Virus School Notes

Viruses: What is a Virus? ● A virus is a tiny, nonliving particle that enters and then reproduces inside a living cell. ● No organisms are safe from viruses. Characteristics of Viruses: ● Most biologists today consider viruses to be nonliving because viruses do not have all the characteristics essential for life. ● Viruses are not cells and do not use their own energy to grow or to respond to their surroundings. ● Viruses also cannot make food, take in food, or produce wastes. ● The only way in which viruses are like organisms is that they are able to multiply. ● Although viruses can multiply, they do so differently than organisms. Viruses can multiply only when they are inside a living cell. ● The organism that a virus enters and multiplies inside is called a host. ● A host is an organism that provides a source of energy for a virus or another organism. ● A virus acts like a parasite, an organism that lives on or in a host and causes it harm. ● Almost all viruses destroy the cells in which they multiply. Virus Shapes: ● Viruses vary widely in shape. ● Some viruses are round, while some are rod-shaped. ● Other viruses are shaped like bricks, threads, or bullets. ● There are even viruses that have complex, robotlike shapes, such as the bacteriophage. ● A bacteriophage is a virus that infects bacteria. In fact, its name means “bacteria eater.” Virus Sizes: ● Just as viruses vary in shape, they also vary in size. ● Viruses are smaller than cells and cannot be seen with the microscopes you use in school. ● Viruses are so small that they are measured in units called nanometers (nm), which is one billionth of a meter. (video clip) The Structure of a Virus ● All viruses have two basic parts: a protein coat that protects the virus and an inner core made of genetic material. ● A virus’s genetic material contains the instructions for making new viruses. ● Like keys, a virus’s proteins fit only into certain “locks,” or proteins, on the surface of a host’s cells. How Viruses Multiply ● After a virus attaches to a host cell, it enters the cell. ● Once inside a cell, a virus’s genetic material takes over many of the cell’s functions. ● It instructs the cell to produce the virus’s proteins and genetic material. ● These proteins and genetic material then assemble into new viruses. ● Some viruses take over cell functions immediately. Other viruses wait for a while. (video clip) Active Viruses: ● After entering a cell, an active virus immediately goes into action. ● The virus’s genetic material takes over cell functions, and the cell quickly begins to reproduce. ● When it is full of new viruses, the host cell bursts open, releasing hundreds of new viruses as it dies. Hidden Viruses: ● Other viruses do not immediately become active. Instead, they “hide” for a while. ● After a hidden virus enters a host cell, its genetic material becomes part of the cell’s genetic material. ● The virus does not appear to affect the cell’s functions and may stay in this inactive state for years. ● Each time the host cell divides, the virus’s genetic material is copied along with the host’s genetic material. ● Then, under certain conditions, the virus’s genetic material suddenly becomes active. ● It takes over the cell’s functions in much the same way that active viruses do. ● Soon, the cell is full of new viruses and bursts open. ● The virus that causes cold sores is an example of a hidden virus. ● It can remain inactive for months or years inside nerve cells in the face. ● While hidden, the virus causes no symptoms. Review Questions: 1. What is a virus? - A nonliving particle that invades a cell and reproduces. 2. What basic structures do all viruses share? - An inner core of genetic material surrounded by a protein outer coat. 3. Scientists hypothesize that viruses could not have existed on Earth before organisms appeared. Use what you know about viruses to support this hypothesis… - Viruses cannot exist without organisms because to reproduce they must have a host cell they can invade. 4. Compare and contrast active and hidden viruses. - Both are viruses that are going to invade and reproduce in a host cell. The difference is that an active virus immediately takes over the cell and a hidden virus stays “hidden” and waits a while to take over. 5. Do you think that the Influenza virus is an active or hidden virus? Explain your reasoning. - Active because soon after “catching” it from someone, the symptoms appear.