viruses

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VIRUS

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Imagine Something that does not grow, respond, or eat. Something that is neither living or non-living. Something not made up of cells. Something not included in a kingdom. Can you think of some figure? INTRODUCTION

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For an organism to be classified as being alive it must ·  Reproduce  ·  Obtain and use energy  ·  Grow, develop, and die  ·  Respond to the environment Is a Virus a Living Organism?

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A virus is not a cell, it does not need food. It does need material to reproduce, but it does not require energy, technically. Viruses do not Grow Have homeostasis Metabolize Viruses do Infect cells and use the cell to make more viruses Cause disease in many organisms Viruses are not living organisms

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In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. These infectious particles are genetic entities that lie somewhere in the grey area between living and non-living states. The first clue to the existence of viruses

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Viruses can have very different shapes and sizes. They’re extremely small, ranging from 100 to 2,000 Angstrom. Viruses are particles of molecular size and are essentially a  single or a double strand of nucleic acid – either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) depending on the type of virus - surrounded by an extremely thin protein coating. What does a virus look like?

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The Capsid - The Capsid of a virus is basically its "brains." It contains an outer protein coat which is wrapped around a central core of a highly complex chemical called nucleic acid. Typically, the capsid is divided into distinct subunits called capsomeres. The capsid proteins are arranged into are 3 forms of symmetry, icosahedral, helical and complex. The Body - Viruses have a highly complex symmetry. Attached to the head (capsid) is a rod like structure that consists of a retractible sheath surrounding a central hollow core. The Tails - At the very end of the core is a spiked plate carrying 6 slender tail fibers which help anchor the virus to its host.   Lipid Membrane – a membrane around the capsid in many kinds of viruses; helps the virus enter cells (“enveloped” viruses; without the membrane, the virus is “naked”).Made of proteins, lipids, and glycoproteins Virus Structure

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At the simplest level, the function of the outer shells (CAPSID) of a virus particle is to protect the fragile nucleic acid genome from: Physical damage - Shearing by mechanical forces. Chemical damage- UV irradiation (from sunlight) leading to chemical modification. Enzymatic damage - Nucleases derived from dead or leaky cells or deliberately secreted by vertebrates as defense against infection. Protein coat - Capsid

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Icosahedral (isometric) capsids: An alternative way of building a virus capsid is to arrange protein subunits in the form of a hollow quasi-spherical structure, enclosing the genome within. The criteria for arranging subunits on the surface of a solid are more complex than those for building a helix. An icosahedron has 12 vertices with 20 triangular sides, termed facets, like a geophysical dome. SHAPES OF VIRUS

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Poliovirus Foot & Mouth Disease Virus (FMDV) Icosahedral (isometric) capsids

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Tobacco mosaic virus (TMV) is representative of one of the two major structural classes seen in viruses of all types, those with helical symmetry. The simplest way to arrange multiple, identical protein subunits is to use rotational symmetry & to arrange the irregularly shaped proteins around the circumference of a circle to form a disc. Multiple discs can then be stacked on top of one another to form a cylinder, with the virus genome coated by the protein shell or contained in the hollow centre of the cylinder. Helical capsids:

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Helical capsids

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These viruses have oval or 'brick-shaped' particles 200-400nm long. The external surface of the virion is ridged in parallel rows, sometimes arranged helically. The particles are extremely complex & have been shown to contain more than 100 different proteins. Complex Virus Structures

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How are viruses classified?

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Viruses are classified on the basis of their following properties: Nature of viral genome: RNA or DNA Structure of genome : double or single stranded, linear or circular Capsid symmetry: icosahedral, helical or complex Presence or absence of envelope Replication strategies How are viruses classified?

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Adsorption - Recognition of virus and host cell – attachment. Penetration/ Uncoating - Entrance of virion into host cell. Replication (Synthesis) - Copy and manufacture of viral components. Assembly – Assembly of virus particles. Release - Escape of virus. Viral Multiplication

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A bacteriophage is any one of a number of viruses that infect bacteria. Typically, bacteriophages consist of an outer protein capsid enclosing genetic material. The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA along with either circular or linear arrangement. Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of carrying out both. BACTERIOPHAGE

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Lytic Cycle: The virus enters the cell, replicates itself hundreds of times, and then bursts out of the cell, destroying it. In the lytic cycle, the virus reproduces itself using the host cell's chemical machinery. The red spiral lines in the drawing indicate the virus's genetic material. The orange portion is the outer shell that protects it. How do viruses replicate?

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Lysogenic Cycle: The virus DNA integrates with the host DNA and the host’s cell helps create more virus DNA. An environmental change may cause the virus to enter the Lytic Cycle. In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions.

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Viruses enter bacterial cells by punching a hole in the cells wall and injecting its DNA Viruses enter plant cells through tiny rips in the cell wall. Viruses enter animal cells by endocytosis. Viruses Enter Living Cells

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Bacteriophages – attack & destroy bacteria Baculovirus – ebola-like virus that attacks insects Could use for pest control in crops Cabbage loopers eat cabbage crops Virus can kill pests in days (it’s really gross) … and then there are those that are not so good…. Viruses can be beneficial…

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Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer - though definitely not all - have been linked to viruses. Animal viruses

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Adenovirus Papillomavirus Herpesvirus Hepatitis B virus Parvo virus Electron Micrograph Images of DNA containing Viruses

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Rift Valley Fever Virus Entero viruses Paramyxo viruses Rotavirus Influenza virus Mimi virus

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Viruses are simple biological entities and are therefore important to the study of molecular and cellular biology. Study of viruses helped our understanding of the basic mechanisms of molecular genetics. Viruses are used as a tool to cure bacterial infections and are now used in curing genetic disorders. Used in producing vaccines against viral diseases. Significance of viruses

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Not viruses, but virus like particles: Viroids are ultramicroscopic, single-stranded molecules of RNA without any protein coat. They infect plants. Prions are proteinaceous infectious particles, cause degenerative diseases in animals -e.g., scrapie in sheep-bovine spongiform encephalopathy in cattle-Creutzfeldt-Jakob disease in humans Viroids and Prions

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Retroviruses belong to the Retroviridae family of viruses. The genetic material of retroviruses consists of ribonucleic acid (RNA), instead of deoxyribonucleic acid (DNA). Viruses of this type also contain reverse transcriptase. Retroviruses are known to lead to certain types of cancers in both humans and animals, as well as a range of viral infections. Human Immunodeficiency Virus (HIV), the virus that causes Acquired Immune Deficiency Syndrome (AIDS), is one example of a retrovirus. Retroviruses are prone to mutation. For this reason, viruses in this family often become resistant to antiviral drugs within a relatively short period of time. Antibiotics are not effective against retroviruses What are Retroviruses?

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Human Immunodeficiency Virus (HIV)Acquired Immuno Deficiency Syndrome (AIDS)

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Viral envelope – lipid bilayer; glycoproteins protrude from surface Glycoproteins enable virus to recognize surface proteins of special immune cells and to enter the cell (like a key to the cell’s door) 2 strands RNA – only 9 genes; 3 are found in many viruses (structural proteins) Reverse Transcriptase – turns RNA into DNA (this makes HIV a retrovirus); DNA instructs cell to make more viruses Basic Structure

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Virus enters cell through endocytosis Virus replicates RNA to DNA with reverse transcriptase HIV Making Factories

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DNA enters nucleus & binds with host DNA New virions exit cell through exocytosis to infect other cells (notice cell isn’t destroyed) mRNA is created (carries instructions for making new viral proteins) and leaves nucleus Uses host cell’s enzymes to make new viruses

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In the US, there is better than a 1/1000 chance of contracting HIV during unprotected sex A person can be contagious for more than 10 years before any sign of the disease is apparent HIV becomes AIDS when the number of immune cells drops below a predetermined number No one dies from HIV or AIDS; people die from secondary infections (ranging from the common cold to cancer) More than 3 million people (size of Chicago) die each year There are approx. 14,000 new cases of HIV worldwide every day Think about it…

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Presented by… Mani Vinay Jwalant