Micro. Ch.5

Virology

The study of virises.

Virus

Genetic element that can multiply only in a living (host) cell.

Obligate intracellular parasite

That needs a host cell for energy, metabolites, and certain enzymes.

Viron

Extracellular form of a virus.

What does a Viron include?

Contains nucleic acid genome surrounded by a protein coat and, in some cases, an envelope (membrane).

When does replication/multiplication occur in viruses?

infection, entry into the host cell.

What are possible components of a virus structure?

Capsid, capsomere, nucleocapsid, enveloped virus.

What are the components in a naked virus?

Nucleic acid and capsid.

What are the components in a enveloped virus?

Nucleic acid, capsid, glycoprotein, and envelope

What structural components are in both a naked and enveloped virus?

Nucleic acid and capsid

What are capsids composed of?

capsomeres

What is part of the enveloped virus structure that is not part of a naked virus?

Glycoprotein and envelope

Capsid

The protein shell that surrounds the genome of a virus.

Capsomere

Protein subunit of the capsid.

Nucleocapsid

Combination of nucleic acid and capsid of a virion.

Enveloped virus

Virus that has a bilayer membrane around the nucleocapsid.

What does the envelope bind to?

The cells in the host.

Are viruses diverse in shape?

Yes, viruses can come in many shapes and sizes.

Are viruses bigger or smaller than prokaryotes?

Most viruses are smaller than prokaryote cells.

What is the size of viruses?

Size ranges from 0.02 to 0.3 um.

How do viral genomes compare to cells?

Most viral genomes are smaller than those of cells.

What is the shape of viral genomes?

Genomes are usually linear

What are viral genomes composed of?

Composed of either DNA or RNA.

How are nucleocapsids constructed?

Symmetrically

What kind of symmetry can nucleocapsids have?

Helical symmetry and Icosahedral symmetry.

Helical symmetry

Rod shaped viruses.

Example of a virus that has helical symmetry:

Tobacco mosaic virus

For helical symmetry, what is the relationship with length?

The length of the virus is determined by the length of the nucleic acid genome.

Icosahedral symmetry

“spherical” viruses

Example of virus that has icosahedral symmetry.

Human papillomavirus

What is the structure of the enveloped viruses?

-A lipid bilayer with embedded “spike” proteins surrounding the nucleocapsid.

Why are protein spikes important for enveloped viruses?

Protein spikes allow attachment to the host cell.

Complex viruses

Virions composed of several parts, each with separate shapes and symmetries.

Bacteriophages

Bacterial viruses

Are bacteriophages complex viruses?

Yes, they are.

What is an example of a bacteriophage?

Phage T4 because there is an icosahedral “head” with helical “tails”.

How do enveloped viruses look under a microscope?

Fuzzy looking because they the envelope covers the definite shape.

What do most bacteriophages contain?

dsDNA genomes

Most bacteriophages are naked or enveloped?

Naked

What is the structure for many Virions?

-Structurally complex

-Contains heads, tails, and other components.

What enzymes do virions contain that are critical for infection?

-Lysozymes

-Nucleic acid polymerases

-Neuraminidases

What kind of virions contain the lysozyme virus?

Bacteriophages

What kind of virions contain nucleic acid polymerase enzymes?

RNA Viruses

What kind of virions contains neuraminidase enzymes?

Animal viruses

Lysozyme

-Makes hole in cell wall of bacteria

-Lyses bacterial cell upon exit of new virions.

Nucleic acid polymerases

-RNA replicase

-Reverse transcriptase

Why is RNA replicase interesting?

-RNA is replicated into another strand of RNA

-RNA-dependent RNA polymerase

Why is reverse transcriptase interesting?

-RNA-dependent DNA polymerase-makes DNA from RNA template

Neuraminidases

Destroy glycoprotein and glycolipids in the host cytoplasmic membrane, thus liberating newly formed virions from the host cell.

Do virus replicate in every type of cell?

No, Viruses replicate only in certain type of cells or organism.

What viruses are easiest to cultivate?

Bacterial viruses are easiest to cultivate and are used as a model system because its more ethical than using humans.

How are various animal viruses cultivated?

Cultivated in chicken eggs and/or tissue or cell cultures.

Titer

Number of infectious particles per volume of fluid.

Plaque assay

Analogous to the bacterial colony and is a way to measure virus infectivity.

Plaques

Clear zones resulting from host cell lysis that develop on lawns of cells.

What are lawns?

Lawns can be bacterial or tissue cultures.

How does a plaque occur?

Results from infection by a single virus particle.

What is a major difference between prokaryotic and eukaryotic viruses?

Major difference is nucleic acid entry in prokaryotes vs. virion entry in eukaryotes.

Phases of viral replication:

1. Attachment

2. Penetration

3. Synthesis

4. Assembly

5. Release

What happens during the attachment phase in viral replication?

Attachment of the virus to a susceptible host cell.

What happens during the penetration phase in viral replication?

Penetration of the virion or it nucleic acid.

What happens during the synthesis phase in viral replication?

Synthesis of viruses nucleic acid and protein by host cell machinery, as redirected by virus.

What happens during the assembly phase in viral replication?

Assembly of capsids and packaging of viral genomes into new virions (maturation).

What happens during the release phase in viral replication?

The mature virions from the host cell are released.

How is virus replication typically characterized?

Characterized by a one-step growth curve.

What is the latent period followed by?

Maturation, assembly, and release

What is included in the eclipse?

Early enzymes, nucleic acids, and protein coats

Is the attachment of virion to the host cell specific?

Highly specific

What does the attachment of virion to the host cell require?

Requires complementary receptors on the surface of a susceptible host and its infecting virus.

What do the receptors include?

Proteins, carbohydrates, glycoproteins, lipids, or lipoproteins

What do the receptors do?

They carry out normal functions for the host cells, which includes uptake proteins, cell-to-cell interaction, and motility.

Bacteriophage T4

A virus of E. Coli, which has a complex penetration mechanism.

How does the virion attach to the host cells?

Tail fibers that interact with polysaccharides on the E. Coli outer membrane.

Once the virion attaches to the host cell, what happens next?

The Tail fiber retract, and the tail core makes contact with the E. Coli cell wall.

Once the tail core makes contact with the cell wall, what next?

Lysozyme forms small pores in the peptidoglycan.

Once there are small holes in the peptidoglycan, what next?

Tail sheath contract, and viral DNA is injected into cytoplasm.

What are the parts the T4 genome can be divided into?

Early, middle, and late proteins.

Early and middle proteins

Enzymes needed for DNA replication and transcription.

Late proteins

Head and tail proteins and enzymes required to release mature phage particles.

Process for Mature T4:

1. Production of early viral proteins

2. Replication of viral genomes

3. Production of middle/late viral proteins

4. Assembly of virions

5. Production of lysozymes

When packaging the T4 genome, what is produced first?

Precursor of bacteriophage head (the prohead) is assembled.

When packaging the T4 genome, what is assembled along with the prohead?

DNA packaging motor is assembled.

When packaging the T4 genome, what is pumped into the head under pressure using ATP?

dsDNA viral genome

When packaging the T4 genome, what is added once the head is filled with DNA?

The T4 tail, tail fibers, and other components.

What are two parts of the bacteriophage life cycles?

1. Virulent (lytic)mode

2. Temperate (lysogenic) mode

Virulent (lytic) mode

Virus lyses the host cell after infection

Temperate (lysogenic) mode

Virus is in a stable relationship with the host cell.

lysogeny

State where most virus genes are not expressed, and the virus genome is integrated into and replicated with the host chromosome.

With lysogeny, what is the virus called?

Prophage

Under certain conditions, what could a prophage do?

The prophage may revert to the lytic pathway and begin to produce virions.

Lytic pathway

1. Attachment of the virus to the host cell

2. injection of viral DNA

3. lytic event is initiated.

4. Phage components are synthesized, and virions are assembled.

5. Lysis of the host cell and release of new phage virions.

lysogenic pathway

1. Attachment of the virus to the host cell

2. injection of viral DNA

3. Viral DNA is integrated into the host cell

4. Viral DNA is replicated with host DNA at cell division.

What is it called when the lysogenized cell with the prophage transfer to the lytic pathway?

Induction

Where do many animal viruses replicate?

Nucleus of their host cells.

Enveloped bacteriophages are_____, but _____of enveloped animal viruses.

rare, many kinds

How do enveloped viruses obtain their lipid bilayer?

From the host cell upon exiting the host.

Are animal viruses’ genome diverse?

Yes, there are DNA viruses and RNA viruses

What are the consequences of virus infection in animal cells?

1. lysis

2. Persistent infections

3. Latent infections

4. Transformation

5. Cell fusion

Lysis

Host cell lyses upon release of virions

Persistent infections

-Release of virions from host cell does not result in cell lysis

-Infected cell remains alive and conditions continues to produce virus.