Chapter 13- Characterizing and Classifying Viruses, Viroids, and Prions Flashcards
What is the definition of a virus?
A miniscule, acellualr, infectious agent having either DNA or RNA
Causes many infections in human, animals, plants, and bacteria
Causes of the diseases the plague the industrialized world- cold, flu, herpes, AIDS, and SARS
What are some characteristics of viruses?
Cannot carry out any metabolic pathway
Neither grow nor respond to the environment
Cannot reproduce independently- recruit cell’s metabolic pathways to increase numbers
No cytoplamic membrane, cytosol, or organelles
Have an extracellular and intracellular state
What are some characteristics of the extracellular state?
Called virion
Protein coat (capsid) surrounds the nucleic acid, nucleic acid and capsule known as nucleocapsid
Some have phsopholipid envelope
Outermost layer provides protection and recognition sites for host cells
What are some characteristics of the intracellular state?
Capsid is removed
Virus exists as nucleic acids
What is the genetic material of viruses?
Show more variety in nature of their genomes than do cells
May be used as a means of classification
EITHER DNA OR RNA, NEVER BOTH- dsDNA, ssDNA, ssRNA, dsRNA
May be either linear and segmented or single and circular
Much smaller than genomes of cells
What are the types of hosts for viruses?
Only infect a particular host’s cells- due to affinity of viral surface proteins for complementary proteins on host cell surface
May be so specific they only infect particular kind of cell in a host (HIV)
Generalists- infect many kinds of cells in many different hosts including bacteria, arachea, protozoa, fungi, plants, animals, and humans (flu virus, West Nile)
What is the history of viruses?
Ivanowski- 1892, demonstrated that viruses are acellular through the conduction of an experiment
Stanley- 1935, isolated and characterized tobacoo mosaic viruses
What are general characteristics of capsid morphology?
Capsids- protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells
Composed of proteinaceous subunits called capsomeres
Capsomere may be made of single or multiple types of proteins
What are the characteristic shapes of a virus?
Polyhedral, complex, helical
What are enveloped viruses?
A virus with an outer envelope surrounding the capsid is an enveloped virion
If lacking an envelope- non-enveloped or naked virion
Matrix proteins fill the region between capsid and envelope
How are viral envelopes acquired by a virus?
Acquired from host cell during viral replication or release
Envelope is a portion of membrane system of host, composed of phospholipid bilayer and proteins
Some proteins are virally coded- glycoproteins (spikes)
Envelope’s proteins and glycoproteins often play role in host recognition
What are general characteristics of DNA viruses?
All replicate in the nucleus- except for the Pox virus (cytoplasm)
All are double strranded- except for parvoviridae (single) and hepadnaviridae (partial single or double stranded)
Includes:
Poxviridae
Herpesviridae
Papillomaviridae
Polyomaviridae
Adenoviridae
Hepadnaviridae
Parvoviridae
What are some characteristics of RNA viruses?
All single stranded except for Reoviridae
+- viral RNA can be used as mRNA without modification
- viral RNA cannot be used by cell, must be modified
(See chart in notes)
What are characteristics of viral replication?
Dependent on hosts’ organelles and enzymes to produce new virions
Lytic replication- replication cycle that usually reasults in death and lysis of host cell
Stages- Attachment, Entry, Synthesis, Assembly, and Release
What is burst time?
Period of time require to complete the entire process of synthesis, assembly, and release
What is burst size?
Number of new virions released per lysed host cell
What is the lysogenic life cycle of viruses?
Modified replication cycle- infected host cells grow and reproduce normally for generations before they lyse
Temperate phages/ lysogenic phages
Prophages- inactive phages
Lysogenic conversion- results when phages carry genes that alter phenotype of bacterium
Switch from inactive to active during induction
What are some characteristics of animal viruses?
Same basic pathway as bacteriophages
Differences result from:
Presence of envelope, eukaryotic nature of animal cells, lack of cell wall
How do animal viruses attach to the host cell?
Chemical attraction
Have no tails of tail fibers- instead have glycoprotein spikes or other attachment molecules that mediate attachment
Three types of attachment-
Direct penetration- non-enveloped viruses
Membrane fusion- envelope fuses with cell membrane
Endocytosis- take in envelope and virus
How are animal viruses synthesized?
Each type of animal virus requires different strategy depending on nucleic acid
DNA viruses- replicate in nucleus, except for Pox virus
RNA virus- replicate in cytoplasm
Must consider- how mRNA is synthesized, what serves as template for nucleic acid replication
How does the replication of dsDNA viruses occur?
Similar to normal replication of cellular DNA and translation of proteins
dsDNA - mRNA - viral proteins
How does the replication of ssDNA viruses occur?
New strand of complementary DNA is made which then binds to ssDNA virus to dsDNA, replication then proceeds the same
ssDNA - dsDNA - mRNA - viral proteins
How does replication of +ssRNA viruses occur?
+ssRNA serves as mRNA- translated into viral proteins
Complementary strand is synthesized by viral RNA polymerase, produces -ssRNA, serves as template to create +ssRNA genome
How does replication of -ssRNA viruses occur?
Transcribe complementary strand (+ssRNA) using RNA dependent RNA transcriptase, used to make viral proteins
-ssRNA replicated from complementary strand for genome
How does replication of dsRNA viruses occur?
Contains +ssRNA and -ssRNA, produced by unwinding
-ssRNA transcibed by viral RNA polymerace to make new genome
+ssRNA is translated to produce viral proteins
How does a retrovirus replicate?
Positive sense RNA virus
However, do not use genome as mRNA, synthesize a DNA intermediary using reverse transcriptase enzyme
DNA serves as template for synthesis of +RNA molecules which acts as mRNA for protein synthesis
DNA able to integrate into chromosomes
Ex: HIV
How are animal viruses assembled and released?
DNA viruses- in nucleus
RNA viruses- in cytoplasm
Number of viruses produced depends on the size and initial health of host cell and type of virus
How are enveloped viruses released?
Released through budding
Viral glycoproteins found on cell membrane, capsid fuses with the cytoplasmic membrane, cell does not lyse
Results in continuous release of virus
How are naked viruses released?
Released by exocytosis or lysis
Sudden, sharp increase in number of viruses
What is meant by latency of animal viruses?
Defined as when an animal virus remains dormant in host cells, may be prolonged for years without viral activity
Some latent viruses do not become incorporated into host chromosome
Incorporation of provirus into host DNa is permanent
How are viruses cultured?
3 ways:
In mature organisms- bacteria, plants, and animals
Embryonated chicken eggs- inexpensive, among largest of cells, free of contaminating microbes, nourishing yolk
Cell culture
What are some characteristics of cell cultures?
Consists of cells isolated from an organism and grown on a medium or in a broth
2 Types:
Diploid cell cultures- created from embroynic plant, animal, or human cells, do not last more than 100 generations
Continuous cell cultures- longer lasting, derived from tumor cells, HeLa cell lines
What are some characteristics of viroids?
Extremely small, circular pieces of RNA that are infectious and pathogenic in plants
Similar to RNA viruses but lack capsid
May appear linear due to H-bonding within the molecule
Several plant diseases are caused by viroids
What are some characteristics of prions?
Proteinaceous infectious agents first decribed by Stanley Prusiner in 1982
Lack nucleic acid
How does a prion cause disease?
Cellular PrP- cytoplasmic membrane protein which plays a role in normal activity of brain, made by all mammals, normally composed of alpha helices
Prion PrP- composed of beta sheets
Prion PrP converts cellular PrP into prion PrP by inducing a conformational change
What are the diseases caused by prions?
Fatal neurological degeneration, fibril deposits in brain, loss of brain matter
Large vacuoles from in brain- characteristics spongy appearance
Spongiform encephalopathies- BSE, vCJD, kuru
Prions only destroyed by incineration or autoclaving in 1N NaOH
