Transcript Viruses
Viruses What is a virus? Terminology Characteristics of a virus How are viruses structured? Structures and Shapes Retroviruses What is a virus? • An agent that causes disease in plants and animals • Segments of nucleic acids (genetic material like DNA or RNA) contained in a protein coat What is a virus? Viruses are generally considered to be nonliving because they DO NOT have all the characteristics of life. Viruses are different from living things in 3 ways: 1. DO NOT grow 2. CANNOT maintain homeostasis 3. DO NOT metabolize What is a virus? • Viruses are similar to living things in 2 ways: 1. They contain nucleic acid (DNA or RNA) 2. They contain proteins (the capsid) Vocabulary • Pathogen- agent that causes disease • Host- an organism that shelters and nourishes a virus or another organism • Capsid- the protein coat of a virus • Envelope- surrounds the capsid and helps viruses enter cells Vocabulary con’t • Bacteriophage - a virus that attacks a bacteria (word parts - phage = to eat) • Prophage/Provirus - a viral DNA segment inserted into a bacterial cell Vocabulary Con’t • Retrovirus - a virus that replicates by first transcribing RNA into DNA (HIV) Vocabulary Con’t • Lysis - the breaking apart of the cell to release new viruses (word parts! lyse = to burst) • Viroid - a single strand of viral RNA that causes plant diseases (TMV) • Prion - protein that infects animals Characteristics of a Virus When trying to find the cause of tobacco mosaic disease (disease that stunts the growth of tobacco plants) scientists discovered something other than bacteria was causing TMD Concluded it was smaller than a bacterium and named it virus after the Latin word meaning “poison” Characteristics of a Virus • 1935, Wendell Stanley concluded that TMD was a chemical rather than an organism/cell • Today we know that Stanley had discovered the first virus Viruses have the following characteristics: – Smaller than prokaryotic cells (like bacteria) – Not normally classified (debate over living vs. nonliving) – Live and reproduce ONLY inside other living cells – Cause diseases such as colds, influenza (flu), AIDS, smallpox, measles, chicken pox, rabies, mumps, and mononucleosis How are Viruses Structured? • Viruses have an outer layer called a capsid that is made of proteins • Inside the capsid is genetic material such as DNA or RNA (but it never contains both as the same time) How are viruses structured? • Some viruses have an envelope outside of their capsid as well as proteins that stick out to aid in entering host cells (similar to marker or receptor proteins on cells) Structure and Shapes • 5 basic shapes of a virus: – Helical – Spherical – Polyhedral – Binal – Filovirus Structures and Shapes • Helical: – RNA or DNA is coiled in a long narrow capsid Ex. Tobacco Mosaic Virus Structures and Shapes • Spherical: – Typically studded with receptors, may be enveloped Ex. Influenza Virus (Flu) Structures and Shapes • Polyhedral: – Geometric in appearance Ex. Adenovirus (causes the common cold) Structures and Shapes • Binal: – Polyhedral capsid – Helical tail Ex. Bacteriophage Structures and Shapes • Filovirus: – No distinct uniform shape – Threadlike loops Ex. Ebola virus HIV a Retrovirus HIV- Human immunodeficiency virus Structure: • Spherical • Contains RNA • Retrovirus • Exterior wall made of glycoprotein HIV Retrovirus • Function: – causes AIDS (acquired immune deficiency syndrome) – binds to human immune cells at specific receptor sites – is a special type of virus called a retrovirus – virus that gets its genetic information copied backward from RNA to DNA – when a retrovirus infects a cell, it produces a DNA copy of its RNA – retroviruses are responsible for some types of cancer in animals, including humans Human Diseases Caused by Viruses Category Disease Sexually transmitted disease AIDS (HIV), genital herpes Childhood diseases Measles, mumps, chicken pox Respiratory diseases Common cold, influenza (flu) Skin diseases Warts, shingles Digestive tract diseases Gastroenteritis Nervous System diseases Polio, viral meningitis, rabies Other diseases Smallpox, hepatitis, Avian Bird flu, Ebola, SARS, West Nile How Viruses Spread • Viruses are transmitted from one host organism to another by many different vectors. – transmit – to pass along – vector – the substance or path used to pass the virus along • Viruses, like other pathogens, are commonly transmitted through body fluids, including the spittle in your sneezes and coughs, mucus produced by many parts of your body such as your nose and lungs, and even the ear wax in your ears. They can be transmitted through contaminated food and drinks as well. Some require contact with body fluids such as blood, semen, or vaginal secretions that you do not usually encounter. • SOME viral infections can be prevented through vaccines, such as the flu vaccine available each year and the HPV vaccine available from your doctor. A vaccine is often a weakened form of the virus injected to prevent infection. Children in the United States are also commonly vaccinated against measles, mumps, chicken pox, and polio. There are vaccines available for some forms of meningitis, rabies, smallpox, and some forms of hepatitis, but they are only given to people at risk of getting the diseases such as first responders, medical personnel, soldiers, and even some teachers. • Unfortunately, viruses mutate rapidly and vaccines lose their ability to be 100% effective in preventing disease. This is why we have a new flu vaccine every year. It takes over a year to manufacture flu vaccine, therefore scientists have to predict what will be most effective in the future. Sometimes they have excellent predictions and almost everyone who gets the vaccine stays well. Sometimes, they don’t have excellent predictions and many people get sick. It’s still important to get the vaccine because there is often more than one flu virus going around and protection against at least some of the virus is better than none at all. • Once we do get sick with a viral infection, we don’t have many effective ways to “cure” the infection. We do have some antiviral medications which help prevent viruses from replicating as rapidly as they would without it, and interferons which mimic molecules naturally produced by your body in response to viral infections. Antibiotics DO NOT have any effect on viruses and should never be given or taken for an infection such as influenza (which is a respiratory illness, NOT a digestive system illness) or a cold. • Our inability to vaccinate against and to cure viral infections is why it is so important that you always cover your mouth when you sneeze or cough, don’t share drinks or eat after someone else, wash your hands frequently, and even avoid sharing earbuds!