Chapter 13: Viruses

Lysogenic cycle steps

1. Bacteriphage attaches to host cell's receptor sites 2. Phage DNA injected into host cell 3. Host cell multiplies with viral DNA ** can become lytic if external factors interfere

Describe the structure of bacteriophages (3)

1. Icosahedral head 2. Contractile tail with collar and sheath that is a helical shape 3. Tail fibers that come from the base plate

How do the various viruses and bacteriophages exit the host cell?

Nonenveloped viruses rupture the plasma membrane (lysis) Enveloped viruses bud out

The shape of a virus is determined by either ____ or ___.

Nucleic acid or capsid

Name two types of capsids

Polyhedral and Helical

Viruses may be grown in what type of cell lines in cell culture? (3)

Primary cell lines: derived from tissue cells, which only lasts a few generations Diploid cell lines: developed from human embryos, which lasts for about 100 generations Continuous cell lines: transformed cancerous cells that can be maintained for an indefinite number of generations.

What are the functions of the enzymes carried by retroviruses

Protease: performs proteolysis of immature viral proteins Integrase: integrates viral DNA into host cell chromosome, creating provirus reverse transcriptase: catalyzes the formation of DNA from an RNA template

Retrovirus

RNA virus that uses reverse transcriptase to transcribe DNA from RNA

What are two ways that animal viruses penetrate the host cell?

Receptor-mediated endocytosis or fusion .

Viruses that infect bacteria are called

bacteriophages

Nucleocapsid

capsid with its enclosed genome

spikes

carbohydrate-protein complexes that project from the surface of the envelope

The envelope ______ and ______ are encoded by HOST cell genes and are present in the plasma membrane before the budding of a new animal virus.

carbohydrates and lipids

Cytopathic effect

cell deterioration caused by virus infection of cells in the monolayer of cell cultures

Plaque

clearings in a lawn of bacteria caused by the destruction of cells by viruses

Helix

coiled spring shape of virus

How do prions cause disease?

conversion of normal host proteins into infectious forms

Define a temperate virus and give an example of one:

doesn't kill its host cells immediately ex: a cell undergoing lysogenic replication cycle

When can a cell become lytic:

if external factors (mutagens like radiation, chemicals) interfere

Viruses have a _______ (limited/wide) host range. Why?

limited They can infect a limited variety of hosts

Prions

misfolded, infectious proteins that cause the misfolding of normal proteins in animals

Lysogenic and Lytic Cycle are two ways bacteriophages ________.

multiply

Viruses

nonliving particle composed of a nucleic acid and a protein coat they can cause may disease and can change how a cell functions

What types of coverings can nucleocapsids have?

nucleocapsids may correspond to a naked core or be surrounded by a membranous envelope.

Viruses are what type of parasites?

obligate intracellular

Icosahedron

polyhedral, 20 triangular faces that form the shape of a virus

Are prions infectious?

prion comes from PROteoinaceous INfectious particle yes, they can be transmitted from one human/animal to another

glycoprotein

projections used to attach to the host cell (may not be present in all viruses) they are made of protein containing sugar chains

What are three enzymes a retrovirus carries with it into a cell?

protease, integrase, and reverse transciptase

Capsid

protein coat around the nucleic acid that serves as protection

Capsomeres

protein subunits of each capsid

Viral envelope

serve as a membrane-like structure around the capsid ( a lipid based membrane that is taken from the host) and aids in host infection

Viroid

short, circular, single strand of RNA without a capsid that can replicate! cause errors in regulatory systems controlling plant growth

syncytia

a multinucleated cell that can result from multiple cell fusions of uninuclear cells

Steps to retrovirus replication

1. Retrovirus enters via fusion between attachment spikes and the host cell receptors 2. Uncoating releases two viral RNA genomes and the viral enzymes: - reverse transcriptase - protease - integrase 3. Reverse transcriptase copies viral RNA to produce double-stranded DNA 4. Viral DNA is transported to the host cell's nucleus and is integrated into host cell chromosome as a PROVIRUS by integrase. ** provirus may be replicated when the host cell replicates 5. Transcription of the provirus to produce new RNA, capsid, enzymes and envelope protein for new retrovirus 6. Viral proteins are processed via viral protease; some viral proteins are moved to the host plasma membrane 7. Mature retrovirus leaves the host cell, acquiring an envelope and attachment spikes while it BUDS out

What are various cytopathic effects of viruses?

1. They deteriorate cells from monolayer 2. They induce cell piling following infection.

Lytic cycle steps

1. Virulent bacteriophage attaches to host cell's receptor sites 2. Phage DNA is injected into host cell and is circularized 3. Phage DNA and proteins are synthesized by host cell's machinery 4. New phages self-assemble 5. Host cell lyses, releasing new phages

reverse transcriptase

An enzyme encoded by some certain viruses (retroviruses) that uses RNA as a template for DNA synthesis.

when viruses are grown in living animals, what special conditions have to be considered?

Animal inoculation are performed to either study the immune system's response to viral infections or to identify/isolate viruses from a clinical specimen It must be known that some viruses cannot be grown in animals or can be grown but do not cause to the disease: - some don't show symptoms (cannot be used to study effects or treatment) - could take years to determine effectiveness of vaccine when using human subjects - may provide a reliable model for studying viral replication but they do not provide models for vaccine development

Describe how bacteriophages are detected and counted by the plaque method.

Bacteria and bacteriophage are mixed together in one media (liquid or solid). Each virus infects a bacterium, multiplies and releases several hundred new viruses. The new viruses infect other bacteria in the vicinity, making more new viruses After several viral multiplication cycles, all the bacteria in the area surrounding the original virus are destroyed, producing plaques. Simultaneously, uninfected bacteria multiply and produce a turbid background Each plaque theoretically corresponds to a single virus suspension and is measured in terms of plaque-forming units (PFU)

What is the difference between the attachment of bacteriophages and animal viruses?

Bacteriophages - attachment site on the virus of the virus attaches to a complementary receptor site on the bacterial cell Animal viruses - they attach to complementary receptor sites but the receptor sites of the animal cells are proteins and glycoproteins of the plasma membrane. Animal viruses do not have appendages such as tail fibers.

Describe the two ways that animal viruses leave their host cell

Budding: envelope consisting of protein, lipid, and carbohydrates + assembled capsid; envelope developing around the assembled capsid and pinch off the host cell envelope

_______ in cells cultures can be detected and counted in much the same way as plaques caused by bacteriophages on a lawn of bacteria and reported as PFU/mL

CPE (cytopathic effect)

In some viruses, capsomeres function as enzymes as well as structural supports. Of what advantage is this to the virus?

Enzymatically active capsomeres catalyze the early interactions between virion and target cell, especially the penetration step. They are not dependent on cellular functions (ie. endocytosis) Because they are actively participating in viral attachment and penetration, it increases the potential target cell range since it decrease the dependence on cell expression of specific proteins

If viruses that normally form envelopes were prevented from budding, would they still be infectious?

No because the envelope contains necessary proteins, carbohydrates and lipids necessary for viral entry and multiplication.

How are the two types of capsids different

Helical viruses are long, hollow cylindrical shaped. Icosahedral viruses are regular shaped polyhedrons with 20 triangular faces and 12 corners. Each face forms an equilateral triangle

HIV attacks only specific types of human cells, such as certain white blood cells and nerve cells. What determines if a virus can enter a particular cell type or not?

It must attach to specific receptor sites on the cell to initiate fusion or endocytosis. The virus must have the right attachment spikes to initiate entry.

What does it mean for a virus to be persistent or latent and how are these events important?

Latent virus: a virus in equilibrium with the host and not actually produce disease for a long period of time Persistent virus: a virus that causes an infection that occurs gradually over a long period These events are important because persistent viral infections are detectable infectious virus that gradually build up and latent virus infections appear suddenly.

How are the two types of capsids similar?

They are made up of capsomere units and enclose viral nucleic acids and enzymes.

What are three ways viruses grown in the lab?

They can be cultured in three different ways: living animals, embryonated eggs, or cell cultures.

Briefly describe the action of an oncogenic virus

They integrate their genetic material into the host cell's DNA and it gets replicated with the host cell's chromosome. With this, they initiate transformation of tumor cells.

Since viruses lack metabolic enzymes, how can they synthesize necessary components?

They must take over the metabolic machinery of the host cell.

The envelope protein of an animal virus that is incorporated into the plasma membrane of the host cell is encoded by __________.

VIRAL genes

when viruses are grown in embryonated eggs, what special conditions have to be considered?

Virus suspension or suspected virus-containing tissue is injected into the fluid of the egg that is most appropriate for its growth.. if viral growth occurred, they the embryo would die via embryo cell damage, formation of typical pocks or lesions on the egg membranes This was a convenient and inexpensive way of isolating and growing viruses to create vaccines

Discuss the connection between viruses and cancer; include a possible mechanism for viruses that cause cancer.

Viruses and cancer both alter normal functions of a cell. The relationship between viruses and cancer was first discovered when studying leukemia and noticing that it can be transferred from an infected to an uninfected individual viruses that cause cancer may integrate their genomes into the host cell, which allows the transformation of cells resulting in properties/functions that differ from normal functioning cells. Integration into the host cell genome makes certain that the virus gets replicated and spread along with the host cell chromosome.

If a naked +ssRNA from a virus were injected into a cell by itself, could it cause a lytic infection?

Yes because it can act as mRNA which can be translated into RNA-dependent RNA polymerase.

What is the function of spikes?

attachment to the host cell virus identification

when viruses are grown in cell cultures, what special conditions have to be considered?

since cell cultures are generally homogeneous collections of cells, they are more convenient to work with than whole animals or embryonated eggs. It is the preferred type of growth medium for many viruses because they can be handled much like bacterial cultures. It MUST be free of microbial contamination.

Viruses are ________ than ribosomes.

smaller

Provirus

the genetic material of a virus as incorporated into, and able to replicate with, the genome of a host cell.

What dictates the host range of animal viruses?

the specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication.

Prophage

viral DNA Is integrated into the host's genomes

What needs to be uncoated in the cytoplasm

viral nucleic acid

If you were involved in developing an antiviral drug, what would be some important considerations? (can a drug "kill" a virus)

viruses invade the host cells and use the cell's machinery for their survival so attacking the viruses in turn attack the host cells.

Virulent

viruses that causes disease