Viruses 1 What to Know: Learning Objectives Overview: What are some basic facts about viruses? Structure: What are the shapes? Determine why virus are not alive. Virus & Disease: How do they get into cells and then get reproduced? How do they get treated? Describe a few common or well-known virus. AIDS, the common cold, hepatitis, influenza Reproduction: What is the lytic & lysogenic pathways? What are viroids & prions? 2 Viruses Objectives Overview: What are some basic facts about viruses? Structure: What are the shapes? Determine why virus are not alive. Virus & Disease: Describe a few common or well-known virus. How do they get into cells and then get reproduced? How do they get treated? AIDS, the common cold, hepatitis, influenza Reproduction: What are the lytic & lysogenic pathways? What are viroids & prions? Viruses are: Infectious particles (not living cells). Very small in the biological world (hundreds times smaller than the cells they infect). Science is still not sure how or why viruses came about, but they are integral in all species no known benefit to living species Major sources of disease in the living world . Major scientific debate over whether virus helped cause variation in species or not. Typically named after the disease they cause or the 3 tissues they infect. Virus Classification Classification is not typical: Defies biological classification. “Grouped” various ways 1. Morphology: Shape & structures 2. What is infected: Plants, animals, bacteria 3. Genetic composition: DNA, RNA, single or double-stranded 4. Pathology: nature of the disease 1. Deadly, inconvenient, highly contagious or not, how it’s transmitted, etc. 4 Virus Structure Simple virus structure *Genetics… DNA, this is the direction to make more virus RNA, single or double-stranded. *capsid… protein complex housing that forms its outer shell + May or may not have an additional covering (envelope) + Many have protein capsid with surface markers (like keys) that are designed to fit locks in the cells of species they have evolved to infect. * All virus have these two features 5 6 Viral Shapes Each have their own form but can be categorized: Groups: Helical Polyhedral Enveloped/Spherical Other. See Your Handout! 7 Helical Polyhedral Enveloped Others 8 Other Notable Virus Classification Virus can be called “vectors” Vectors: Transmitters of genetic information Vectors transmit several ways: 5 Main categories; As DNA (3 types) or RNA (2 types) For example: plasmids are vectors; genetic information is transferred from one bacteria to another. Retrovirus: An RNA virus that infects host cells by reverse-transcribing DNA from RNA! HIV (human immunodeficiency virus): this nasty retrovirus pathogen disguises itself by changing the surface markers when the immune system starts targeting it. This makes it difficult to eradicate without expensive antivirus treatment. Viroids & Prions Viroids: infectious RNA molecules that are transmitted like viruses Prions: infectious protein particles 9 Viruses: Living or Not? Considered to be non-living (debated) Does not meet all of the characteristics of life In common with Living -Contains genetic material (RNA/DNA) -Reproduce (in host) -Has organization Why they are Non-Living -Do not have cells -Do not respond to stimuli -Do not use energy -Do not grow & develop -Cannot reproduce on their own 10 11 Human Disease Virus *Common cold Rhinovirus Chicken pox Varicella-zoster virus *Rabies Rabies virus Infectious Hepatitis Hepatitis virus *Measles Measles virus Polio Polio virus *Influenza Influenza virus *Ebola Ebola virus Mumps Mumps virus *AIDS Human immunodeficiency virus *Genital warts/Cancer Human papillomavirus Plant Disease Tobacco mosaic Tobacco mosaic virus Potato leaf roll Potato leaf roll Virus Tomato leaf curl virus Tomato leaf curl virus Virus & Disease By virtue of their very existence, virus are infectious particles. They MUST infect a living host to reproduce. In the process, they interrupt host life functions in a variety of ways. This interruption causes the disease. Their pathology (characteristics of their disease process) *These can be tested 12 Transmission of Viral Disease Virus are pathogenic… They cannot reproduce unless they attack another cell. …and carcinogenic. Once in the cell they can have a tendency to cause irreparable genetic damage that can lead to cancer. HPV Possible ways to become infected are Bites Physical contact Body fluid Mother to child Contact in the air Sexual contact Environmental exposure & Cervical cancer Hepatitis T B & C & Liver cancer Lymphotropic & Leukemia 13 Viral Reproduction Virus can be destroyed, but they can’t reproduce. How do more virus get made? Viruses must have a living host cell to reproduce Example: Bacteriophages infect bacteria Example: Herpes Simplex 1 infects lip cells Virus insert their genetic information inside the host cell and use the host cell to make more virus particles. This process is broken down into 2 pathways. The Lytic & Lysogenic Pathways of Viral Infections. 14 Viral Replication Lytic cycle Viral DNA is injected into host cell Contains instructions needed to make more viruses Host cell replicates viral DNA and makes the viral capsids (protein coats) New viruses are assembled inside host cell Cell bursts open releasing new viruses 15 Lytic Cycle D. Cell bursts open releasing new viruses C. New viruses are assembled Lytic Cycle inside host cell A. Viral DNA is injected into host cell B. Host cell replicates viral DNA and makes the viral capsids (protein coats) 16 Viral Replication Lysogenic cycle Viral DNA is injected into the host cell Viral DNA inserts itself into the host’s DNA Remains inactive for days, months, or years As the cell reproduces, more cells are produced that have the viral DNA in them Eventually, when the conditions are favorable (like when your immune system is weakened) the virus will enter the lytic cycle 17 Lysogenic Cycle 18 Lysogenic & Lytic Cycles: These integrate for dormant-type virus E Lysis of host cell lets new virus particles escape. A Virus particle binds, injects genetic material. A1 Viral DNA is A2 Chromosome and inserted into host integrated viral DNA chromosome by are replicated. viral enzyme action. Lytic Pathway D Accessory parts are attached to viral coat. C Viral proteins selfassemble into a coat around viral DNA. Lysogenic Pathway B Host replicates viral genetic material, builds viral proteins. A4 Viral enzyme excises viral DNA from chromosome. 19 A3 Cell divides; recombinant DNA in each daughter cell. Treatment of Virus Antibiotics: Do not work on viruses… they are not living Virus treatment options: Antivirals: specific for infection Tamiflu®, Relenza®, etc. Body’s natural defenses: Immune system Rest Antioxidants “Feed a cold, starve a fever”? Vaccines are taken beforehand to prevent infections by introducing the immune system to viral recognition signals. Subject to controversy… Common Virus Vaccines Hepatitis A Hepatitis B Influenza (the “Flu”) Measles, Mumps, Rubella Pertussis (whooping cough) Rabies Polio Tetanus Meningitis 20 Video clip Write 5 facts from the video http://www.npr.org/blogs/krulwich/2011/06/01/11 4075029/flu-attack-how-a-virus-invades-your-body Get into groups, divide the chapter up into sections, read through and gather information, answer the questions. 21 Here's a better, longer answer than the one in the video. First, some new viruses get caught in mucus and other fluids inside your body and are destroyed. Other viruses get expelled in coughs and sneezes. Second, lots of those new viruses are lemons. They don't work that well. Some don't have the right "keys" to invade healthy cells so they can't spread the infection. And third, as the animation shows, your immune system is busy attacking the viruses whenever and wherever possible. 22
