Viruses and Viral Like Agents

What is a virus? Organism that causes diseases Common cold Flu AIDS Bird flu Polio Electron micrographs of viruses. HIV Aeromonas virus 31 Influenza virus Orf virus Herpes simplex virus Smallopx virus Viruses are made of genetic material (DNA or RNA) packaged in a protein shell. They are small (10 nm-3 microns in diameter) and have icosahedral, helical or other complex shapes. Colored electron micrographs of some disease causing viruses are seen in this slide.

Viruses are non-living In the 1930s, it was discovered that viruses are nonliving agents composed of nucleic acids and proteins. FIGURE 2: Size relationships among microorganisms and viruses

What are viruses??? About 5, 000 viruses have been identified, but there is believed to be around 400, 000 different viruses. Believed to be the most abundant biological entity on Earth!! Viruses are very small- they do not “grow,” just reproduce They cannot reproduce on their own (obligate, intracellular particles) Most can only be seen with the electron microscope, to survive they must infect and take over a host cell in order to reproduce. They lack the chemical machinery to generate energy and synthesize large molecules. Viruses do not have organelles, have no cytoplasm, and no cell nucleus or nucleoid. They do have: nucleic acid core and a protein coat.

What are Viruses??? The viral genome contains either DNA or RNA, but does not have both (like we do). They have a protein coat called a capsid that gives shape to the virus. The capsid is what provides protection to the cell. The amino acids make it resistant to temperature, pH and other environmental fluctuations. Some capsids have special proteins called spikes, that help the virus attach to host cells and penetrate cell membranes. Most viruses have a membrane that surrounds the nucleocapsid that is flexible called an envelope.

FIGURE 6: Various viral shapes Viruses are grouped by the shape of their nucleocapsid symmetry FIGURE 6: Various viral shapes

Structures compared From Medical Microbiology, 5th ed., Murray, Rosenthal & Pfaller, Mosby Inc., 2005, Fig. 6-4.

Properties of Viruses Various morphologies polyhedral helical spherical filamentous complex Ebola virus Rabies virus Poliovirus Herpes virus Coronavirus Lassa virus

Viral Structure Nucleic acid core 2) Capsid layer the genetic material of the virus. instructions to make all the viral parts and viral enzymes. either DNA or RNA. 2) Capsid layer made of protein that surrounds and protects the nucleic acid core.  can also be involved in attaching the virus to the cell membrane of its host.

Structure Continued 3) Envelope not seen in all viruses. made of lipid or fat. surrounds the capsid layer. can also be used to attach to the CM of the host cell. 4) Spike proteins embedded in the envelope of the virus. have a specific shape. used to attach the virus to the host’s CM.

FIGURE 5: The components of viruses