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Viruses PowerPoint Presentation

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Description : General description of viruses

Tags : viruses,flu

Published on : Oct 22, 2022 | Views : 763 | Downloads : 2

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Series of English Courses Dr. NEBBACHE Salim
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Viruses
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Section 18.1 Summary – pages 475-483 Viruses are nonliving infectious agents composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium. What is a virus? Most biologists consider viruses to be nonliving because they don’t exhibit all the criteria for life. They don’t carry out respiration, grow, or develop. All viruses can do replicate and they can’t even do that without the help of living cells. A cell in which a virus replicates is called the host cell.
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How are viruses named? Viruses, such as rabies viruses and polioviruses, were named after the diseases they cause. Today, most viruses are given a genus name ending in the word “virus” and a species name. A virus that infects a bacterium is called a bacteriophage, or phage for short.
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Section 18.1 Summary – pages 475-483 Viral Structure A virus has an inner core of nucleic acid, either RNA or DNA, and an outer protein coat called a capsid. Capsid Nucleic acid Envelope Some relatively large viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids. Envelopes are made from the same type of materials of the cell membrane.
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Section 18.1 Summary – pages 475-483 Viral nucleic acid is either DNA or RNA and contains instructions for making copies of the virus. Some viruses have only four genes, while others have hundreds. Nucleic acid Capsid Viral Structure The arrangement of proteins in the capsid of a virus determines the virus’s shape as well as the type of host cell it can infect.
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Section 18.1 Summary – pages 475-483 Viral Shape and Size Nucleic acid Capsid Polyhedral viruses resemble small crystals. Helical or coiled springs. Virus size ranges from 20 nanometers to 250 nanometers. Virology is the study of viruses. Virologist is a person who studies viruses.
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Section 18.1 Summary – pages 475-483 Viral Replication Before a virus can replicate, it must enter a host cell. A virus recognizes and attaches to a host cell when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s plasma membrane. In the T4 bacteriophage, a protein in the tail fibers recognizes and attaches the T4 to its bacterial host cell. In some viruses, the attachment protein is in the capsid or in the envelope.
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Section 18.1 Summary – pages 475-483 T4 Bacteriophage Capsid Nucleic acid Tail Tail fiber
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T4 Bacteriophage
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Section 18.1 Summary – pages 475-483 T4 Bacteriophage
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Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells. Viral Replication In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.
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Section 18.1 Summary – pages 475-483 Viral Replication Cycles Once attached to the plasma membrane of the host cell, the virus enters the cell and takes over its metabolism. Only then can the virus replicate. Viruses have two ways of getting into host cells. The virus may inject its nucleic acid into the host cell like a syringe injects a vaccine into your arm. The capsid of the virus stays attached to the outside of the host cell. 2. The envelope virus enters after attachment. The plasma membrane of the host cell surrounds the virus and produces a virus-filled vacuole inside the host cell’s cytoplasm. Then, the virus bursts out of the vacuole and releases its nucleic acid into the cell.
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Section 18.1 Summary – pages 475-483
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Section 18.1 Summary – pages 475-483
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Section 18.1 Summary – pages 475-483 Viral Replication Cycles Translocation
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Section 18.1 Summary – pages 475-483 Lytic cycle Steps The virus attaches to the host cell The virus injects its nucleic acid into the host cell. The host cell uses its own supplies to make copies of the viral nucleic acid and proteins. New viruses are assembled and ready to be released. The host cell breaks open and releases the new viruses to continue the cycle of infection. The new viruses can then infect and kill other host cells. This process is called a lytic cycle. Typical lytic viruses take about 30 minutes to complete the cycle and release 200 new viruses. The viruses are called virulent because they cause disease immediately.
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Section 18.1 Summary – pages 475-483 Nucleic acid Bacterial host cell Bacteriophage Bacterial DNA B. Entry The bacteriophage injects its nucleic acid into the bacterial cell. A. Attachment C. Replication D. Assembly E. Lysis and Release The host’s metabolic machinery makes viral nucleic acid and proteins. New virus particles are assembled. The host cell breaks open and releases new virus particles. Lytic cycle
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Section 18.1 Summary – pages 475-483 Lysogenic cycle Not all viruses kill the cells they infect. Some viruses go through a lysogenic cycle, a replication cycle in which the virus’s nucleic acid is integrated into the host cell’s chromosome. A lysongenic cycle begins in the same way as a lytic cycle. The virus must attach to the host cell and inject its nucleic acid. Viral DNA that is integrated into the host cell’s chromosomes is called a provirus (Prophage). A provirus may not affect the functioning of its host cell, which continues to carry out its own metabolic activity.
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Section 18.1 Summary – pages 475-483 Lysogenic cycle However, every time the host cell reproduces, the provirus is replicated along with the host cell’s chromosome. Therefore, every cell that originates from an infected host cell has a copy of the provirus. The lysogenic phase can continue for many years. However, at any time, the provirus can be activated and enter a lytic cycle. Lysogenic viruses usually wait for an opportune time. A time when the body is not well rested, stressed or fatigued. This type of virus is called temperate because it does not kill or make sick immediately.
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Section 18.1 Summary – pages 475-483 LYSOGENIC CYCLE LYTIC CYCLE The provirus leaves the chromosome. Viral nucleic acid and proteins are made. The cell breaks open releasing viruses. A lysogenic virus injects its nucleic acid into a bacterium. Bacterial host chromosome A. Attachment and Entry B. Provirus Formation Provirus The viral nucleic acid is called a provirus when it becomes part of the host’s chromosome. C. Cell Division Although the provirus is inactive, it replicates along with the host cell’s chromosome. Lysogenic cycle
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Thank you