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VIRUSES AND VIRAL LIKE AGENTS

WHAT IS A VIRUS? Intracellular particles that causes diseases Common cold Flu AIDS Bird flu Polio Electron micrographs of viruses. HIV Aeromonas virus 31 Influenza virus Orf virusHerpes simplex virusSmallopx virus

VIRUSES ARE NON-LIVING In the 1930s, it was discovered that viruses are nonliving agents composed of only nucleic acids and proteins. Viruses cause diseases. FIGURE 2: Size relationships among microorganisms and viruses

CHARACTERISTICS OF 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 nucleic acid core. 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.

ORIGIN OF VIRUSES There Are three hypotheses for the origin of viruses. The regressive evolution hypothesis Viruses are degenerate life-forms, basically they are derived from intracellular parasites that have lost life functions. The cellular origins hypothesis Viruses are derived from subcellular components and macromolecules that escaped from cell walls and replicated inside hosts. The independent entities hypothesis Viruses coevolved with cellular organisms from a self- replicating molecule present on primitive Earth.

VIRAL STRUCTURE 1)Nucleic acid core 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 -not seen in all viruses. -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

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

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

PROPERTIES OF VIRUSES Various morphologies polyhedral helical spherical filamentous complex Ebola virusRabies virus PoliovirusHerpes virusCoronavirusLassa virus

SPREAD OF INFLUENZA VIRUS

INFECTION STRATEGIES Lytic Cycle: The virus enters the cell, makes copies of itself and causes the cell to burst. Uses the materials and cell machinery of the host cell to replicate itself. Lysogenic Cycle The virus enters the cell, integrates itself into host DNA and replicates along with the host. The imbedded virus is called a prophage After a period of time, the prophage will enter the lytic cycle

FIGURE 14.8: Bacteriophage replication

Figure 10A: Replication of a DNA and RNA

LATENT INFECTIONS Many DNA viruses will establish latent infections that are characterized by repression of most viral genes, so the virus lies dormant. For example herpesviruses can generate latent infections, and then when some stress come about the DNA of the virus can produce a new infection. Retroviruses like HIV can also lie dormant Called a provirus Each time the host cells then replicates the provirus is copied.

FIGURE 11: The formation of a provirus by HIV

Viruses are host specific – a protein on the surface of the virus has a shape that matches a molecule in the plasma membrane of its host, allowing the virus to lock onto the host cell.

Human adenoviruses growing within the nucleus of an infected cell

VIRUSES AND TUMORS About 60% – 90% of cancers are due to some type of carcinogen. But some viruses can act as carcinogens. Some viruses have been isolated from human cancers and they can cause transformations of normal cells into tumor cells. Examples: herpes viruses can cause cervical cancers, Epstein-Barr virus is linked to Burkitt lymphoma, Human papilloma virus can cause cervical cancers as well...these viruses are called oncogenic viruses.

EMERGING VIRUSES Many emerging viral infections are due to viruses that appear for the first time in a population, or developing the ability to expand their host range. Most times this due to the viruses finding new populations in different geographical areas. Some new viruses can form through genetic recombination. Influenza virus does this that’s why vaccinations only last for one year. Viruses can also mutate to cause changes in their DNA or RNA These mutations can cause the virus to be resistant to drugs or allow them to affect other populations. Increased contact with animals can also allow viruses to “jump” from one species to another.