Viruses PDF

What is a virus? Virus: An infectious particle that is nonliving. The word virus comes from the Latin word meaning “______.” poison  All viruses are parasites. All viruses require a host. Parasites:...

What is a virus? Virus: An infectious particle that is nonliving. The word virus comes from the Latin word meaning “______.” poison  All viruses are parasites. All viruses require a host. Parasites: Parasites live in or on other living organisms, causing them harm. Host: The host is the living organism the parasite lives on. Characteristics of Viruses extremely small Viruses are _____________. Most can be seen only with an electron microscope _________________.  A viruses is active only when _______________. inside a living cell When removed from a living cell, it _______ ceases all activities but retains its ability They may be crystallized and to ___________. infect the cell stored indefinitely, but even after long periods of time, they retain their ability to infect a living cell. Viruses vary widely in terms of size and structure, but they all have one thing in common: They enter living cells and use the machinery of the cell to produce more viruses. The parts of the virus: 1 – Capsid 2 – Base plate 3 – Tail fibers 4 – Sheath 5 – Collar 6 – DNA or RNA Viruses are non-cellular. 1. They are not made of cells. 2. Viruses consist of two main parts: DNA or RNA surrounded by a protein coat. 3. The protein coat is called the capsid.  The capsid is made of The _______ proteins that enable the Viral virus to enter a host cell. Capsid The capsid has a particular ______ shape that must match _________ receptors on the surface of a ________. host cell When the virus attaches to these receptors, the cell is “tricked” into letting the virus inside. Since viruses must bind precisely to _______ proteins on the __________, cell surface they are highly specific to the cells they infect. Papaya Ringspot Virus Plant viruses can only infect plant cells. Animal viruses can only infect animal cells. Polio Virus Viruses of eukaryotes are usually tissue specific. Example: Human cold viruses infect only the cells lining the upper respiratory system, ignoring all other tissues. Bacteriophages are viruses that infect only certain types of bacteria. Viruses can reproduce, but only _______________. inside a living cell They reproduce inside a cell by getting the cell to produce viral parts instead of cell parts.  Viruses: Are They Living or Nonliving? Living Characteristics of Viruses: 1. They can reproduce but only inside a living cell. 2. They can mutate or change. 3. They have DNA or RNA. Their genome may consist of only four genes, or up to a hundred genes.  Viruses: Are They Living or Nonliving? Non-living Characteristics of Viruses: 1. They are non-cellular. 2. They have no metabolism. They have no food or energy requirements. 3. They can be crystallized and dehydrated and stored indefinitely. They come to "life" only when injected inside a living cell.  Viral Reproduction : How Do They Do It? 1. Since viruses have no enzymes and no cell parts, they force the host cell to ___________________. start making viral parts 2. A viral infection begins when: the genetic material (DNA or RNA) of a virus makes its way into a host cell. 3. Once inside, the virus _______ hijacks the cell, reprogramming the cell.  4. The viral genome takes over the _______ host cell and makes the host cell start producing viral ________. parts viral DNA and 5. The host cell will begin to make copies of the ________ producing the _____________. protein capsids 6. The host cell assembles the parts into viruses. 7. The reproductive cycle ends with the exit of hundreds or thousands of viruses from the infected host cell. destroys the host cell. 8. This often ________ Each of these viral progeny has the capacity to infect neighboring cells, thereby spreading the infection.  The Two Reproductive Possibilities: 1. Once a virus is inside a host cell, two different processes may occur. 2. Some viruses replicate themselves immediately, killing the host cell. 3. Other viruses replicate themselves in a way that does not destroy the host cell. 4. These two processes are called: a) The lytic cycle b) The lysogenic cycle The Lytic Cycle In a lytic infection, a virus enters a cell, forces the cell to make viral copies, and causes the cell to burst. Bacteriophage T4 is an example of a bacteriophage that causes a lytic infection.  Attachment. Tail fibers The Lytic are used to attach to receptor sites on the surface of the host cell. Cycle Entry. Phage DNA is injected. Empty capsid Release. The cell remains outside. Host swells, bursts, and cell DNA is destroyed. releases 100’s of new viruses. Synthesis. The host cell is directed to produce viral genomes and Assembly. The viral DNA or protein capsids. RNA is assembled inside the protein coat.  The Lysogenic Cycle 1. In this type of viral reproduction the host cell makes copies of the viral genetic material indefinitely. 2. The virus incorporates its DNA into the DNA of the host cell. The viral DNA is then ________ replicated along with the host cell’s own DNA. 3. Lysogenic viruses do not __________________. kill the cell right away A lysogenic virus may remain _______ inactive for some period of time.  Phage attaches and injects its DNA. Daughter cell with prophage Many cell divisions may Bacterial occur, producing chromosome a large population of Bacteriophage bacteria that are (phage) infected with the prophage. The host LYTIC LYSOGENIC cell bursts, Bacterial cell divides by binary Certain features fission normally. The viral releasing the determine whether: genome is copied and passed to new viruses. Lytic cycle Lysogenic cycle daughter cells. is induced Is entered Prophage Viral DNA is incorporated into the bacterial chromosome, forming a prophage. New viruses are produced.  The Lysogenic Cycle The name of the Prophage: viral DNA embedded into the host cell’s DNA. The prophage may remain part of the host __________________ for many generations before becoming active. Eventually, certain environmental conditions (chemicals, radiation) may trigger the switchover from the ________ lysogenic cycle to the ____ lytic cycle. Comparison of Viruses and Cells Cell membrane, Have DNA or RNA cytoplasm, nucleus, and a protein capsid cytoplasmic organelles Reproduce independently, Only within a host cell either sexually or asexually DNA or RNA DNA None Yes, in multicellular organisms No Yes No Yes Yes Yes  Viral Diseases There is sometimes little that can be done to cure a viral infection. Antibiotics are effective against _______ bacteria but not against _______. viruses A few new drugs have been developed that interfere with the reproduction of the virus, but they only seem to slow the effect of the virus. The battle against viral diseases lies in the use of vaccines. Contain a harmless Vaccines: variation of the pathogen. Our immune system launches a response to the harmless form, recognize it the thereby learning to _________ next time that we are ________ exposed to it. When we are exposed to the "real" pathogen, our immune system can respond __________ much faster learned to since it has already _______ recognize the pathogen.  Viral Colds Diseases Include: Hepatitis AIDS Flu West Nile Virus Chicken pox