VirusesChapter 19

HIV virus

Ebola virus

H1N1 flu virus

T4 bacteriophage

Smallpox virus

Virus: an infectious particle consisting of little more than gene packaged in a protein coat.

Why are they important? Basic molecular biology concepts were discovered using viruses.

DNA and mRNA Translation and transcription Enhancers

Viruses can be used as experimental systems. The study of viruses has led to the development of techniques

that enable scientists to manipulate genes and transfer them from one organism to another (gene therapy).

Biological control (population control). Studying viruses helps scientists understand how viruses cause

disease.

A Borrowed Life

Polio HIV (Retroviridae) Influenza/Avian flu

(Orthomyoviridae) Ebola (Filoviridae) West Nile (Flaviviridae) Hepatitis (several families) Herpes (Herpesviridae) HPV (Papillomaviridae) SARS (Coronaviridae) Smallpox (Poxviridae) Common Cold

Diseases Caused by Viruses

In 1883, Adolf Mayer discovered that he could transmit tobacco mosaic disease from plant to plant by rubbing sap from diseased leaves onto healthy leaves. Suggested caused by small bacteria that were

invisible under a microscope. In 1892, Dmitri Iwanowsky passed infected sap

through a filter designed to remove bacteria. Sap still caused disease. Iwanowsky still clung to the idea that bacteria

caused the disease.

Discovery of Viruses…Using tobacco mosaic disease (stunts growth and

gives leaves a mosaic coloration)

In 1899, Martinus Beijerinck filtered infected sap through a porcelain filter known to trap bacteria; sap still caused disease. He also noted that the agent that caused disease could not be cultivated on a nutrient plate.

He had the courage to champion a new concept.

Called the agent Contagium vivum fluidum (contagious living fluid).

Beijerinck’s suspicions were confirmed in 1935 when the American scientist Wendell Stanley crystallized the infectious particle, now known as the tobacco mosaic virus.

Won the Noble Prize in 1946. This data freaked scientists out; a chemical

thing was behaving as if it was living.

Smallest viruses are 25nm in diameter. Smaller than a ribosome. Parvovirus

Largest viruses are 400 nm in diameter. Barely visible under a light microscope. Mimivirus

Size of Viruses

Nucleocapsid (virion)Definition: viral genome and capsid

Viral Genomes Genetic Material

Double stranded DNA (dsDNA) Single stranded DNA (ssDNA) Double stranded RNA (dsRNA) Single stranded RNA (ssRNA).

Genome is usually single linear or circular molecule of nucleic acid.

Genomes can consist of 3 to several hundred genes.

Viral Capsids The protein shell

enclosing the viral genome is called a capsid. Capsids are built from a

large number of protein subunits called capsomers.

Shapes: helical (rod-shaped), icosahedral, complex.

Proteins arrange helically. Allows equivalent bonds

between proteins and nucleic acids.

Bonds are not strained. Proteins have a groove to

interact with the nucleic acid.

Example: Tobacco Mosaic Virus

Helical Viruses

20 sided solid, in which each side is an equivalent triangle. Most stable solid

approaching a sphere. Energetically stable shape. Least distortion of protein

subunits or bonds between proteins.

Example: Adenovirus (respiratory virus of animals).

Icosahedral Virus

Combination of icosahedral and helical features.

Example: bacteriophage T4 Elongated icosahedral

head. Tail has helical symmetry.

Complex Virus

Relevance of Virus Structure

Molecular nature of surface components. Important for vaccine

development. Viruses initiate entry into

the cell by attaching to receptors.

Nature of biomolecular interactions.

What determines virus structure?

Protein-protein interactions. Protein structure protein

sequence viral gene Protein-nucleic acid

interaction.

Hepatitis B Virus

West Nile Virus