Viruses Flashcards
Describe the basic characteristics of viruses, including shape, nucleic acid organization and
components ( think conditions for living)
Miniscule, acellular infectious agents having either DNA or RNA
Cause many infections of humans, animals, plants, and bacteria
NO cytoplasmic membrane, cytosol, or organelles
- Contains protein coat or may contain envelope
- Have extracellular and intracellular states
- Does Not have cellular structure
Neither grow nor respond to environment
- viral particles assemble into viron, exhibit tropism (organisms turns toward direction of stimulus), and use host actin tails
Cannot carry out metabolic pathway → can redirect host cell metabolic pathways
- No biochemical reactions going on inside
Cannot respond to environment → attach host cell receptors for entry, assemble, and exit host cell and affect others
- Does Not respond to stimuli
Cannot reproduce independently
- Do replicate in host cell, like many parasitic organisms that require a host
Viruses are NOT alive
Viruses infect all cell types
Bacteriophages ( phages)
Viruses that infect bacteria
Most are eukaryotic viruses ( plants, animals…)
Classified into families based on genome structure, life cycle, morphology, genetic relatedness
Viral basic structural components and states
(for each structure be able to expand)
Virion size range is 10-400 nm in diameter and most viruses must be viewed with an electron microscope
all virions contain
- nucleocapsid composed of Nucleic acid ( DNA or RNA)
- protein coat (capsid)
- envelope
( some viruses consist only of a nucleocapsid others have other components)
- envelope proteins
- extracellular state
- intracellular state
- genetic material 9 Standedness)
what are the two types of viruses characterized by envelopes
- virions having envelopes = enveloped viruses
- virions lacking envelopes = naked viruses
characteristics of Viruses capsids
- Protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells
- Composed of proteinaceous subunits called capsomeres (can be made by single or multiple types of proteins) or protomers
- Capsids are helical, icosahedral, or complex
describe a helical Capsid
Helical: shaped like hollow tubes with protein walls, capsomers self assemble, size of capsid is a function of nucleic acid
describe a Icosahedral capsid
Icosahedron: regular polyhedron with 20 equilateral faces (resembles a sphere); composed of capsomers → ring or knob-shaped units made of 5-6 protomers
Pentamers (pentons) → 5 subunit capsomers
Hexamers (hexons) → 6 subunits capsomers
describe a complex capsid
Complex: viruses that don’t fit into the category of having helical or icosahedral capsids.
Ex: poxviruses (largest animal virus) and large bacteriophages (binal symmetry; head resembles icosahedral and tail is helical)
size and morphology
Not sure what we need to Know but see table
Characterize the viral envelope
Many viruses are bound by an outer, flexible, membranous layer called the envelope
Acquired from host cell during viral replication or release
- Envelope is portion of the membrane system from host
Composed of phospholipid bilayer and proteins
- Some proteins are virally encoded glycoproteins (spikes)
characterize viral envelope proteins
Viral envelope proteins:
Envelope proteins, which are viral encoded, may project from the envelope surface as spikes or peplomers
Involved in viral attachment to host cell → ex: hemagglutinin of influenza virus
Used for identification of virus
Bc the glycoproteins generate antibodies ( ask)
May have enzymatic activity or other activity → ex: neuraminidase of influenza virus
May play a role in nucleic acid replication
characterize the extracellular states of the viruses
Extracellular state:
Called virion
Protein coat (capsid) surrounding nucleic acid
Nucleic acid and capsid also called nucleocapsid
Some have phospholipid envelope
Outermost layer provides protection and recognition sites for host cells
characterize the intracellular state of the viruses
Intracellular state:
- Capsid removed
- Virus exists as nucleic acid j -DNA and RNA - there is no way to differentiate between it and your DNA
Describe the genetic material of viruses
Shows more variety in nature of their genomes than do cells
Primary way scientists categorize and classify viruses
May be DNA or RNA, but never both
Can be dsDNA, ssDNA, dsRNA, ssRNA
Strandedness
May be linear and segmented or single and circular
Much smaller than genomes of cell
what are the two types of criteria used to classify viruses
Classification based on numerous characteristics
Determined by ICTV (international committee on taxonomy of viruses):
David baltimore
describe the ICTV
Nucleic acid type
- Presence or absence of envelop
- Capsid symmetry
-Dimension of viron and capsid
describe the David Baltimore classification
Focused on viral genomes and process used to synthesize viral mRNA
7 life cycle groups based on
dsDNA
ssDNA
dsRNA
ssRNA (+ or – strand)
Retrovirus (ssRNA and dsDNA)
DNA viruses are categorized
KNOW HOW
ON THE EXAM: GIVEN strand, be able to tell what kind of genetic material it is (strandedness slide)
Describe lytic replication
Replication cycle that usually results in death and lysis of host cell
Stages:
1. Attachment
2. Entry
3. Synthesis
4. Assembly
5. Release
Phage injects its DNA into cytoplasm and directs the synthesis of many new phages. The cell lyses and releases the new phages. New phages can then bind to bacterial cells
describe Lysogenic replication
The phage DNA injects its DNA into the cytoplasm of the bacterial cell. The phage DNA integrates into the host chromosome. Then the prophage DNA is copied when the cell divides. Exposure to stress such as UV light triggers excision from the host chromosome.
Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium (lysogenic ALWAYS feeds into Lytic)
Fusion of bacteriophage DNA with that of the host genome
Typically reserved to describe bacteriophages
Prophages: inactive phages
Provirus: inactive virus
know two adv. of Lysogeny
temperature phage changes phenotype of its host
what are the two advantages of lysogeny
Phage remains viable but may not replicate
Multiplicity of infection ensures survival of host cell
understand the effects pf temperature phage changing the phenotype of its host
Bacteria become immune to superinfection
Phage may express pathogenic toxin or enzyme
similarities of the lytic and lysogenic cycle
Both lytic and lysogenic replication cycles are mechanisms of viral reproduction, they occur within a host cell, and both cycles may produce thousands of original copies of original virus
Describe Viral entry
Varies between naked or enveloped virus
Three methods:
1. Fusion of viral envelope with host membrane; nucleocapsid enters
- Endocytosis in vesicle; endosome aids in viral uncoating
- Injection of nucleic acid (Penetration)
3 mechanisms of entry of animal viruses
- Direct injection of nucleic acid
- Membrane diffusion
- endocytosis
Describe Viral Exit
Budding
- Viral proteins are first incorporated into host membrane
- Nucleocapsid may bind to viral proteins
- Envelope derived from host cell membrane, but may be golgi, ER, or other
- Virus may use host actin tails to propel through host membrane
exocytosis
Cell death (lysis)
- Viral proteins may attack peptidoglycan or membrane
- Cell explodes
Describe DNA viruses + their replication
Categorized by
- Their genomic structure
- Presence of an envelope
- Size and shape of their capsid
- The host cells they attack
3 types
- dsDNA → virus can follow normal replication flow (DNA → RNA → protein)
- ssDNA → host enzymes are used to synthesize a complementary strand to genome strand, thus producing dsDNA which can then follow normal replication flow
- dsDNA-RT (double stranded DNA retroviruses)
Describe RNA viruses+ their replication
Only infective agents that store genetic information in RNA molecules
Categorized by
- Their genomic sequence
- Presence of envelope
- Size and shape of their capsid
4 types:
- +ssRNA → acts like mRNA, can be translated directly to make viral proteins
- RNA retroviruses (+ssRNA that convert their genome to DNA)
- (–ssRNA) → cannot translate until replicated into +ssRNA by RNA dependent RNA polymerase (RdRP)
- dsRNA → uses –ssRNA as template to replicate +ssRNA
- Positive RNA acts like mRNA
(can be used by a ribosome to translate protein)
- Negative RNA must first be transcribed as mRNA to be processed by a ribosome
(Requires viral RNA-dependent RNA polymerase)
Describe animal viruses
- Zoonoses (learning objective in itself so know this)
Diseases naturally spread from animal host to humans
EX: rabies, black plague, lyme disease, west nile virus
Acquire zoonoses through various routes
Direct contact with animal or its waste
Eating animals
Bloodsucking arthropods
Humans are usually dead-end host to zoonotic pathogens
Compare and contrast lytic and lysogenic replication cycles:
Lytic is where the viral proteins are being produced and then the lyse and rupture. lysogenic is just replication of genetic material and it later excises itself once it goes into the lytic
Lytic replication:
-Replication cycle that usually results in death and lysis of host cell
Stages:
- Attachment
- Entry
- Synthesis
- Assembly
- Release
(Phage injects its DNA into cytoplasm and directs the synthesis of many new phages. THe cell lyses and releases the new phages. New phages can then bind to bacterial cells)
Lysogenic replication:
The phage DNA injects its DNA into the cytoplasm of the bacterial cell. The phage DNA integrates into the host chromosome. Then the prophage DNA is copied when the cell divides. Exposure to stress such as UV light triggers excision from the host chromosome.
(viral proteins are not being replicated in bacteriophages???)
Infected host cells grow and reproduce normally for generations before it lyses
Viral genetic material is integrated into the chromosome
RNA vs DNA viruses and replication of both
DNA viruses
Categorized by
Their genomic structure
Presence of an envelope
Size and shape of their capsid
The host cells they attack
3 types:
dsDNA → virus can follow normal replication flow (DNA → RNA → protein)
ssDNA → host enzymes are used to synthesize a complementary strand to genome strand, thus producing dsDNA which can then follow normal replication flow
dsDNA-RT (double stranded DNA retroviruses)
RNA viruses
Only infective agents that store genetic information in RNA molecules
Categorized by
Their genomic sequence
Presence of envelope
Size and shape of their capsid
4 types:
dsRNA → uses –ssRNA as template to replicate +ssRNA
+ssRNA → acts like mRNA, can be translated directly to make viral proteins
–ssRNA → cannot translate until replicated into +ssRNA by RNA dependent RNA polymerase (RdRP)
RNA retroviruses (+ssRNA that convert their genome to DNA)
Differentiate between animal viruses and bacteriophages
( be able to expand on each step this is only the skeletal structure)
Animal Viruses
- Replication
- Attachment
- Synthesis
- Assembly and release
Latency of animal viruses
bacteriophages
- attachment
- replication
- synthesis
- assembly and release
Describe potential effects of viral infection on host cells, including latency, inclusions and oncogenesis
Latency:
- Viruses remain hidden or dormant inside host cell which cause latent infections
- May cause an acute infection before becoming dormant
- Ex: varicella zoster virus causes chickenpox (affects many areas of the body) but 10-12 days later becomes dormant and stops replicating → after many years of dormancy the virus is reactivated and causes shingles (nerve cell specific)
- Latent viruses may remain dormant by existing as circular viral genome molecules outside of host chromosome or by becoming proviruses by integrating into host genome
Inclusions and oncogenesis:
Inclusions:
- Aggregates of proteins found in neurons
- Caused by the presence of viruses
- Represent sites of viral multiplication
- Occur in the cytoplasm
Oncogenesis:
- Process of normal cells turning into cancer cells
- Activation of oncogenes
- Series of genetic and cellular changes
- Cells divide in uncontrolled manner
- Cancer causing
Describe the role of oncogenesis in viruses. Discuss the functioning of oncogenes.
- Oncogenesis allows viruses to encode for proteins that break down the host cell genome and prevent cell death which leads to uncontrolled cell growth.
- Oncogenes are a gene that when activated have the potential to cause cancer because they are pleiotropic and can regulate growth, apoptosis, differentiation, cell cycle, and cell proliferation.
—-Metastasis: spread of cancerous cells throughout the body
—-Benign: tumor remains in place
—–Malignant: abnormal growth of neoplastic cells - form lump of tissue
( look at blue on the doc for more info)
Discuss ways to culture and visualize viruses
Culturing viruses in mature organisms:
1. In bacteria
2. In Plants and animals
In embryonic chicken eggs:
1. Inexpensive
2. among the largest of cells
3. Free of contaminating microbes
4. Contain nourishing yolk
In cell tissue culture:
1. Consists of cells isolated from an organism and grown on a medium or in a broth
2. Two types of cell cultures:
a. Diploid cell cultures
b. Continuous cell cultures
Visualize Cells
1. Plaque assays:
a. Dilutions of virus preparation made and plated on lawn of host cells
b. Number of plaques counted
c. Results expressed as plaque-forming units (PFU) / mL
- Viral plaques:
a. Viral plaques in a lawn of bacterial growth
Define Viroid, virusoid, and prion. Compare these subviral agents to viruses and other living organisms.
Viroid: A subviral particle lacking a capsid and consist solely as RNA
a. extremely small, circular pieces of RNA that are infectious and pathogenic in plants
b. Similar to RNA viruses, but lack capsid
c. Do not encode gene products
d. May appear linear due to H bonding
Virusoid: A subviral particle lacking a capsid, consist solely as RNA, and requires a virus for replication
a. Infectious agents composed only of closed, circular ssRNAs
b. Do not encode gene products
c. Requires virus helper
d. Requires host cell DNA-dependent RNA polymerase to replicate
e. Cause plant diseases → found in nucleolus or chloroplasts; disease caused by RNA silencing
Prion: A misfolded, infectious protein, only affecting the brain
- Proteinaceous infectious agents ( interacts with another protein and causes it to misfold which then interacts with another protein causing it to misfold = domino effect) changes in weak chemical interactions through protein protein interactions)
- Cellular PrP protein
a. Made by all mammals
b. Function unknown
c. Normal structure with a-helices called
cellular PrP
Prion PrP
-Disease-causing form with B-sheets called
prion PrP
Prion PrP converts cellular PrP into prion PrP by inducing conformational change
These compare to viruses and other organisms in the way that they only consist of either RNA or proteins whereas viruses and other organisms can consist of DNA and RNA and proteins all in one.
Are viruses alive? Discuss why the status of the virus as a living organism is controversial. How does viral infection lead to morbidity and mortality?
No viruses are not alive because they do not meet all the requirements (cannot carry out metabolic pathways and cannot reproduce independently and require a host)
Others consider them to be the least complex living entities because they:
a. Use sophisticated methods to invade cells
b. Have the ability to take control of their host cells
c. Are able to replicate themselves
Others think it’s a complex pathogenic chemical that lacks characteristics of life
Characterics of life:
Growth
Metabolism
Responsiveness
Reproduction
Cell Structure
Viral infection leads to morbidity and mortality:
Viruses cause 20-25% of human cancers
Some carry copies of oncogenes as part of their genomes
Some promote oncogenes already present in host
Some interfere with tumor repression when inserted into hosts repressor gene
Specific viruses are known to cause 15% of human cancer
Once viral infections invade a host cell they can begin to replicate and destroy more and more cells
They can also activate oncogene which could cause uncontrolled cell growth.
