What Is A Virus? What Is A Viral Infection? A virus (from the Latin virus meaning toxin or poison) is a microscopic organism consisting of genetic material (RNA or DNA) surrounded by a protein, lipid (fat), or glycoprotein coat. Some microbiologists classify viruses as microorganisms, while others don't because they are "nonliving" and describe viruses as microscopic infective agents. Viruses are unique microorganisms because they cannot reproduce without a host cell. After contacting a host cell, a virus will insert genetic material into the host and take over that host's functions. The cell, now infected, continues to reproduce, but it reproduces more viral protein and genetic material instead of its usual products. It is this process that earns viruses the classification of "parasite". What are "friendly" viruses? Most of us know about friendly bacteria that exists in our intestines and help us digest food. Scientists from San Diego State University reported in the Proceedings of the National Academy of Sciences (May 2013 issue) that humans also carry friendly viruses that help protect us from dangerous bacteria, including E. coli. Jeremy Barr and team believe their discovery may change how several diseases are treated. They discovered that mucus contains bacteriophages, viruses that actively protect their hosts form harmful bacteria by destroying them. Barr wrote "Taking previous research into consideration, we are able to propose the Bacteriophage Adherence to Mucus -or BAM -- is a new model of immunity, which emphasizes the important role bacteriophage play in protecting the body from invading pathogens . . . . . We envision BAM influencing the prevention and treatment of mucosal infections seen in the gut and lungs, having applications for phage therapy and even directly interacting with the human immune system." How are viruses spread? Viruses may spread vertically (from mother to child) or horizontally (from person to person). A virus's ability to spread depends on the makeup of the virus. Some viruses can spread by simple contact, exchanges of saliva, coughing, or sneezing. Some require sexual contact, while others go through the fecal-oral route via contaminated food or water. Still other viruses require an insect like a mosquito to carry the virus from person to person. What diseases are caused by viruses? Several human diseases are caused by viruses. These include: Smallpox The common cold Measles Chickenpox Hepatitis Influenza Human papilloma virus Shingles Herpes Polio Rabies Ebola Hanta fever HIV (the virus that causes AIDS) Cold sores SARS (Severe respiratory syndrome) Dengue Epstein-Barr virus What are the essential characteristics of viruses? Viruses are the most primitive cellular and non-cytoplasmic infectious agents. Russian botanist D.J. Iwanowski (1892) first discovered virus in an infected tobacco plant. However, M.W. Beijerink (1898) coined the term virus. Then American chemist W.M. Stanley (1935) isolated pure crystal of Tobacco Mosaic Viruses (TMV) and concluded that viruses are made of nucleoproteins. infectious property of a virus is due to its nucleic acid. General Characteristics of Viruses: - Viruses are a cellular, non-cytoplasmic infectious agents. - They are smaller than bacteria, and this can pass through bacteriological filter. - Viruses are transmissible from disease to healthy organisms. - All viruses are obligate parasites and can multiply only within the living host cells. - Viruses contain only a single type of nucleic acid either DNA or RNA. - Viruses are host specific that they infect only a single species and definite cells of the host organisms. - Viruses are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions. Biological position of viruses: Viruses lack a cytoplasmic membrane and they do not have the basic component of a cell. They can only replicate inside the host cell. Outside the host cell, they are non-living. Thus, viruses show characters of both living and non-living. - Non-living Characters of Viruses: Generalized Structure of Viruses: - Shape and size: The shape varies considerable. They may be spherical or golf ball-like, rod-shaped, tadpole-like, helical or polyhedral. Plant viruses are smaller than bacteria. - Chemical structure and function: Viruses have a very simple structure. The core of the viruses is made upon of nucleic acid, which is surrounded by a protein coat called capsid. The nucleic acid always contains only a single kind of nucleic acid i.e. either DNA or RNA. The Capsid or the protein coats: It is made up of many identical protein sub-units called capsomeres. The capsomeres are composed of either one or several type of proteins. Capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus. The host specificity of virus is due to proteins of the capsid. Following characters of viruses assign them as non-living: (a) They can be crystallized. (b) Outside the cell, they behave like inert chemicals. (c) They do not show growth, development, nutrition, reproduction, etc. (d) They can be precipitated. Living characters of viruses: (a) They multiply within host cells. (b) They possess genetic material, either DNA or RNA. (c) There are definite races or strains. (d) They exhibit mutations. Because of the above reasons, viruses form unique bridge between living and non-living things. DO NOT WRITE ON THE PAPER I. The Basics of a Virus – “What is a Virus? What is a Viral Infection?” a) What is the definition of a virus? b) Where does this term come from? c) How can and do viruses spread (list four ways)? II. “What are the Essential Characteristics of Viruses?” a) List three non-living characteristics of a virus. 6pts. b) List four living characteristics of a virus. 8pts. c) Based on the above characteristics, do you think viruses are living or non-living entities? Explain. 10 pts. d) True or false 1. Virus has 2 nucleoids DNA and RNA. 2. The cell wall of the virus is very thick. 3. Virus constantly mutates. 4. Virus is contagious. 5. Beijerink discover virus in tobacco. 6. Virus is small than bacteria. 7. Virus cytoplasm is yellow. 8. Virus is parasites. 9. All viruses are round. 10. Virus reproduce using a host cell. III. Classify in virus or bacteria (COPY AND ANWER) ___ flu ___ bad breathe ___ yogurt ___ common cold ___ AIDS ___ Strep throat ___ E. coli ___ Digest oil ___ Warts ___ Anthrax ___ Rabies ___ Small pox ___ Non cellular ___ unicellular ___ parasites ___ Ebola ___ Has cell wall ___ Polio ___ HIV IV. Match the letter on the diagram with the best statement ____ Virus nucleic acid is injected into host cell. ____ The virus nucleoid acid is copied. ____ The host cell explodes, releasing more viruses. ____ The virus attaches itself to the host cell. ____ cell reproduce more viruses. V. Multiple choice 1. The study of viruses is a part of biology because a. they belong to the kingdom Monera. c. they are living organisms. b. they are about to become extinct. d. they are active inside living cells. 2. A typical virus consists of a. a protein coat and a cytoplasm core. b. a carbohydrate coat and a nucleic acid c. a protein coat and a nucleic acid core. d. a polysaccharide coat and a nucleic acid 3. All viruses have a. cytoplasm. b. mitochondria. c. ribosomes. d.DNA or RNA, but not both. 4. HIV can be transmitted a. Through sexual contact. b. Through the sharing of no sterile needles. c.to infants during pregnancy or through breast milk. d. All of the above VI. Draw and label 1. Draw a “robot” virus. Label its protein coat and its genetic material. 2. Draw a rod-shaped bacterium. Label its cytoplasm, genetic material, cell membrane, cell wall, and flagellum (or flagella).