Name four characteristics of viruses
- acellular
- cannot carry out any metabolic pathway
- cannot grow or respond to environment
- cannot reproduce independenly
What are the two distinctive states of viruses?
- Virion (extracellular)
- Virus (intracellular)
What is the name of the protein coat that surrounds a Virion's nucleic acid core?
The capsid
Differentiate between a capsid and an envelope.
Capsids are made of protein (potentially several different ones), while an envelope is a phospholipid membrane (similar to the cytoplasmic membrane of a cell)
True or False:
Virions have cytosol and organelles.
FALSE
What is primarily used to classify viruses ("taxonomically")
Genetic material. DNA, RNA, single-strand or double-strand.
e.g. ssDNA, ssRNA, dsDNA, dsRNA
Virus DNA/RNA can be in linear strands or circular strands depending on whether the intended host is a eukaryote or prokaryote.
Which of the two arrangements applies to which organism?
Linear - Eukaryotes
Circular - Prokaryotes
What important characteristic of Viruses determines whether or not it can infect a host?
The viral surface proteins or glycoproteins (of the envelope or capsid) must match the host proteins or glycoproteins (of the cytoplasmic membrane or cell wall)
What is the term for a virus that can infect multiple types of cells or tissues in a variety of hosts?
Generalist
What is the term for a virus that infects bacteria?
bacteriophage (or just "phage")
What is the typical size range for viruses?
Viruses are ultramicroscopic and require an electron microscope to view.
Their size range is generally 24 nm - 500 nm (both of those numbers represent min/max)
True or False:
ALL types of organisms are susceptible to viral attack, including viruses.
TRUE
Which scientist first proved that the tobacco mosaic virus was caused by something "smaller than a bacterium"?
Dmitri Ivanowsky
Which American scientist pioneered electron microscopy?
Wendell Stanley
Define capsomere.
proteinaceous subunit of a capsid.
(capsid can be comprised of capsomeres of a single type of protein across the entire surface, or have sections of capsomeres comprised of different proteins)
Virion shape is also used to help classify viruses. Name the three basic shapes of virions.
- helical (appear as a "tube" or "stick")
- polyhedral (appear spherical, or icosahedral-20 sided shape)
- complex (wide variety of "other" shapes)
For enveloped viruses, how is the envelope acquired?
It is acquired from the cytoplasmic membrane, nuclear envelope, or endoplasmic reticulum membrane of the infected host (upon replication or release)
Name four criteria used to help scientists classify viruses.
- nucleic acid (most important)
- presence of envelope (versus "naked" or capsid)
- shape
- size
What is the specific epithet for viruses, and how is it written?
- specific epithet: common English name
- written: italics
What organization is responsible for the taxonomy of viruses?
ICTV (International Committee on Taxonomy of Viruses)
True or False:
Viruses don't have any established taxonomic kingdom, division or class.
TRUE
What is the general name for the replication cycle of viruses?
The "Lytic Replication Cycle"
What are the 5 steps of the Lytic Replication Cycle?
- Attachment
- Entry
- Synthesis
- Assembly
- Release
(Think "caesar" without the "c")
What happens in the "Attachment" phase of the Lytic Replication Cycle?
the virion attaches to the host cell
What happens in the "Entry" phase of the Lytic Replication cycle?
the virion or its genome enters the host cell and destroys the host DNA
What happens in the "Synthesis" phase of the Lytic Replication cycle?
the virus uses the host's ribosomes and enzymes to generate virally coded nucleic acids and proteins for the necessary virion components.
What happens in the "Release" phase of the Lytic Replication cycle?
New (complete) virions are released from the host cell (resulting in host cell lysis or death)
Explain the primary differences between the Lytic Replication Cycle and Lysogeny
Lysogeny includes all 5 steps from the Lytic Replication Cycle. However, in Lysogeny, the host DNA isn't immediately destroyed upon infection in the "entry" stage. The virus fuses with the DNA of the host cell and the host cell replicates otherwise normally such that all daughter cells carry a copy of the viral DNA. After an environmental trigger, the Lytic Replication Cycle suddenly resumes with sythesis, assembly and release.
What is another name for the quescent virus, or inactive bacteriophage during Lysogeny?
prophage
Explain the phenomenon of lysogenic conversion.
Previously harmless bacteria can become pathogenic when the viral DNA fuses with the bacterial DNA during Lysogeny.
In Lysogeny, what term describes the separation of the (formerly) inactive virus from the host DNA, destroying it in the process?
induction
What are the 5 steps of the replication of animal viruses?
- Attachment
- Entry
- Synthesis
- Assembly
- Release
(same basic five steps as Lysogenic Replication Cycle of bacteriophages. "caesar" without the "c".
What is the term that describes an animal virus losing its capsid or envelope?
Uncoating
For which type of animal virus (ssDNA, +/-ssRNA, dsDNA, dsRNA) does the viral DNA enter the host nucleus, fuse with its DNA and uses the cellular enzymes to replicate the viral genome as if it were normal cellular DNA?
dsDNA
(it already resembles the double-helix shape of animal DNA, so it just latches directly onto the host DNA and goes straight to work assembling new viruses)
For which type of animal virus (ssDNA, +/-ssRNA, dsDNA, dsRNA) does the virus enter the host nucleus, use host enzymes to produce a complementary string of nucleic acids, then fuse with host DNA to start synthesizing and assembling new viruses?
ssDNA
(since animal DNA is double-stranded the single-stranded DNA virus first needs to become double-stranded so it can fuse with the host DNA and resume the replication cycle)
For which type of animal virus (ssDNA, +/-ssRNA, dsDNA, dsRNA) is the virus able to act directly as host mRNA and start translating viral polypeptides?
(+)ssRNA
(positive single-stranded RNA can act directly as host mRNA and start synthesizing and assembling new viruses)
For which type of animal virus (ssDNA, +/-ssRNA, dsDNA, dsRNA) does the virion require a special enzyme to transcribe an animal-compatible version of the virus before acting as mRNA?
(-)ssRNA
(negative single-stranded RNA viruses are incompatible with the +RNA of animals, so they use the enzyme RNA-dependent RNA transcriptase to convert themselves to +ssRNA to carry out the lytic replication cycle for more -ssRNA viruses.)
What is the enzyme found inside the capsid of virions that helps convert -ssRNA to +ssRNA?
RNA-dependent RNA transcriptase
Which category of animal virus (ssDNA, +/-ssRNA, dsDNA, dsRNA) does a "retrovirus" fall under?
+ssRNA
(Retroviruses differ from stereotypical +ssRNA viruses because they must use a DNA intermediary to transcribe viral mRNA)
Briefly compare (+/-)ssRNA viruses with dsRNA viruses.
dsRNA viruses are a combination of the traits of + and - ssRNA viruses. The +ssRNA component's viral genome can act directly as host mRNA, but the -ssRNA component must use RNA-dependent RNA transcriptase to synthesize and assemble for -ssRNA components of the dsRNA
Explain the term "persistent infection".
Enveloped viruses are shed slowly and steadily while the host cell is still alive. Host lysis isn't immediate.
Some animal viruses may remain dormant in cells in a process known as ________.
latency
What is another name for a "latent virus"?
provirus
Define the term neoplasia.
Uncontrolled cell division in multicellular animals.
What is another name for a mass of neoplastic cells?
Tumor
What type of tumor remains in one place and generally isn't harmful?
benign tumor
What type of tumor invades neighboring tissues and can spread throughout an organism?
malignant tumor (or cancer)
What is the name for the process by which a malignant tumor spreads?
metastasis
What is the term that describes genes that normally play a role in cell division but are typically repressed?
protooncogenes
Around what percentage of human cancers is caused by viruses?
20-25%
In what way can a virus cause human cancer?
- viruses carry oncogenes as part of their genomes
- some can promote existing oncogenes in the host
- some can interefere with host's natural oncogene repression
In what three types of laboratory media are viruses typically grown in?
- mature organisms (mice, bacteria)
- embryonated (fertilized) eggs
- cell cultures
What are the two most common types of cell cultures used for laboratory study of viruses?
- diploid cell cultures (created from embryos of animals or plants)
- continuous cell cultures (derived from tumor cells. Longer lasting than diploid cultures)
What is a Viroid?
- circular pieces of viral RNA
- lack a capsid
Who was the scientist that discovered prions?
Stanley Prusiner
How do prions differ from viruses?
Prions lack nucleic acids. They are infectious proteins. Normal cooking and sterilization techniques are ineffective against prions.
How do prions infect healthy cells?
They corrupt a cytoplasmic membrane protein called "PrP" that is found in lipid rafts of the cytoplasmic membrane.
Name the primary physical difference between bacteriophages and animal viruses
Animal viruses do not have tails or tail fibers.
What are the three main ways an animal virus can infect a host?
- direct penetration (receptors match)
- membrane fusion (viral membrane merges and remains part of cytoplasmic membrane)
- endocytosis by cytoplasmic membrane
Most DNA viruses assemble in the ___________ of the host cell.
nucleus
Most RNA viruses assemble in the __________ of the host cell
cytoplasm