Viruses, Viroids, and Prions

Viruses, Viroids, and Prions

Learning Objectives

  • Differentiate a virus from a bacterium.

  • Describe the chemical and physical structure of both an enveloped and nonenveloped virus.

  • Define viral species.

  • Describe how bacteriophages and animal viruses are cultured.

  • List three techniques used to identify viruses.

  • Compare and contrast the multiplication cycle of DNA and RNA-containing animal viruses.

  • Distinguish persistent viral infections from latent viral infections.

  • Differentiate virus, viroid, and prion.

Viruses

  • A microscopic particle that gets inside a cell and often destroys the cell.

  • Viruses are tiny, smaller than the smallest bacteria.

  • Viruses change rapidly, making it difficult to know how many types exist and to fight them.

Viruses Are Not Living Things

  • Viruses are complex assemblies of molecules, including proteins, nucleic acids, lipids, and carbohydrates.

  • On their own, viruses can do nothing until they enter a living cell.

  • Without cells, viruses cannot multiply; therefore, viruses are not living things; however, for simplicity, we will refer to viruses as "living."

  • Viruses are obligate intracellular parasites (O.I.Ps), meaning they must have a living host cell to multiply.

Are Viruses Living?

  • Viruses contain protein and genetic material, like living things.

  • Viruses do not eat, grow, break down food, or use oxygen.

  • Viruses cannot function on their own, and can only reproduce inside a living cell (the host).

  • The host is an organism from which a parasite takes food or shelter.

  • The virus uses the host's cell as a tiny factory and forces the host to make viruses rather than healthy new cells.

Features of Viruses

  • Viruses contain only one type of nucleic acid, either DNA or RNA (not both).

  • Viruses have a protein coat made of lipids, carbohydrates, and proteins.

    • Must have a host cell.

    • Uses the cell's machinery to copy itself and makes specialized structures that can transfer the viral information to other cells.

    • Lack personal enzymes

Host Range

  • Viruses can have a host range, which is a spectrum of host cells the virus can infect.

  • Viruses that infect bacteria are bacteriophages (phages).

  • The outer surface of a virus must chemically interact with a specific receptor site on the host cell.

Viral Size

  • Viruses are very small compared to bacterial cells.

  • An electron scanning microscope is needed to view viruses.

Viral Structure

  • Virion = complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat that protects it from the environment.

Nucleic Acid

  • Prokaryotes and Eukaryotes use RNA and DNA as genetic information.

  • Viruses use RNA or DNA - not both.

    • It is single or double stranded

Capsid and Envelope

  • Protein coat = capsid

  • Combination
    of",

#### Capsid and Envelope - Protein coat = capsid - Combination
of capsid and envelope = nucleocapsid$$ - **Capsomeres:** protein subunits that make up the capsid - **Envelope:** covers the capsid; contains spikes - **Spikes:** projections that stick out from the envelope surface #### General Morphology - Based on capsid structure - Helical viruses - Polyhedral viruses - Enveloped viruses - Complex viruses #### Viral Taxonomy - Virus classification is based on: - Nucleic acid type - Replication strategy - Morphology - Genus names end in -virus. - Family names end in -viridae. - Order names end in -ales. - A viral species is a population of viruses with similar characteristics that occupies a particular ecological niche. The subspecies are designated by a number. - Example: HIV-1

Isolation, Cultivation, and Identification of Viruses

  • Viruses utilize living cells; therefore, growing viruses can be difficult.

    • Viruses Can Be Grown In:

    • Living animals.

    • Embryonated eggs.

    • Cell cultures.

Growing Viruses in Living Animals

  • Some viruses can only be grown in living animals - mice, rabbits, guinea pigs.

  • Animal inoculation may be used to study the disease process or for identification.

Growing Viruses in Embryonated Eggs

  • If a virus will grow in an embryonated egg, this can be a convenient and inexpensive form of isolation.

  • A hole is drilled in the shell of the egg, and virus is injected into the egg.

  • Viral growth is indicated by the death of the embryo, embryo cell damage, or by lesions on the egg membranes.

Growing Viruses in Cell Cultures

  • Cell Cultures: Cells grown in culture media in the laboratory.

  • Tissues are treated with enzymes to separate the cells.

  • Cells are suspended in a solution that provides the cells with the osmotic pressure, nutrients, and growth factors needed for growth.

  • Normal cells grow in a monolayer across the glass or plastic container; viruses infecting the monolayer cause the cells to deteriorate as they multiply.

Viral Identification

  • Serological Tests

    • Detect antibodies against viruses in a patient.

    • Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot.

  • Nucleic Acids

    • RFLPs

    • PCR

Viral Multiplication

  • A virus does not have the machinery for self-replication; therefore, to multiply, a virus must invade a host cell and take over the host’s metabolic machinery.

  • The multiplication of viruses can be demonstrated with a one-step growth curve.

Viral Multiplication - Animal Viruses

  • Attachment: viruses attach to cell membrane

  • Penetration: by endocytosis or fusion

  • Uncoating: by viral or host enzymes

  • Biosynthesis: production of nucleic acid and proteins

  • Maturation: nucleic acid and capsid proteins assemble

  • Release: by budding (enveloped viruses) or rupture

Multiplication Cycle - Animal Virus

  • Attachment: Virus binds to host cell surface receptors.

  • Penetration: Receptor-mediated endocytosis.

  • Uncoating: Viral nucleic acid is released from the capsid.

  • Biosynthesis: Viral DNA is replicated, and some viral proteins are made.

  • Maturation: Capsid proteins are synthesized.

  • Release: Viruses are released.

Multiplication of DNA Viruses

  • DNA viruses multiply in the nucleus of the host cell using viral enzymes to make new DNA.

  • Synthesize capsid in the cytoplasm using host cell enzymes.

  • The capsid proteins migrate into the nucleus and assemble with the viral DNA to form virions.

Multiplication of RNA Viruses

  • RNA viruses multiply in the cytoplasm.

  • Sense strand RNA (+ strand) acts as mRNA - Ribosomes can translate it into viral proteins right away.

  • Antisense strand RNA (- strand) Viruses need to transcribe a + strand from their - strand before viral proteins can be made.

RNA Viruses

  • ssRNA viruses produce DNA

    • Use reverse transcriptase to produce DNA from the viral genome

    • Viral DNA integrates into the host cell chromosome as a provirus

  • HIV

Persistent Viral Infections

  • Disease processes that occur gradually over a long period.

  • Typically fatal.

  • Two types:

    • Latent Infections

    • Persistent Infections

Latent Viral Infections

  • Virus remains in asymptomatic host cell for long periods.

  • May reactivate due to changes in immunity.

  • Examples:

    • Herpesviruses

    • Chickenpox virus

Persistent Viral Infections

  • Disease processes that occur gradually over a long period.

  • Typically, infections are fatal.

  • Measles virus: may remain in brain cells for many years, resulting in encephalitis.

Prions

  • Proteinaceous infectious particle.

  • Inherited and transmissible by ingestion, transplant, and surgical instruments.

Prion Diseases

  • Spongiform encephalopathies: large vacuoles develop in the brain.

    • Sheep scrapie

    • Creutzfeldt-Jakob disease

    • Mad cow disease

Plant Viruses and Viroids

  • Plant Viruses:

    • Enter through wounds or via insects

  • Viroids:

    • Short pieces of naked RNA

    • Cause potato spindle tuber disease