Chapter 13- Characterizing and Classifying Viruses, Viroids, and Prions Flashcards
(36 cards)
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
