Viruses & Subviral Particles Flashcards
Viral Structure
- genetic material
- capsid (protein coat)
- sometimes an envelope
Viral Envelope
- surrounds capsid
- composed of phospholipids and virus specific proteins
- sensitive to heat/detergents
What is a virus?
obligate intracellular parasite
Do envelopes make viruses more or less resistant?
- envelopes make a virus less resistant to sterilization
- virus without an envelope is more likely to persist on surfaces for an extended period of time
Can viruses reproduce independently?
No, so considered obligate intracellular parasites
Explain the role of a host cell in viral reproduction
- virus must express and replicate genetic information within a host cell
- virus must use host cell b/c they lack ribosomes to carry out protein synthesis
List the basic steps of virus reproduction
(1) virus hijacks cell’s machinery
(2) virus replicates
(3) virus produces viral progeny (virons) which can be released to infect other cells
Bacteriophages
- viruses that specifically target bacteria
- do not enter bacteria but inject genetic material into bacteria
- made of nucleic acid genome, capsid, tail sheath, tail fibers
Tail Sheath
act like syringe in injecting genetic material into bacterium
Tail Fibers
help bacteriophage recognize and connect to cell
What is the viral genome made of?
- double stranded DNA
- single stranded DNA
- double stranded RNA
- single stranded RNA
Positive Sense Virus
- single-stranded RNA virus
- genome can be directly translated to functional proteins by ribosomes of host cell
- use host enzymes for protein translation but RNA dependent RNA polymerase to replicate genome
Negative Sense Virus
- singe-stranded RNA virus
- require synthesis of a RNA strand complementary to the negative sense RNA strand which can be used as a template for protein synthesis
- must carry a RNA replicase in viron to ensure synthesis of complementary strand
Retrovirus
- enveloped, single stranded RNA virus
- viron contains 2 identical RNA molecules
- carry an enzyme known as reverse transcriptase
- DNA integrates into host cell genome where it replicates/transcribes its own DNA
Reverse Transcriptase
synthesizes DNA from single stranded RNA
Steps of Viral Life Cycle (4)
(1) Infection
(2) Translation and Progeny Assembly
(3) Progeny Release
(4) Lytic and Lysogenic Cycles
Two Basic Steps of Viral Life Cycle:
(1) attachment (adsorption) – specific binding, not yet infected
(2) injection (penetration) – genomes goes inside host
Describe Viral Infection
- virus binds to specific receptor on host cell
- enveloped viruses fuse with plasma membrane of cell and release viron
How do Bacteriophages infect cells?
use of their tail fibers to anchor themselves to host cell and then use tail sheath to inject viral genome into host cell
Describe Viral Translation and Progeny Assembly
- translocation of genetic material to correct location in cell
- most DNA viruses go to nucleus to be transcribed into mRNA which goes to the cytoplasm and is translated into proteins
- translation of RNA to protein (usually structural capsid proteins) uses ribosomes + tRNA + amino acids + enzymes of host cell
Positive-Sense RNA Virus Translation
genetic material from these type of viruses stays in the cytoplasm where it is directly translated to protein by host cell ribosomes
Negative-Sense RNA Virus Translation
these type of viruses require synthesis of complementary RNA strand via RNA replicase which then allows for translation of proteins
Retrovirus Translation
DNA from these types of viruses travels to nucleus where it is integrated into host genome
Describe the different ways Progeny Release occurs
- viral invasion initiates cell death resulting in spilling of viral progeny
- OR host cell may lyse as a result of containing many virons
- OR virus can leave cell by fusing with its plasma membrane via process known as extrusion (keeps host cell alive and allows for virus to stay in host cell in a productive cycle)
Lytic Cycle
- bacteriophage makes maximal use of cell’s machinery with little regard for survival of host cell
- host cell eventually swells with new virons and lyses so other bacteria can be infected
Virulent
term used for bacteria in the lytic cycle
What happens if virus doesn’t lyse bacterium during the lytic cylce?
then the virus may integrate into host genome as a provirus or prophage and begin the lysogenic cycle
List the steps of the Lytic Cycle (3)
(1) Transcribe and Translate viral genome – hydrolase (destroys host cell genome) and capsid proteins allow for synthesis of viral proteins to occur
(2) Replication of viral genome – automatic assembly of genome with capsid proteins
(3) Lysis of host and release of new viral particles – use of lysozyme which puts holes in cell wall so bacteria can no longer resist influx of water by osmosis
Lysogenic Cycle
- virus is replicated as the bacterium reproduces because it is part of host genome
- environmental factors (radiation, light, chemicals) eventually cause provirus to leave the genome and revert to a lytic cycle
Superinfection
- simultaneous infection of a bacteria
- can become less susceptible to this if bacterium is already infected with one strain of phage
List the steps of the Lysogenic Cycle (3)
(1) integration of viral genome with host genome
(2) normal host activity including reproduction – use of repressor to turn off synthesis of viral genes and make sure only host transcription/translation occurs
(3) excision and lytic cycle – requires removal of repressor, triggered by stress (pH, salt, heat), viral genome is transcribed/translated to enter lytic cycle
What can occur during the excision step of the lysogenic cycle?
- transduction can occur, meaning that there is removal of part of the host gene along with the viral gene
- mixed viral and host genome can be transferred to next host and leads to genetic diversity for bacteria
What must occur to a retrovirus before it enters into the lysogenic cycle?
- virus must go through a cycle to convert its single stranded RNA into a double stranded DNA version of itself
- single stranded RNA of retrovirus can’t be inserted into double stranded DNA of host genome
What are 2 problems that occur with retroviruses?
(1) once in the host genome, they are permanently there
(2) very rapid mutation of these viruses
What is one way that RNA Polymerase differs from DNA Polymerase?
RNA Polymerase has definite stop sites
Productive Cycle
- same as lytic cycle but does not have lysis of host cell at end
- budding of new virus from host cell which allows virus to acquire a coating of the host cell’s lipid bilayer (envelope)
- can only occur in cells without a cell wall – animal viruses!
List the advantages of the Productive Cycle
- leads to creation of more virus because cell not lysed in end
- enveloping of virus gives virus some immune protection and allows for easier infection of next host
Prions
- infectious proteins, nonliving, no DNA or RNA, no membrane or organelles
- resistant to things that normally destroy proteins like salinity, heat, pH, etc
How do Prions cause disease?
- they trigger misfolding of other proteins
- usually involves the conversion of a protein from an alpha helical structure to a beta pleated sheet
- leads to reduced solubility of protein and reduced ability of cell to degrade misfolded protein
Viroids
- small pathogens consiting of short circular single-stranded positive sense RNA that infects plants
- similar to viruses but no capsid
- replicate independently
- can bind to many RNA sequences and silence genes in plant genome which prevents synthesis of proteins and causes metabolic/structural derangement in plant cell
