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Download AP Biology Notes Outline Chapter 19: Viruses OVERVIEW OF
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AP Biology Notes Outline Chapter 19: Viruses OVERVIEW OF VIRAL GENETICS A virus is a parasitic infectious agent that is UNABLE to survive outside a host organism. This is because viruses DO NOT CONTAIN RIBOSOMES FOR PROTEIN SYSNTHESIS, NOR DO THEY CONTAIN ENZYMES FOR METABOLISM. They are completely dependent on their host. Once a virus infects a cell, it takes over the cell’s machinery and uses it to produce whatever it needs to survive and reproduce. How a virus acts after it enters a cell depends on what type of virus it is. Classification of viruses is based on several factors: o Genetic Material: DNA, RNA, protein, etc? o Capsid: Type of capsid? o Viral Envelope: present or absent? o Host Range: what type of cells does it affect? All viruses have a genome (DNA or RNA) and a protein coat (capsid). Genome is an entire set of genes (DNA or RNA) Capsid is the protein coat that encloses the viral genome. Capsids are built from a large number of protein subunits called capsomeres. Viral Envelopes: accessory structures that help viruses attach their host – these are membranes cloaking the capsid. Envelopes are derived from the membrane of the host cell and will always contain some components of the virus and some of the host cell. Each virus has a host range…the range of cells that the virus is able to infect. For example, HIV infects the T cells of our body, and bacteriophages only infect bacteria. A) Tobacco mosaic viruses have a helical capsid with the overall shape of a rigid rod. B) Adenoviruses have a polyhedral capsid with a glycoprotein spike at each vertex. C) Influenza viruses have an outer envelope studded with glycoprotein spikes. The genome consists of eight different RNA molecules, each wrapped in a helical capsid. D) T4 Bacteriophages have a complex capsid consisting of a polyhedral head and tail apparatus. AP Biology Notes Outline Chapter 19: Viruses RETROVIRUSES http://www.sumanasinc.com/webcontent/animations/content/lifecyclehiv.swf A very special type of virus is one called a retrovirus. RETROVIRUSES ARE RNA VIRUSES THAT CARRY AN ENZYME CALLED REVERSE TRANSCRIPTASE. Once in the cytoplasm of the host cell, the RNA virus uses this enzyme and “reverse transcribes” its genetic information from RNA into DNA, which then enters the nucleus of the cell. In the nucleus, the newly transcribed DNA incorporates into the host DNA and is transcribed into RNA when the host cell undergoes normal transcription. The mRNA produced from this process gives rise to new retrovirus offspring, which can then leave the cell in a lytic pathway. A well-known example of a retrovirus is the HIV virus of AIDS. VIRAL REPRODUCTION: THE LYTIC AND LYSOGENIC CYCLES http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html Once inside the cell, a DNA virus can take one of two pathways – a lytic or lysogenic pathway. In a lytic cycle, the cell actually produces many viral offspring, which are released from the cell, thus killing the host cell in the process. In a lysogenic cycle, the virus falls dormant and incorporates its DNA into the host DNA as an entity called a provirus OR prophage. The viral DNA is quietly reproduced every time the cell reproduces itself, and this allows the virus to stay alive from generation to generation without killing the host cell. Viruses in the lysogenic cycle can sometimes separate out from the host DNA and enter the lytic cycle (like a bear waking from hibernation). http://biology.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=biology&cdn=education&tm=48&gps=195_41 3_1276_493&f=11&su=p897.9.336.ip_&tt=2&bt=1&bts=1&zu=http%3A//student.ccbcmd.edu/courses/ bio141/lecguide/unit3/viruses/lytsum.htmL AP Biology Notes Outline OVERVIEW OF VIRAL REPLICATION 1. Attachment 2. Penetration 3. Replication and Synthesis 4. Assembly 5. Release Virus uses host’s nucleotides and enzymes to replicate itself. At the same time, other host resources are used to make new capsid proteins by transcription and translation. The new viral genomes and capsids are assembled into new virus particles; when the number exceeds the cell’s surface area limitations, the cell bursts open and new viruses are released to infect other cells – exponential increase. Chapter 19: Viruses AP Biology Notes Outline Chapter 19: Viruses OTHER IMPORTANT INFORMATION Viruses can come in many shapes and sizes. viroids are plant viruses that are only a few hundred nucleotides in length prions are incorrectly folded forms of brain cell proteins that can convert other normal host proteins into misshapen proteins…mad cow disease is caused by a prion – diseases of this type are degenerative and tend to cause brain disfunction (dementia, muscular control problems, and loss of balance). CONTROLLING VIRUSES Diseases causes by viruses are difficult to treat. Drugs are only used to treat SYMPTOMS, not cure the disease (just make patient feel better for short duration). The only methods to control viruses are to PREVENT illness via vaccines and antibody production, use of interferon in body. Antibodies are made by host’s immune system after infection occurs (if host survives the infection). Help inactivate viruses and destroy harmful bacteria - Are specific for viruses or bacteria - Once an antibody is produced that recognizes a specific virus or bacteria, then that strain will be ineffective on that individual organism Vaccines are harmless variants or derivatives of pathogenic microbes - Stimulate the immune system to mount defenses against a specific pathogen - Developed by Edward Jenner – cowpox used to develop smallpox vaccination - Vaccinated or immunized again disease Ex. MMR, DPT, polio, smallpox, influenze,rabies, hepatitis C Interferons are chemicals in the body that are activated when cells are attacked Cell under seige produces interferon which binds to neighboring cells’ cell membranes to warn them of the dangerous pathogen Antibiotics are powerless against viruses! Antibiotics kill bacteria by inhibiting enzymes or processes specific to the pathogens; since viruses have no metabolism of their own, the antibiotics do not work. Only drugs that have any effect on viruses are ones that interfere with nucleic acid synthesis – AZT (with HIV), acyclovir (with herpesvirus)…or with protein production (protease inhibitors with AIDS). Emerging viruses that cause new outbreaks of disease are usually existing viruses that manage to expand their host territory. o AIDS o Hantavirus o Ebola (hemorrhagic fever) o Nipah virus o Influenza What contributes to spread of emerging viruses? Mutation Spread from one species to another Dissemination from small, isolated population