that is a group of clustered, regularly interspaced, short, palindromic repeats
DNA sequence with spacers, repeats, spacers, repeats, etc.
repeats
short segments of DNA and they are palindromes(reads the same forwards and backwards)
spacers
DNA from previous infections by other phages
CAS
CRISPR-associated genes) (Cas9,Cas1,Cas2) will make Cas proteins
CRISPR-Cas system
bacterial immune system
when a phage infects a bacterial cell, it injects its viral genome into the cytoplasm of the host cell
go through lytic or lysogenic cycle or the CRISPR-cas system
infection by a phage triggers:
transcription of repeats from Crispr gene region of DNA to make RNA transcript/RNA guide
transcription and translation of Cas gene
region of DNA to make Cas protein
RNA guide binds to Cas proteins to form a complex
Complementary guide RNA binds to DNA from invading phage
Cas proteins cut phage DNA
phage DNA degrades (virus no longer replicates)
scientists use CAS9 for
gene therapy or gene editing
gene editing
altering DNA sequences of a gene for a specific predictable way
gene therapy
used to treat genetic diseases by repairing a gene that may have a harmful mutation
how is CRISPR-CAS9 used for gene editing
cas9 protein and guide rna come together to form a complex
the complex is introduced into a cell
guide rna is engineered to guide the cas9
protein to a target gene
cas9 protein has active sites that can cut DNA so once the complex is introduced into cell, the guide RNA will bind to a complementary sequence —> part of target gene —> then the protein cuts
where on the target DNA sequence does Cas9 protein cut?
protospacer-adjacent motif (PAM)
short conserved 2-6 base pair sequence that ends in NGG
after cutting of gene using CRISPR-Cas9
the broken strands of DNA are repaired by the cell in one of two ways
could be inserting or deleting random nucleotides and altering the nucleotide sequence enough that it is no longer functional
repair a gene that has a harmful mutation —> you could put in a repair enzyme to help the DNA sequence becomes functional again —> editing gene
PCR amplification
make many many copies of DNA
gel electrophoresis
figuring out which segment of DNA you have and how long/heavy it is
DNA negatively charged
go towards positive electrode
