Microbiology - Chapter 13: Viruses, Viroids, and Prions

Icosahedral: 20 flat triangles. Helical: Capsomeres: arranged in a helix. Complex viruses: phage, icosahedral nucleocapsid (head) and helical protein (tail).

What are the three shapes of a virus?

Viroids are small single-standed RNA molecules that form a closed ring. It is only found in plants and cause serious disease (potato spindle tuber, chrysanthemum, stunt, citrus exocortis, cucumber pale fruit, and cadang-cadang). They enter through wound sites.

What are viroids?

budding.

In addition to lysis, animal viruses may exit the host cell by

spike proteins AND matrix proteins.

In the region of budding, the cytoplasmic membrane acquires

Viruses that can cause cancer in humans.

What is an oncovirus?

proteins.

Capsids are composed of

Acute infections are characterized by the sudden onset of symptoms of relatively short duration. Persistent infections can continue for the lifetime of the host, with or without symptoms. Acute infections can be compared to lytic infections by bacteriophages, but even though the cells often die, the host may survive because of immune response. Persistent infections can either be chronic infections or latent infections. Chronic infections are characterized by the continuous production of low levels of viral particles. Latent infections are analogous to lysogeny in bacteriophages in that the virus genome remains in the host cell but is not active, so no new virus particles are formed.

Compare and contrast acute infections and the two types of persistent infections caused by animal viruses.

Oncogenic viruses are viruses that cause cancer in humans. Oncolytic viruses target and kill cancer cells.

Compare and contrast oncogenic viruses and oncolytic viruses.

All known viroids infect only plants and cause serious disease. They enter the plants through wound sites rather than binding to receptors. Prions are linked to slow, degenerative diseases that are always fatal. Prion diseases can occur in humans and animals.

Compare hosts of viroids and prions.

Viroids consist solely of a small single-stranded RNA molecule that forms a closed ring. They are about 1/10 the size of the smallest infectious viral RNA genome known. Prions are composed only of protein and they have no nucleic acid.

Describe the chemical structure of viroids and prions.

At a minimum, a. virion consists of nucleic acid surrounded by a protein coat. Virions contain either RNA or DNA. This classifies viruses as either RNA or DNA viruses. The protein coat is called the capsid that protects the viral nucleic acid from enzymes and toxic chemicals in the environment; it is composed of precisely arranged identical protein subunits called capsomeres. The capsid together with the nucleic acid it encloses is called the nucleocapsid. Viruses have protein components that allow the virion to attach to specific receptor sites on host cells. Phages have tails and animal viruses have spikes. Some viruses have an outer lipid bilayer called an envelope. Non-enveloped viruses do not have that. Enveloped obtain their outer layer from the host cell and have a viral matrix protein that links the envelope to the nucleocapsid. Viruses are either icosahedral, helical, or complex.

Describe the general features of viral structure.

Prion proteins accumulate in neural tissue. For unknown reasons, neurons die and brain functions deteriorate as the tissues develop characteristic holes. The result was a sponge-like appearance of the brain tissues. The infectious protein is a misfolded version of the normal protein, but that altered shape has significant consequences. It makes the infectious protein resistant to degradation by the host cell proteases that normally destroy older proteins as new ones are synthesized. This makes the molecules insoluble and makes them aggregate in cells. Infectious prion proteins are PrP^SC and the counterparts are PrP^C. PrP^SC accumulation is the outcome of the protein's interaction with PrP^C. PrP^C will have changed folding making them PrP^SC.

Describe the process by which prions accumulate in tissues.

A tumor is an abnormal growth of tissue resulting from a malfunction in the normally high regulated process of cell growth. Some are benign and do not spread. Others are cancerous or malignant and can spread. Control of cell growth and division involves coordination of 2 sets of genes: those that stimulate growth, called protoncogenes, and those that inhibit growth, termed tumor suppressor genes. Mutations that either increase the expression of protoncogenes or decrease the expression of tumor suppressor genes are the most common cause of abnormal cell growth. An oncogene is a proto-oncogene that has been changed in a way that promotes uncontrolled growth, leading to tumor formation. Spontaneous and induced mutations can convert proto-oncogene into oncogene. A single change in the DNA sequence of regulatory genes is probably not enough to cause a tumor; multiple changes at many sites are needed. Most tumors are caused by mutations in host genes that regulate cell growth, but some are caused by viruses. In contrast to cancer causing viruses, other viruses offer potential for treating cancers.

Describe the roles of proto-oncogenes and tumor suppressor genes in controlling cell growth, and discuss how some viruses bypass this control.

This is important from a medical standpoint and depends on virally encoded enzymes as potential targets for antiviral drugs. Interfering with the activities of these enzymes causes antiviral meds to slow the progression of a viral infection. This gives the host immune system time to eliminate the virus before illness occurs. It is a 5 step process: attachment, genome entry, synthesis, assembly, and release.

Explain animal virus replication.

Enveloped viruses enter by fusion or endocytosis. Non-enveloped viruses can't fuse and enter by endocytosis. The entire virion enters the cell and nucleic acid separates from the protein coat through uncoating.

Explain animal virus replication: entry and uncoating.

Provirus integrates into host chromosome or replicates separately, much like a plasmid, cannot be eliminated/can later be reactivated. Examples are varicella zoster virus (VSV) and herpes simplex type 1 virus (HSV-1).

Explain latent infections.

Viral proteins will become envelope spikes insert into host cytoplasmic membrane. Viral matrix protein coats inside of cytoplasmic membrane. Nucleocapsid extrudes from host cell and becomes coated with matrix proteins/envelope with protein spikes. New virus is then released.

Explain the process of animal virus release.

A virion is a viral particle that is a complete virus in its inert non-replicating form outside a host cell. Nucleic acid is surrounded by a protein coat, capsid. Capsid is made of simple identical subunits called capsomeres. Capsid and nucleic acids are called nucleocapsid and nucleic acid is either DNA or RNA, but not both; may be circular or linear, single-stranded or double. Spikes attach to receptor sites on host cells and phages attach by tail fibers. Enveloped viruses are surrounded by a lipid bilayer obtained from host cell; matrix protein is between nucleocapsid and envelope. Non-enveloped, naked, viruses are more resistant to disinfectants.

Explain viral structure.

cause diseases of humans.

Prions contain only nucleic acid without a protein coat. replicate like HIV. integrate their nucleic acid into the host genome. cause diseases of humans. cause diseases of plants.

The majority of tumors are caused by oncogenic viruses but some may be caused by mutations in host genes that regulate cell growth.

Select the FALSE statement regarding viruses and cancer. The majority of tumors are caused by oncogenic viruses but some may be caused by mutations in host genes that regulate cell growth. Some viruses carry an oncogene—infection with these viruses interferes with the host cell's own growth-control mechanisms. The most common viral causes of tumors are certain DNA viruses such as hepatitis B virus. Viruses that insert their genome into the host cell's chromosome may cause changes at the insertion site, converting a proto-oncogene into an oncogene. Viruses that lead to cancer formation are called oncogenic viruses.

Onocoviruses are viruses that can cause cancer in humans. They may arise directly from viral infection or when the genome acts as an oncogene or when viral genome inserts into the host chromosome in a way to convert a proto-oncogene to an oncogene. Most virus-induced tumors are associate DNA viruses, but some RNA are also oncogenic. There are currently 7 human oncogenic viruses. Cancers caused by viruses don't develop quickly and present 15-40 years after infection. Small percent of oncovirus-infected people develop the associated cancer. Some virus-associated cancers are preventable by vaccines (human papillomaviruses, HPVs).

What are cancer causing viruses?

Viruses that infect bacteria are called bacteriophages or phages. They are easy to grow in the lab as a good model for how animal viruses interact with their host. They are a vehicle for horizontal gene transfer. They can kill bacteria and this is important ecologically and medically.

What are phages?

Prions are proteinaceous infectious agents. They are composed solely of protein and no nucleic acids. It is linked to slow, fatal diseases in humans and animals. Prion proteins accumulate in neural tissue, neurons die, tissues develop holes, brain function deteriorates, and characteristic appearance gives rise to general term for all prion diseases transmissible spongiform encephalopathies. Cells make normal form of neuronal protein. Infectious prion proteins are PrP ^SC. Resistant to proteases; insoluble, forms aggregates; appears that PrP^SC changes folding of PrP^C converting it to PrP^SC. Prions are resistant to heat and chemical treatments.

What are prions?

Phages have tail fibers for attachment, and many animal viruses have protein structures called spikes on their surface.

What are spikes?

Some viruses do not neatly fall into categories. Acute infections: rapid onset; short duration, burst of virions released from infected host cell, immune system gradually eliminates virus (influenza, mumps, and poliomyelitis). Persistent infections: continue for years or lifetime, may or may not have symptoms, may be chronic or latent. Chronic infections: continuous production of low levels of virus particles, carries can lack symptoms and transmit virus. Latent infections: viral genome remains silent in host cell, can reactivate to cause productive infection, and the viral genome may be integrated into host cell chromosome as a provirus.

What are the categories of animal virus infections?

A tumor is abnormal growth. Cancerous or malignant tumor can metastasize and benign won't. Proto-oncogenes stimulate cell growth and division; tumor suppressor genes inhibit growth division. Mutations in these genes cause abnormal and/or uncontrolled growth, but multiple changes at different sites are required. Oncogene is proto-oncogene that has been changed to promote uncontrolled growth, virus may carry, numerous events including spontaneous and induced mutations an lead to conversion into an oncogene. Some viruses offer potential for treating cancer.

What are viruses and human tumors?

Onocolytic viruses specifically target and kill cancer cells. They destroy cancer cells directly by stimulating the host's cancer-fighting immune cells and they genetically engineer other well-characterized viruses to develop oncolytic versions for therapy. In 2015, the FDA approved a genetically engineered herpes simplex virus type 1 to treat advanced inoperable melanoma. The engineered virus is injected into melanoma lesions. it preferentially infects and multiplies in the tumor cells, ultimately causing them to lyse.

What do onocolytic viruses do?

A virus that infects bacteria; often called a phage.

What is a bacteriophage?

Protein coat that protects the viral nucleic acid from enzymes and toxic chemicals in the environment; it is composed of precisely arranged identical protein subunits called capsomeres.

What is a capsid?

With respect to viral infections, an infection in which the viral genome is present in the host cell but not active, so new viral particles are not being produced.

What is a latent infection?

The capsid together with the nucleic acid is enclosed by nucleocapsid.

What is a nucleocapsid?

An infectious protein that causes a neurodegenerative disease.

What is a prion?

A complete virus in its inert non-replicating form outside a host cell; also referred to as a viral particle.

What is a virion?

An infectious agent of plants that consists only of RNA.

What is a viroid?

Genetic information, either DNA or RNA, contained within a protective protein coat.

What is a virus?

Viruses are obligate intracellular agents that contain genetic material, RNA or DNA, inside a protective protein coat. Inert particles mean they have no metabolism, replication, or motility. Genome hijacks the host cell's replication machinery. Inert outside cells; inside, direct activities of cell. They are infectious agents and not organisms. Viruses need live organisms as hosts and can't grow in a pure culture or be seen with a light microscope. They are classified based on the cell type they infect: eukaryotic or prokaryotic.

What is a virus?

A proto-oncogene that has been changed in such a way that it promotes uncontrolled growth, leading to tumor formation.

What is an oncogene?

They target and kill cancer cells, a characteristic that makes them potentially useful in cancer treatment.

What is an oncolytic virus?

Acute infections of viruses are a result of productive infection

Which of the following statements is true? Acute infections of viruses are a result of productive infection Chronic infections of viruses generally lead to long-lasting immunity. Acute infections usually followed by chronic infections Latent infections are caused by repetitive infection of the same viruses Latent infections result from integration of viral nucleic acid into the host.

Disinfectants damage the lipid bilayer of the envelope

Why are non-enveloped viruses generally more resistant to disinfectants than are enveloped viruses?