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BIOL 1406 HCC-SW/Stafford Campus J.L. Marshall, Ph.D Chapter 19 – Viruses *Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for the exams. A Borrowed Life Figure 19.1 shows Escherichia coli, a bacterium, being attached by a virus called T4 bacteriophage. This virus infects the bacterial cell and causes the bacterium to make more viral particles. A virus is an infectious particle that is composed of at least a nucleic acid center surrounded by a protein coat. Viruses are known to cause disease in the cells they infect. Yet, viruses do not display the characteristics of life, so they are considered non-living entities. But, viruses have been used in research to understand basic molecular biology. Concept 19.1 : A virus consists of a nucleic acid surrounded by a protein coat Viruses were discovered near the end of the 19th century. The Discovery of Viruses: Scientific Inquiry Research was done during the late 1800’s and early 1900’s on the tobacco plant that lead to the discovery of viruses (Figure 19.2). Structure of Viruses Viruses are not cells. They are infectious particles consisting of a nucleic acid core and protein coat; some viruses have a membranous envelope. Viral Genomes The genomes of viruses can be dsDNA, ssDNA, dsRNA, and ssRNA. A virus is commonly referred to as a DNA virus or RNA virus, based on its genome. Capsids and Envelopes The protein shell surrounding the nucleic acid core is called the capsid. The capsid gives a virus its overall shape. The protein units that comprise the capsid is called capsomeres (Figure 19.3a and b). Some viruses contain viral envelopes, which are derived from the host cell membrane (Figure 19.3c). Some viruses also have glycoprotein spikes. And, some viruses carry accessory enzymes that aid in their infectious process. Viruses that infect bacteria are called bacteriophages, or phages (Figure 19.3d). 1 BIOL 1406 HCC-SW/Stafford Campus J.L. Marshall, Ph.D Concept 19.2 : Viruses replicate only in host cells Viruses are obligate intracellular parasites as they are completely dependent on infecting a host cell to male more viral particles. Each virus has a host range, a specific species the virus can infect. Viruses identify their host by the host cell’s membrane proteins. In multicellular organisms like humans, viruses also identify specific cells they will infect, like HIV and its specific white blood cell. General Features of Viral Replicative Cycles Once viruses enters cells (cross the cell membrane), the virus takes over the cell causing it to re-direct its cellular efforts to making more viral particles (Figure 19.4). Viruses use the host cell to make the viral components, then the virus is assembled and is released from the host cell. Often, when the virus exits the host cell, it causes destruction of the host cell. Replicative Cycles of Phages Phages have two alternative replicative cycles: lytic and lysogenic. The Lytic Cycle The phage cycle that causes death of the host cell is called the lytic cycle. When these phages exit the cell, they do so by lysing the bacterial cell membrane and cell wall. A phage that only replicates by a lytic cycle is called a virulent phage (Figure 19.5). Bacteria are not completely susceptible to phages. Bacteria can mutate such that viruses no longer recognize their receptor protein. And, bacteria have enzymes called restriction enzymes that will “cut” foreign nucleic acids, such as viral nucleic acids. The Lysogenic Cycle The lysogenic cycle of a phage infection is where the phage DNA is incorporated into the host’s chromosome. This type of phage is called a temperate phage. The most well studied temperate phage is phage (lambda) that infects E. coli (Figure 19.6). Lambda phage can go through a lytic cyle, or incorporate its DNA into E. coli’s DNA, so that now the viral DNA is known as a prophage. As long as the environment is positive for it will remain part of the E. coli chromosome. Replicative Cycles of Animal Viruses There are a number of different viruses that cause disease in humans. Viruses are classified based on the type of nucleic acid they have, and whether it is single stranded or double stranded (Table 19.1). 2 BIOL 1406 HCC-SW/Stafford Campus J.L. Marshall, Ph.D Viral Envelopes Animal viruses with an envelope use it to enter its host’s cell. These viruses use their glycoproteins to gain entry (Figure 19.7). When some viruses are ready to exit, they do so by a process similar to exocytosis. RNA viruses make use of the ribosomes in the host cell cytoplasm to begin the viruses synthesis process. DNA viruses make use of the host cell nucleus to maintain their viral DNA. RNA as Viral Genetic Material Some RNA viruses are “seen” by the host cell as mRNA that needs to be translated. Some RNA viruses must make a copy of their RNA before it can being the translation process. Yet, there are some RNA viruses that go through a DNA intermediate. These viruses are called retroviruses and carry with them an enzyme that is significant in their infectious process called reverse transcriptase. Their replication process is RNA --> DNA (Figure 19.8). An example of this type of virus is the human immunodeficiency virus (HIV) that is known to cause acquired immunodeficiency syndrome (AIDS). Concept 19.3 : Viruses, viroids, and prions are formidable pathogens in animals and plants Complex entities that are smaller than viruses can cause infections, such as viroids and prions, in plants and animals. Viral Diseases in Animals Viruses can cause a wide array of symptoms from the common cold, to polio. Some viral infections are temporary, and some are permanent. A vaccine is a harmless variant or a defective pathogen that is used to stimulate the immune system to mount protection against the more virulent form of the virus. Viral infections can not be cured by antibiotics; antibiotics are only effective against bacteria. But, there are antiviral medicines that work to inhibit viral replication steps. Emerging Viruses Emerging viruses suddenly become apparent. These are viruses that seem to come from nowhere, like HIV. Viral diseases that occur in high frequency in a population (higher than normal) cause an epidemic, like the H1N1 flu virus of 2009 (Figure 19.9a). If the viral disease spreads world wide, then it becomes a pandemic (Figure 19.9b). 3 BIOL 1406 HCC-SW/Stafford Campus J.L. Marshall, Ph.D Viral Diseases in Plants Viral diseases in plants affect agriculture by causing brown spots, stunted growth, and damage to flowers and roots(page 405). Plants acquire the virus by horizontal transmission, where it acquires the virus from the environment, usually as a result of tissue destruction of the plant. Plants can also acquire the viral infection by vertical transmission, where they inherit the viral infection from a parent. Viroids and Prions: The Simplest Infectious Agents Viroids are circular RNA particles that infect plants. Viroids tend to stunt plant growth. Infectious proteins are called prions, which tend to cause degenerative brain disease in various animals. In cows its called mad-cow disease. In humans its called Creutzfeldt-Jakob disease. Prions are most likely transmitted in food. Prions have a slow incubation time, and they are indestructible (Figure 19.10). Today, there is no known cure. 4