Chapter 13 Flashcards
Virus Characteristics
- Obligate intracellular parasitic molecules
- non-motile
- non-living
- over 6,000 known to man
Structure of Viruses
- DNA or RNA
- linear or circular
- single or double stranded
- code for enzymes like DNA or RNA polymerase and enzymes to make rest of virus particle
Cell Structure
Virus
do not have organelles or ribosomes
-only protein and nucleic acids
Metabolism
virus
require host for synthesis of nucleic acid and proteins
Nucleic acids
virus
contain DNA or RNA, either single or double stranded
Genes
virus
have 4- 200 genes
bacteria # of genes
3000
humans have ____ genes
30,000
Size
virus
viruses are 100-1000x smaller than cells
Capsid
- found in all viruses
- made of protein subunits called CAPSOMERES
- carries enzymes needed for infection
Common virus shapes
Icosahedral
Helical
Complex
Icosahedral
20 sided polygon
20 triangles, 12 corners
Helical
rod-shaped capsid
Complex
shape
ex. T4 phage: icosahedral head and tail of helical arranged proteins
Envelope
lipid bilayer outside of capsule
- only in some viruses
Naked viruses
lack envelope, most phages
Enveloped viruses
envelope surrounds capsid
- disinfectants damage envelope making virus non-infectious
- matrix proteins found between envelope and capsid, give different shapes to virus
Spikes
- protein structures that allow virus to attach to host
- on capsid, envelope, or tail
- virus subtype based on spikes
purpose of capsid and envelope
protect the nucleic acid from enzymes and toxic chemicals
ex. polio virus - naked virus can survive GI tract
purpose of spikes and capsid
allow virus to attach to host cell
capsid has special proteins for naked cells to help it attach
Classification and Naming of Viruses
- not classified as a kingdom or domain because not living
Two superfamilies
DNA or RNA
human viruses 7 DNA families, 13 RNA families
Family names end in
-viridae
Genus names end in
-virus
Enteric viruses
transmitted by fecal- oral route
ex. polio
Zoonotic viruses
transmitted to humans by animals
ex. rabies, cow pox
Arboviruses
arthropod borne viruses
ex. west nile
bacteriophages
viruses that infect bacteria
AKA phages
Characteristics used in classification
- Genome structure
- type of host
- shape of capsid
- overall size
- presence of cell envelope
- how virus affects host cell
Potential relationship of virus with host
- Productive state
2. Latent or Lysogenic State
Productive state types
Lytic
Chronic
Latent or Lysogenic state types
Latent = animal cells Lysogenic = bacteria cells
Lytic
Productive
Host cell is destroyed by lysis when new viruses released
ex. HIV and lymphocytes
Chronic
Host cell survives and continues to divide while releasing viruses
ex. HIV and macrophage
Productive Lytic - Bacteriophage
T4 virus
T4 = double stranded DNA viruse, infects E.coli
- Attachment/Adsorption
- Penetration/ Genome entry
- Synthesis
- Assembly
- Release
Attachment/Adsorption
- viruses non-motile so they collide with host by chance
- infection depend on specific receptor sites on outside of host cell
Where are receptor sites located on bacteria and animal cells
bacteria - cell wall, flagella and pili
animal cell - cell membrane
type of organisms of cell types that virus can infect, usually very specific
Host range
Penetration/Genome Entry
bacteria
- viral nucleic acids enter host cell
- enzyme lysozyme from tip of T4 virus tail digest part of E.coli cell wall.
- Tail contracts and phage DNA injected through cell wall and cell membrane to cytoplasm
- capsid remains outside of host cell
Synthesis
bacteria
Synthesis and transcription of viral DNA and production of new viral parts
- uses machinery of host cell (ribosomes, some enzymes) to make “phage-encoded” proteins
types of “phage-encoded” proteins
Early proteins
late proteins
Early proteins
nuclease that destroys host cell DNA, proteins that modify host RNA polymerase - so no host proteins are made
Late proteins
structural proteins to make capsid and tail
Assembly
bacteria
- virus particles assembled into mature viruses
- some parts assemble spontaneously
- other parts require extra enzymes and proteins for assembly
Release
Bacteria
- mature viruses are released
- phage-encoded LYSOZYME starts digestion of cell wall from inside
- osmotic pressure causes cell to lyse break
- viruses are expelled
Burst size
number of phages released
about 200 for T4 viruses
Attachment/adsoption
Animal
- Virus recognizes and attaches to host
- Spikes on virus bind to specific receptor sites on host membrane
- in animals, often are two different receptros proteins
- receptors usually have a different purpose for the host
ex. rabies attaches to Acth receptor of nerve cells
Penetration/uncoating
animal
- in animals the whole virus enters the host cell (in bacteria just the nucleic acid enters)
- Two methods:
Endocytosis and fusion
Endocytosis
- whole virus is moved into cell by endocytosis
- virus is engulfed in a vesicle made by host
- occurs with naked and enveloped viruses
- uncoating- host enzymes dissolve capsid and envelope and nucleic acids are released
Fusion
- Viral envelop fuses with host cell membrane
- only capsid and genome enter host
- uncoating occurs to release nucleic acid
Steps of animal productive lytic
- Attachment (adsorption)
- Penetration (genome entry) (uncoating)
- Synthesis -> nuclease (breaks down nucleic acids)
- Assembly
- Release (budding) enveloped viruses
Synthesis
- Synthesis and transcription of viral nucleic acid and production of new viral parts using machinery of host cell just as in phages
- genome is expressed and copied
- viral enzymes made
- capsomeres and spikes made
Synthesis DNA viruses
Double stranded - “normal” mechanisms
Single stranded - made into double stranded before transcription
Synthesis RNA viruses
Single and Double stranded - viral enzyme REPLICASE (RNA polymerase) makes new RNA for new viruses, replicases do not proof-read like DNA polymerase which explains mutations of influenza virus
Synthesis (Revers Transcriptase)
- Some viruses have complex system of genome sythesis
ex. RETROVIRUSES (including HIV) - Viral RNA is made into DNA using viral encoded enxyme called REVERSE TRANSCRIPTASE
- this double stranded DNA is incorporated into host DNA
- Virus can become productive or latent
Assembly
animal
- virus particles assembled into mature viruses
- if the virus is enveloped, then spikes are placed within the cell membrane of the cell
Release
animal
Mature viruses are released
- human virus burst size = 3,000 - 50,000 viruses/one cell
- enveloped vs. Naked viruses
Enveloped Viruses
Release- Animal
- Budding
- viruses released slowly, host cell usually not destroyed
- Host cell can be damaged
Budding
vesicle (from cell membrane or Golgi) surrounds virus, spikes added to membrane
Ways host cell can be damaged in release of enveloped viruses
- Shutdown of metabolism and genetic expression
- Destruction of cell or organelle membranes
- toxicity of viral components
Naked virus - release
Animal
- Cell fills with viruses
- spontaneously lyses
- Release can be caused by virus triggering normal host immune responses
- host cell death
Productive Chronic Bacteriophage
- M13 filamentous, single stranded DNA phage
- virus released without killing host cell
4 steps of Productive Chronic Bacteriophage
- Attachment
- Penetration
- Synthesis
- Assembly
Attachement
Bacteriophage
attaches to F pilus
Penetration
Bacteriophage
injects DNA through pilus to cytoplasm
Synthesis
Bacteriophage
Host DNA polymerase used to synthesize viral DNA, capsomeres put into bacteria membrane
Assembly
Bacteriophage
capsomeres surround the viral DNA as it exits
Viral Relationships with Host Cells
Latent/Lysogenic
- No new viruses are produced by host cell
- Viral nucleic acid incorporated into host chromosome
- incorporated viral DNA called prophage in bacteria and provirus in animal cells. - Host cell continues to divide and viral DNA is replicated with each new cell made
Lysogenic or Latent… what cell type and name
Lysogenic - Bacteria called prophage
Latent - Animal cell called provirus
most viral genes are NOT expressed in ______ state
lysogenic/latent
if genes are expressed the living host cell can produce new types of proteins- gives host new characteristics
Viral repressor proteins
can prevent production of the hosts excision proteins
excision proteins
found on host cell that cut viral DNA out of host chromosome
induction
excision proteins allowed to cut and release viral DNA, virus goes from lysogenic to productive/lytic
temperate virus
AKA lambda phage
a virus that can go from lysogenic to productive lytic
if the bacteriophage is inside it is in what phase
lysogenic phase
types of human viruses
virulent
latent
temperate
virulent viruses
always productive lytic, rapid, severe, destructive, cell death but not always organism death,
Ex. Influenza, mumps, polio
latent viruses
non-active, in dormant state inside cell, incorporate DNA into host chromosome
ex. HIV and herpes
Temperate viruses
can cause lysis or become latenet, environmental conditions may influence which state is present
ex. herpes
Acute infections
-cause disease symptoms for short periods of time
- body can develop life-long immunity
- viruses are productive
- extensive tissue damage and cell death, but is usually localized
- much of damage can be repaired after recovery
Ex. mumps, measles, influenza, cold virus
Persistent infections
- specific virus is present in host for long periods of time, life-long possible
- virus can be productive or latent depending on state of disease
- disease symptoms may not be apparent
- host become carrier of infective viruses
Late complications following acute infection
persistent
relapse can occur years after recovery from an acute infection
Ex. measles and subacute sclerosing panencephalitis (SSPE)
latent infections
persistent
similar infection occurs days to years after recovery from acute infection, viruses are not detected until disease onset.
- Herpes family viruses - shingles, chicken pox, genital herpes, cold sores
- Herpes simplex 1 - cold sores in mouth, move to neurons when latent, environmental triggers cause movement of virus back to mouth
Chronic infections
persistant
continuous production of low levels of viruses often in absence of disease symptoms
ex. Hep B and C - acute phase includes nausea and fever, virus can persist and slowly cause liver cirrhosis
Slow infections
persistant
number or viruses slowly increases over a long period of time
ex. HIV/AIDS
Cancer
abnormal condition of unregulated cell division
Tumor
mass of abnormally dividing cells
Benign tumor
remains within a localized region
Malignant tumor
cells migrate (metastasize) to other location
Regulatory genes
can regulate rate and frequency of cell division - without regulation cell may become cancerous
Two types of regulatory genes
Proto-oncogenes
Tumor-suppressor genes
proto-oncogenes
activate transcription, increase rate of cell division, cancer develops if gene is always on
tumor- suppressor genes
stop cell division, cancer develops if gene is turned off
Regulatory genes are altered by
Mutation - 80% of cancer (50% on tumor-suppressors, 30% on proto-oncogenes)
Viruses - 15% (or more) of all human cancers
last 5% is d/t genetic predispositon
Oncoviruses general info
- viruses that can transform host cell into a cancer cell
- latent - viral genes incorporated into host DNA
- induction can be caused by decreased immunity, stress, fever, UV exposure
Oncovirus definition
disrupt and permanently turn on proto-oncogenes
Retrovirus
RNA viruses use revese transcriptase to make double stranded DNA which is incorporated into host DNA, can modify host cell function including conversion to cancer cell
viroids
- single stranded, circular RNA, no capsid
- no proteins produced
- infect only plants
- mechanisms of disease production unknown
ex. potato spindle tuber
prions
- compose only of proteins
- affect CNS of humans and other animals
- Scrapie in sheep, Bovine spongegiform encephalophathy, and Creutzefeld-Jakob disease in humans
- very stable - resistant to heat and chemicals
Mechanisms unknown - normal host proteins destroyed, nerve cells destroyed.
Person infected with HIV virus by symptoms are not present
HIV infection
person infected with HIV and symptoms ARE present
HIV disease
Last stage of HIV disease, characterized by opportunistic infections
AIDS
Aquired Immunodeficiency Syndrome
Causative Agent of HIV
single stranded RNA virus, enveloped, retrovirus.
HIV-1 is more common in US than HIV-2
