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VIRUSES WHAT IS A VIRUS? Viruses are particles that are not alive. They cause diseases and infections. Viruses are made up of nucleic acids, either DNA or RNA, surrounded by a protein coat. They are smaller than the tiniest bacterium. Most biologists agree that viruses are not alive because they don’t grow, develop, or carry out respiration. All viruses replicate, or make copies of themselves. Viruses need the help of living cells to copy themselves. In order to copy itself, a virus must enter a living cell. The cell in which a virus replicates is called a host cell. WHAT IS THE STURCTURE OF A VIRUS? Viruses differ in size and shape. Some viruses contain as few as four genes. Other viruses have hundreds of genes. Every virus has an inner core of nucleic acid. This core contains the virus’s genetic material. Some viruses contain RNA, some contain DNA. The nucleic acid contains the instructions for making copies of the virus. The outer protein coat surrounding the virus is called a capsid. The arrangement of proteins in the capsid gives the virus its shape. The proteins arrangement also determines what cell can be infected and how the virus infects the cell. Some large viruses contain another layer called an envelope. HOW DOES A VIRUS ATTACH TO A HOST CELL? Before a virus can replicate, it has to enter a host cell. In order to do this, it must first attach itself to the host cell. Every virus has a specially shape device called an attachment protein. Because of the specific shape, each virus can only attach to a few kinds of cells. The attachment process is like two pieces of a jigsaw puzzle fitting together. A virus that infects a bacterium is called a bacteriophage, or phage for short. A protein located in the tail fibers of the bacteriophage recognizes and attaches to its bacterial host cell. DO VIRUSES INFECT MORE THAN ONE SPECIES? Many viruses are species specific. They infect only certain species. For example, smallpox infects only humans. Other viruses are not species specific. The virus that causes the flu infects both human and animals. Viruses that are not species specific are more difficult for scientist to eliminate. VIRAL REPLICATION CYCLES Once a virus attaches itself to the host cell, the virus enters the cell and takes over its metabolism. Only then can the virus replicate itself. There are two ways that viruses get into cells. First, the virus can inject its nucleic acid in to the host cell. When this takes place, the capsid of the virus stays attached to the outside of the host cell. An enveloped virus enters a host cell in another way. After attachment, the plasma membrane of the host cell surrounds the virus. This produces a virus filled vacuole inside the host cell’s cytoplasm. Recall that a vacuole is a membrane bound compartment used for temporary storage of materials within a cell. Once the vacuole is in the cytoplasm, the virus burst out of the vacuole and releases it nucleic acid into the cell. WHAT IS A LYTIC CYCLE? Once the virus is inside the host cell, the virus’s genes are expressed. The substances that produced take over the host cell’s genetic material. When viruses take over the cell, the cell stops producing the materials it needs to live. It uses its own enzymes and energy to make new viruses. The new viruses then burst from the host cell and the host cell dies. The new viruses can infect and kill other host cell. This process is call a lytic cycle. WHAT HAPPENS DURING A LYSOGENIC CYCLE? Not all viruses kill the cells they infect. Some viruses go through a lysogenic cycle. A lysogenic cycle is a replication cycle in which the virus’s nucleic acid is integrated into the chromosome of the host cell. The lysogenic cycle begins the same way as a lytic cycle. The virus attaches to the host cell and its nucleic acid enters the cell. Instead of taking over the genetic material of the host, the viral DNA is included in the host’s cell chromosome. Once that happens, the viral DNA is called a provirus. The cell continues to carry out it own metabolic activity, but every time the host cell reproduces, the provirus is replicated as well. This means that every cell that comes from an infected host cell has a copy of provirus. The lysogenic cycle can last for many years. At any time, though, the provirus can be activated and enter a lytic cycle. When that happens, the virus will replicate and kill the host cell. Next meeting we will discuss some diseases caused by proviruses. We will discuss Retrovirus, what is HIV, Cancer and Viruses, what is a plant virus and the origin of viruses.