Untitled Quiz

Introduction to Viruses

• Viruses are simple, acellular entities.
• They are obligate intracellular parasites.
• They can only reproduce inside living cells.
• Latin for 'poison' or 'venom'.
• Possess either DNA or RNA, but not both.

Viral Structure

• A complete virus particle is a virion.
• Composed of one or more molecules of DNA or RNA, enclosed in a protein coat.
• Some viruses have additional complex layers like carbohydrates, lipids, and additional proteins.

Viral Phases

• Viruses can exist in two phases: extracellular and intracellular.
• In the extracellular phase, they have few, if any, enzymes, and cannot reproduce independently of living cells.
• In the intracellular phase, they replicate nucleic acids and induce host metabolism to synthesize virion components. Eventually, complete virus particles (virions) are released.

Differences Between Viruses and Living Cells

• Simple, acellular organization.
• Presence of either DNA or RNA but not both.
• Inability to reproduce independently from host cells and carry out cell division.

Generalized Structure of Viruses

• Naked virus: Simplest, consisting of a capsid surrounding a nucleic acid strand.
• Enveloped virus: A nucleocapsid surrounded by a flexible membrane (envelope). The envelope often has viral proteins (spikes) inserted into it.

History of Viral Discovery

• Early attempts at preventing viral diseases predate the discovery of viruses themselves.
• Lady Wortley Montagu observed Turkish women inoculating their children against smallpox.
• Edward Jenner developed the first vaccine against smallpox using cowpox material.
• Louis Pasteur used the term "virus" for any living infectious disease agent.
• Charles Chamberland's porcelain bacterial filter allowed the isolation of viruses.
• Dimitri Ivanowski studied tobacco mosaic disease, finding that extracts could cause the disease even after filtering out bacteria.
• Martinus Beijerinck proposed the existence of a filterable virus.
• Friedrich Loeffler and Paul Frosch identified a virus associated with hoof-and-mouth disease of cattle.
• Walter Reed's work on yellow fever helped establish viruses as distinct disease-causing agents.
• Frederick Twort and Felix d'Hérelle discovered bacteriophages.
• Wendell Stanley crystallized the tobacco mosaic virus.
• Development of electron microscopes allowed visualization of viruses.

Viral Size

• Virions range in diameter from about 10 to 400 nm.
• The smallest viruses are slightly larger than ribosomes.
• Poxviruses are relatively large and visible with light microscopes.
• Most viruses require electron microscopes for visualization.

Unique Viral Features

• Lack organelles, cytoplasm, and nuclei.
• Composed of core nucleic acid and a protein coat.
• Genetic material of most viruses is either DNA or RNA; usually coiled or folded to conserve space.
• Nucleic acid is linear or circular.

Capsid

• The protein coat of a virus is called the capsid.
• Provides shape and symmetry to the virus.
• Protective layer.
• Composition of amino acids resist temperature, pH, and other environmental fluctuations.
• Capsomeres are protein subunits forming the capsid.

Nucleocapsid

• The capsid enclosing the genetic material (DNA or RNA).
• Protects viral genetic material.
• Aids in its transfer between host cells.

Spikes

• Some viruses have capsid proteins transformed into spikes.
• Help with virus attachment and penetration.

Virion

• A complete and infectious virus outside its host cell.

Naked vs. Enveloped Viruses

• Naked viruses consist only of a nucleocapsid.
• Enveloped viruses have a membrane (envelope) surrounding the nucleocapsid.

Composition of Envelopes

• Composed of lipids and proteins acquired from the host cell during replication.
• Unique to each virus type.
• Loss of the envelope or alteration in capsid symmetry can affect infectivity.

Animal Virus Envelopes

• Often arise from host cell nuclear or plasma membranes.
• Envelope proteins are encoded by viral genes.
• Often project from the surface as spikes (peplomers).

Viral Shapes

• Viruses are grouped based on nucleocapsid symmetry.
  • Helical: Tightly wound coil (e.g., tobacco mosaic virus, rabies virus).
  • Icosahedral: 20 triangular faces (e.g., herpes simplex virus, poliovirus).
  • Complex: Combination of helical and icosahedral symmetry (e.g., bacteriophages, smallpox virus).

Host Range of Viruses

• Host refers to the organisms a virus can infect, determined by capsid structure.
• Most viruses have a narrow host range (e.g., specific bacteriophages affecting only certain bacterial species).
• Some viruses have a broad host range, affecting numerous warm-blooded animals (e.g., rabies virus).

Tissue Tropism

• Many viruses infect specific cell types or tissues within a multicellular host, even within their host range.
• This is termed tissue tropism.
• Host range and tissue tropism are linked to virus infectivity.

Viral Nomenclature

• Names are usually given based on the disease they cause, the location of isolation, or morphologic factors.

Classification of Viruses

• Based on chemical and morphological criteria, specifically nucleic acid (size, symmetry, enveloped or not).
  • DNA viruses
  • RNA viruses.

DNA Viruses

• May have single or double-stranded genomes; linear or segmented.
• DNA polymerase replicates DNA to DNA copies.
• Most viruses replicate in the host cell's nucleus. Poxviruses are an exception, replicating in the cytoplasm.

RNA Viruses

• Contain either single or double-stranded RNA; linear or segmented.
• Enzymes replicate directly RNA-to-RNA, with some having complimentary + or - strand type.
• Retroviruses replicate indirectly via a DNA intermediate (RNA to DNA to RNA).

Viral Replication

• Viruses invade host cells and hijack their metabolism to reproduce copies of the virus.
• Bacteriophages use a lytic or lysogenic pathway.
• Animals viruses have variation in method and outcome.
• Lysis and budding are two possible release mechanisms for enveloped viruses, naked viruses are released as host cell lyses.
• Viral replication includes steps: attachment, penetration, biosynthesis, maturation, and release.

Bacteriophages

• Viruses that infect bacteria are called bacteriophages.
• The T-even phages are an example of bacteriophages.
• Have the characteristic head and tail structures.
• Tail fibers help identify bacterial species.
• Virulent viruses cause bacterial lysis.

Why Viruses Invade Other Cells

• A virus's nucleic acid often encodes only a few genes needed for viral components and functions.
• Dependence on the host's cellular machinery is high.

Lysis Cycle

• Steps in the bacteriophages' replication cycle: attachment, penetration, biosynthesis, maturation, and release.

Attachment

• Random collisions with bacterial cells.
• Phage tail fibers attach to a complimentary receptor site on the bacterium's cell wall or appendages.

Penetration

• Lysozyme is released to break open cell wall portion.
• Tail core penetrates cell wall once tail sheath contracts.
• Viral DNA is injected into the host cell through the tail core.

Biosynthesis

• Phage DNA uses host bacterial machinery to produce multiple copies of viral genome and capsid.
• mRNA translates and produces phage enzymes and proteins.

Maturation

• Viral components assemble into complete viral particles.
• Assembly follows a step-by-step procedure.

Release

• Mature phage particles burst out of the ruptured bacterial cell.
• Lysozyme degrades the bacteria cell wall in some phages.

Lysogenic Cycle

• Some temperate phages follow the lysogenic cycle.
• Viral DNA (prophage) integrates into bacterial chromosome, thereby replicating with the host.
• The prophage can remain dormant (latent) until conditions stimulate it to start a lytic cycle.

Animal Virus Replication: Attachment

• Animal viruses attach to host cells using spikes distributed on the capsid.

Penetration (Animal Viruses)

• Viral envelope may fuse with the host cell's plasma membrane and release nucleocapsid, or enter by endocytosis.
• Nucleocapsid is uncoated.

Biosynthesis and Maturation (Animal Viruses)

• Viral nucleic acid provides instructions for synthesizing viral components.
• Components may assemble in the nucleus or cytoplasm.

Release (Animal Viruses)

• Enveloped viruses can bud off from the plasma membrane or host cell, forming an envelope.
• Naked viruses are released when the host cell ruptures.

Latent Infection

• Some DNA and retroviruses can cause latent infections, in which viral gene expression is suppressed, and viral components are not produced.
• Examples (including herpes simplex virus) result in a provirus, an integrated viral genome.

Virus-Like Agents

• Viroids are small, infectious RNA molecules that cause plant diseases.
• Prions are infectious proteins with no nucleic acid.
• Examples of prion diseases include mad cow disease, scrapie, and Creutzfeldt–Jakob disease.

Prion Formation, Propagation

• Normal prion protein (PrP^C) is converted to abnormal prion protein (PrP^Sc); the shape change is infectious.
• Prion aggregation in tissues causes spongiform encephalopathies.

Viral Cultivation

• Inoculation into fertilized chicken eggs.
• Cell cultures.

Cell Cultures

• Primary cultures (animal cells separated from a tissue).
• Cells adhere to form a monolayer.
• Cell lines (maintained cells that reproduce in a single layer).

Viral Plaques

• Visible areas of clearing on a cell monolayer.
• Results from virus replication and host cell lysis.
• Used in viral detection.

Phage Typing

• Methodology that uses bacteriophages to detect and identify specific bacterial strains.

Viruses, Cancer

• Viruses can cause or be involved in cancer development.
• Viral genes that cause cancer are oncogenes.

Oncogenic Viruses: Mechanism

• Viruses can introduce oncogenes into host cells.
• Some viral oncogenes can transform host proto-oncogenes into active forms; example V-Oncogenes from retroviruses.
• Oncogenes can lead to uncontrolled cell proliferation; hence promoting tumor development.

Proto-oncogenes

• Normal cellular genes that can be converted into oncogenes via various mechanisms.
• Play roles in cell growth, metabolism, and mitosis.

Virotherapy

• Use of viruses to treat diseases including cancer.
• Some challenges include immune responses, controlling infectivity, ensuring safety, and target cells.