Chapter 5 Flashcards
Biological entity that is known to infect every type of cellular organism.
Virus
Early researchers found that when infectious fluids were passed through porcelain filters designed to trap bacteria, the fluid that came through was still infectious/pathogenic.
; This result proved that an infection would be caused by a fluid containing agent smaller than bacteria, thus introducing the concept of _________ ______.
Filterable virus
This person hypothesized that rabies was caused by a living thing smaller than bacteria
He also proposed the term virus which is Latin for poison
Louis Pasteur
Loeffler and Frosch discovered what?
An animal virus that caused foot-and-mouth disease in cattle
Ivanovski and Beijerinck showed what?
That a disease in Tobacco was caused by a virus.
Seawater contain ___ million viruses per millimeter
10 million
What are some questions that remain about virology.
- Are they organisms; that is, are they alive?
- What role did viruses play in the evolution of life?
- How can they jump from other species to suddenly cause severe disease in humans?
- How can particles so small and simple be capable of causing disease and death?
- What is the connection between viruses and cancer? (our entire genome is stored in DNA and accessed in useable pieces in RNA, so if a virus has nucleic acids it can be detrimental to the RNA in our cells.)
Terminology to describe viruses instead of ‘alive’ and ‘dead’
Active and inactive
The Viral Debate
- Since viruses are unable to multiply independently from the host cell, they are not living things and should be called infectious molecules.
(cannot replicate on their own and require a host cell -> so not alive) - Even though viruses do not exhibit most of the processes of cells, they can direct them, and thus are certainly more than inert and lifeless molecules.
The Vital Role of Viruses in Evolution
- Infect cells and influence their genetic make up.
- Shape the way cells, tissues, bacteria, plants, and animals have evolved.
- 8% of the human genome consists of sequences that come from viruses.
- 10-20% of bacterial DNA contains viral sequences.
What does it mean that viruses are obligate intracellular parasites?
It means that they cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release quantities of new viruses.
Properties of Viruses
- are obligate intracellular parasites of bacteria, algae, fungi, plants, protozoa, and animals
- The amount of viruses on earth is approximately 10x the amount of bacteria and archaea combined (partially because they multiply so rapidly, and because they can infect so many eukaryotes)
- Are ubiquitous in nature and have had a major impact on development of biological life
- Size ranging from 20-1000nm
- Continued on other slides….
Are not cells; structure is very compact and economical
Do not independently fulfill the characteristics of life
Viruses
What is the basic structure of a virus?
Consists of a protein shell (capsid) surrounding a nucleic acid core
-> protein coat with a string of nucleic acids
; nucleic acid can be either RNA or DNA but not both
; nucleic acid can be double-stranded DNA or RNA, or single-stranded DNA or RNA
How do viruses multiply?
By taking control of the host cell’s genetic material and regulating the synthesis and assembly of new viruses
TRUE or FALSE:
Viruses lack enzymes for most metabolic processes and lack machinery for synthesizing proteins.
TRUE
What are the main criteria currently used to group viruses that take into account the actual nature of the virus particles themselves instead of being based on their hosts and the disease they cause?
Hosts and diseases they cause, structure, chemical composition, and similarities in genetic make up
International Committee on the Taxonomy of Viruses (ICTV)
in charge of classifying and categorizing viruses
*it is difficult to classify viruses, and their classifications are constantly changing
What is a reason that viruses can multiply so rapidly?
Because they only have a protein coat and string of nucleic acids to replicate, vs eukaryotes that have multiple structures and organelles to replicate and bacteria/archaea (prokaryotes) that also have more structure to replicate.
Viruses are __________.
; ultramicroscopic (has to have a much higher resolution to be visible)
; range from 20-1000nm in size
Viruses are not ___; and their structure is very ________ and ________.
; cells
; compact
; economical
TRUE OR FALSE:
Viruses do not independently fulfill the characteristic of life.
TRUE
True or False:
Viruses have molecules on their surface that dictate which host cells they should and could attach to.
The molecules also dictate how virulent the virus is.
True
According to the ICTV, how many orders and families of viruses are there?
8 orders and 38 families
*another 84 families not yet assigned to any order.
5.2 Outcome
Discuss the size of viruses relative to other
microorganisms.
smallest infectious agents
more than 2000 bacterial viruses could fit into the average bacterial cell
some range: 0.02micrometers (20 nm) to 150nm
What can the size of a Pandoravirus be compared to?
coccus-shaped bacterial cell
Viral architecture is most often observed using _____ stains in combination with electron microscopy.
Special
Explain the outer surface of a virus
Composed of regular repeating subunits that give rise to their crystalline appearance
-external protein coating
5.3 Outcome
Describe the function and structure(s) of viral capsids
- protein capsid or shell that surrounds the nucleic acid in the central core
- together capsid and nucleic acid called nucleocapsid
- each capsid is constructed of identical subunits called capsomeres (which are made from protein molecules)
- depending on how the capsomeres are shaped and arranged the assembly results in two different types: helical and icosahedral
Many animal viruses also have an additional covering to the capsid called an ______.
envelope (usually a modified piece of a host’s cell membrane)
Viruses that consist of only a nucleocapsid (as opposed to also having an envelope) are called _____ _____.
naked viruses
A fully formed virus that is able to establish an infection in a host cell is often called a _______.
virion
5.2 Outcome
Distinguish between enveloped and naked viruses
An enveloped virus has a nucleocapsid and an envelope which likely originated from the host’s cell membrane
A naked virus only has the nucleocapsid
5.2 Outcome
Explain the importance of viral surface proteins, or spikes
essential for the attachment of viruses to the next host cell
What viruses are complex capsids found in?
only found in viruses that infect bacteria
these viruses are never enveloped
Naked HELICAL nucleocapsids
very rigid, tightly wound, cylinder shaped
ie. tobacco mosaic virus
Enveloped HELICAL nucleocapsids
more flexible, tend to have a looser arranged helix
ie. influenza, measles, SARS-CoV-2, rabies
Helical capsids
rod shaped capsomeres
Icosahedron capsids
three dimensional, 20 sided figure with 12 evenly spaced corners; major variations in the number of capsomeres
Naked icosahedron
clearly see spikes
Enveloped icosahedron
ie. hep B and herpes simplex virus
5.2 Outcome
Describe the possible nucleic acid configurations that viruses may possess.
can have either DNA or RNA but not both.
can have single or double stranded RNA or DNA
-RNA genomes may be segmented
DNA Viruses - List examples
Variola virus
- ds
- causes smallpox
Herpes simplex2
- ds
- causes genital herpes
Parvovirus
- ss
- causes Erythema infectiosum (skin condition)
RNA Viruses - List Examples
Poliovirus
- ss (+) sense
- causes Poliomyelitis
Influenza
- ss (-) sense
- causes flu
Rotavirus
- ds RNA
- causes gastroenteritis
HIV
- ss RNA (carries reverse transcriptase)
- causes AIDS
what type of genome does SARS-CoV-2 have?
single stranded (+) sense RNA virus
this means the virus is ready for immediate translation into proteins
The structure of a virus contains only those parts needed to invade and control a host:
1.
2.
3.
External coating
Core containing one or more nucleic acid strands of DNA or RNA
Sometimes one or two enzymes
Viruses lack any _____-______ machinery found in cells.
protein-synthesizing
The sum total of the genetic information carried by an organism
Genome
The number of viral genes is quite ____ compared with that of a cell, but the number of viral genes can vary.
small
The more people that get vaccinated and few people who get infected allows for what
viruses have less ability to multiply and mutate into something more virulent
; decreased opportunity
Positive-sense RNA:
Negative-sense RNA:
Segmented RNA:
Retroviruses:
- ready for immediate translation
- must be converted before translation occurs
- Individual genes exist on separate pieces of RNA
- Carry their own enzymes to create DNA out of their RNA (going backwards- think retro clothes/music)
*normally do not create DNA out of RNA
Poliovirus and Influenza Virus
single stranded RNA
Variola and Herpes simplex-s viruses
Double stranded DNA
Parvovirus
Single stranded DNA
Rotavirus
double stranded RNA
HIV
single stranded RNA + REVERSE transcriptASE
*ase= enzyme
Enzymes for specific operations within their host cell:
Synthesizes DNA and RNA
Polymerase
Enzymes for specific operations within their host cell:
Replicates a copy of RNA
Replicase
Enzymes for specific operations within their host cell:
Synthesizes DNA from RNA
reverse transcriptase
Some viruses carry away substances form their host cells:
Arenaviruses
pack along host ribosomes
Some viruses carry away substances form their host cells:
Retroviruses
borrow the host’s tRNA molecules
The way the cycle of a virus seizing control of a host cell works dictates what characteristics?
- the way the virus is transmitted
- what it does to the host
- the responses of the immune defenses
- human measures to control viral infections
Step One of 5 Step life cycle of animal viruses
Adsorption: a process of adhering one molecule onto the surface of another molecule
Step Two of 5 Step life cycle of animal viruses
Penetration (viral):
the step in which the virus enters the host cell
Step Three of 5 Step life cycle of animal viruses
Uncoating:
the process of removal of the viral coat and release of the viral genome by its newly invaded host cell
Step FOUR of 6 Step life cycle of animal viruses
Synthesis:
viral genetic materials and proteins are made through replication and transcription/translation
- RNA viruses are replicated and assembled in the cytoplasm
- DNA viruses enter the host’s nucleus and rep. + assembled there
Step FIVE of 6 steps in the life cycle of animal viruses
Assembly:
Capsids and and genetic material are packaged into virions
Step SIX of 6 steps in the life cycle of animal viruses
Release:
the assembled virus particle exits the host cell and moves on to infect another cell
-controlled by size of the virus
-health of the virus
ie. Poxvirus infected cell: 3000-4000 virions
Poliovirus infected cell: 100,000 virions
What is the length of time of the replication cycle in multiplication cycles in animal viruses?
Varies from 8 hours in polioviruses to 36 hours in herpesviruses
Host Range (adsorption)
the limitation imposed by characteristics of the host cell on the type of virus that can successfully invade it
ie. Hep B which only infects human liver cells
Tropisms
specifications of viruses for certain tissues
Penetration/uncoating through endocytosis
an entire virus is engulfed by the cell and enclosed on a vacuole or vesicle
uncoating by enzymes inside the host cell exposing the nucleic acids
ie. herpesvirus
Penetration through direct fusion of the viral envelope with the host cell membrane
envelope merges directly with the cell membrane, liberating the nucleocapsid into the cell’s interior
ie. mumps virus
Life Cycle of dsDNA viruses
early phase:
- viral DNA enters the nucleus, where genes are transcribed into messenger RNA
- RNA transcript moves into cytoplasm to be translated into viral proteins required to replicate the viral DNA
- the host cell’s DNA polymerase is involved in this phase
late phase:
- parts of the viral genome are transcribed and translated into proteins required to form the capsid and other structures
- new viral genomes and capsids are assembled
- mature viruses are released by budding or cell disintegration
5.3 Outcome
Define the term cytopathic effect and provide one
example.
virus-induced damaged to the cell that changes its microscopic appearance
example: Syncytia
- the fusion of multiple damaged host cells into single large cells containing multiple nuclei
example: poliovirus
- cells are killed completely by shrinking, detaching and lyse
5.3 Outcome
Discuss both persistent and transforming infections.
Persistent: some cells maintain a carrier relationship and a virus is not immediately lysed:
- can last from a few weeks to the remainder of a the host’s life
- can remain latent in the cytoplasm
Transforming: Some animal viruses enter a host cell and permanently alter its genetic material, leading to cancer; these viruses are termed oncogenic.
- some ways this works are the virus carries genes that directly cause cancer, or the virus produces proteins that induce loss of growth regulation leading to cancer
- oncoviruses: viruses that can initiate tumors
Provirus
when viral DNA is incorporated into the DNA of the host
-in relation to persistent infections
- chronic latent phase: periodically become activated under the influence of various stimuli
ie. herpes simplex and herpes zoster viruses
5.3 Outcome:
Provide thorough descriptions of both lysogenic and lytic
bacteriophage infections.
Lysogeny: a condition in which the host chromosome carries bacteriophage DNA without the production of virions
- Temperament Phages: undergo adsorption and penetration, do not undergo replication or release immediately (less virulent stage)
- Prophage State: viral DNA inserted into bacterial chromosome, and copied during normal bacterial cell division
- Induction: process whereby a bacteriophage in the prophage state is activated and begins replication and enters the the lytic cycle
Oncoviruses
mammalian viruses capable of initiating tumors:
- Papillomaviruses
- Herpesviruses
- Hep B viruses
- HTLV-I
Bacteriophage
“bacteria eating” virus
- this type was discovered in 1915
- most contain dsDNA
- often make the bacteria they infect more pathogenic for humans
- every bacterial species is parasitized by one or more bacteriophages
Lysogenic Conversion
when a bacterium acquires a new trait from its temperament phage
-Vibrio cholerae - cholera toxin
- Clostridium botulinum - botulinum toxin
- occasionally phage genes in the bacterial chromosome cause the production of toxins or enzymes that the bacterium would not otherwise have
Adsorption
attachment of the virus to its host cell
-has to be attachment and recognition
persistent infections
long term relationships with viruses
-weeks to for a life time
5.4 Outcome:
List the three principal purposes of cultivating viruses
- Isolate and identify viruses in clinical specimens
- Prepare viruses for vaccines
- Do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells.
5.4 Outcome:
Describe three ways in which viruses are cultivated.
in vivo
method of inoculation of living embryos or animals
in vitro
using cells or tissues that are cultivated in the lab for study
Usual choice of animal cultivation of viruses for study
specially bred strains of white mice, rats, hamsters, guinea pigs, rabbits
*insects (invertebrates) or nonhuman primates are occasionally used as well
why is a bird egg containing an embryo a common choice for inoculation studies?
intact and self supporting life unit
- sterile environment
- nourishment
types of in vitro cultivation that are a simple and effective way to grow populations of isolated animal cells in culture
cell culture or tissue culture
-grown in sterile chambers with special media that contain correct nutrients
Monolayer
-in relation to in vitro cultivation by cell culture
a single, confluent sheet of cells that supports viral multiplication and allows researchers to closely inspect for signs of infection
Plaques
in virus propagation methods, the clear zone of lysed cells in tissue culture or chick embryo membrane that corresponds to the area containing viruses
- virus growth show up as clear well-defined patches in the cell sheet
- visible manifestation of cytopathic effects (CPEs)
Detection of viruses in bacteriophages
same technique is used to detect and count bacteriophages as is used in mammalian cells:
- plaque develops when the viruses released by an infected host cell radiate out to adjacent cells
- new cells become infected, die, and release more viruses -> process continues
- plaque manifests as a macroscopic clear spaces that show dead cells
5.5 Outcome
Name three noncellular infectious agents besides viruses.
- Prions
- Satellite Viruses
- Viroids
Prions
a cytopathic protein associated with slow-virus spongiform encephalopathies of humans and animals
ie. Mad cow (bovine spongiform encephalopathy)
ie. Creutzfeldt-Jakob disease (CNS gradual degeneration and death)
ie. Shy-Drager Syndrome
- we do not know the exact mode of prion transmission
- composed primarily of protein with no nucleic acid
Satellite Viruses
- virus like particles that are dependent on OTHER viruses for replication
ie. Adeno-associated virus (AAV): initially thought to only replicate in cells infected with adenovirus, but we now know it can infect cells infected with other viruses
ie. Delta Agent: a naked circle of RNA that is expressed only in the presence of hep b virus -so worsens the severity of sx of hep B
Viroids
- in infectious agent, that unlike a virion, lacks a capsid and consists of a closed circular RNA molecule.
- Although known viroids are all plant pathogens, it is conceivable that animal versions exist
- about one tenth the size of the average virus
- significant in plants like tomatoes, potatoes, cucumbers, citrus trees, and chrysanthemums
5.6 Outcome
Analyze the relative importance of viruses in human
infection and disease
5.6 Outcome:
Discuss the primary reason that antiviral drugs are more
difficult to design than antibacterial drugs
PRIMARY REASON:
difficult to find drugs that will affect viruses without damaging host cells
-viruses can mutate and become resistant
how many viruses are known to infect humans
260
Common causes of acute infections
colds, hepatitis, chickenpox, influenza, herpes, warts
Prominent viral infections worldwide
dengue fever, Rift Valley fever, yellow fever
Infections with high mortality rates
COVID-19, AIDS, Ebola
Infections that cause long-term disability
Polio, neonatal rubella
Connection to chronic infections
T1DM, MS, various cancers, Alzheimer’s, obesity
Almost all antiviral medications are designed to target one of the steps of the viral life cycle
; replication
ie. Integrase inhibitor class of HIV drugs interrupts the ability of HIV genetic info to incorporate into the host cell DNA
-»_space;>prevents the integration of DNA
