Viruses: What is a Virus?

Дата канвертавання27.04.2016
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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.

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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.

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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.

  1. What basic structures do all viruses share?

  • An inner core of genetic material surrounded by a protein outer coat.

  1. 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.

  1. 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.

  1. 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.

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