Viruses and Infectious Disease

Viral Structure and Reproduction

What is a Virus? Viruses are very tiny, nonliving, particles made of either DNA or RNA which is enclosed in a protein coat called a capsid.

What makes them NONliving? Viruses do not exhibit ALL of the characteristics of living things. CharacteristicViruses Living Cells Structure DNA or RNA core, protein capsid Cell membrane, cytoplasm; eukaryotes also contain nucleus and organelles Reproduction Only with the help of a host cell Independent cell division either asexually or sexually Genetic CodeDNA or RNADNA Growth and Development NoYes Obtain and use energy NoYes Change over time Yes

Naming Viruses Most viruses are named after the disease they cause. Some are named for the organ or tissue that they infect.

Viruses that infect bacteria are called bacteriophages, or phage for short. Naming Viruses

Virus Structure Viruses have two basic parts: –Inner core made of nucleic acid (DNA or RNA) –Outer protein coat called a “capsid”.

Virus Structure Some large viruses have an envelope surrounding their capsid. –Made of lipids, just like the plasma membrane of cells. –Makes viruses more infectious because they can more easily infect living cells.

Viral DNA or RNA contains only the instructions for making more copies of the virus. Virus Structure

Virus Structure The arrangement of proteins in the capsid determines the virus’ shape. Tobacco Mosaic virus – long; helical Polyhedral – resemble crystals Shape helps determine what cell the virus infects and how.

Viral Reproduction

The cell in which a virus replicates is called a host cell. Before a virus can replicate, it must attach to the host cell and inject its nucleic acid (DNA or RNA) into the cell.

Viral Reproduction Once initial infection has taken place, a virus will enter one of two reproductive cycles. –Lytic - Lysogenic

Lysogenic Infection Viral DNA integrates itself into host DNA. As the host cell’s DNA replicates itself, it replicates the viral DNA as well. Imbedded DNA is called a prophage. Viral DNA remains inactive for a period of time, slowly incorporating itself into more and more cells as the host cell divides. The host organism may not feel sick or show any signs of infection in a lysogenic infection.

Lysogenic Infection

Lytic Infection The viral DNA enters the host cell and begins to break down the host’s DNA. It then uses the host cells nucleotides to produce more viral DNA copies. Soon, the cell is so full of new copies of the virus that the cell bursts and thousands of viruses leave the cell to infect other cells.

Lytic Infection

Retroviruses Contain RNA as their genetic material. Viral RNA enters the host cell and produces a DNA copy of itself which becomes a prophage. –Ex. HIV (AIDS) Retroviruses may remain dormant (lysogenic) for a very long period of time.

Infectious Disease

What are pathogens? Bacteria and viruses are found everywhere in nature and most are completely harmless. A FEW cause disease. Disease-causing bacteria and viruses are called pathogens.

Bacterial Disease There are 2 ways that bacteria produce disease. –Some damage cells and tissues by breaking down the cells for food Ex. Tuberculosis is inhaled where it destroys lung tissue. Mycobacterium tuberculosis

Bacterial Disease –Some release toxins (poisons) that travel through the body, and interferes with the normal activity of the host. Streptococcus – –Causes Strep Throat –Can release toxins into the bloodstream, causing Scarlet Fever. Tetanus – –Toxins cause severe muscle spasms, paralysis and death.

Preventing Bacterial Disease Vaccine – a preparation of weakened or killed pathogens. –Prompts the body to develop immunity to the disease. –Immunity is the body’s ability to destroy new pathogens.

Antibiotics –Compounds that block the growth and reproduction of bacteria. Ex. Penicillin and tetracyclins Preventing Bacterial Disease

Controlling Bacterial Growth Sterilization by heat –Most bacteria cannot survive high temperatures for very long.

Controlling Bacterial Growth Disinfectants –Chemical solutions that kill bacteria –Overuse of antibacterial compounds increases the likelihood that bacteria will evolve to become resistant to them which makes them more difficult to kill.

Food Storage and Processing –Refrigeration Food stored at low temperatures lasts longer because it takes bacteria longer to multiply. –Canning Preserves food almost indefinitely –Food is sterilized by heat and then immediately placed in sterile jars or cans. Controlling Bacterial Growth

Viral Disease Viruses produce disease by disrupting the body’s normal equilibrium. –Viruses attack and kill living tissue. Viral Diseases cannot be treated with antibiotics. The best treatment is prevention, usually through the use of vaccines.

Common Infectious Diseases BacterialViral Lyme DiseaseCommon cold TetanusInfluenza DiptheriaSmallpox Bacterial MeningitisWarts Strep throatAIDS (HIV) Tooth decayChickenpox Measles Polio Hepatitis A, B, and C West Nile