Viruses

Viruses are particles of nucleic acid, protein, and in some cases lipids. They can only reproduce by infecting living cells. There are many types of viruses that vary in size and structure. Despite the differences, all viruses enter living cells and, once inside, use the machinery of the infected cell to produce more viruses. Typical viruses are made up of a DNA or RNA core and are surrounded by a protein coat. The protein coat covering the virus is called a capsid. **Bacteriophages** Most viruses are highly specific to the cells they infect. Plant viruses do not infect animal cells; most animal viruses only infect certain species of animals; and bacterial viruses infect only certain types of bacteria. Bacteriophages are viruses that infect bacteria. There are two ways for bacteriophage to infect a bacteria; lytic and lysogenic infections. **Lytic Infections** Lytic infections occur when a virus enters the cell and makes copies of itself, causing the cell to burst. The lytic cycle begins when a virus attaches to bacterium’s cell wall. Then it injects its own DNA into bacterium. The bacteriophage begins to take over the cell, causing synthesis of new bacteriophage proteins and nucleic acids. The proteins and nucleic acids assemble into complete bacteriophage. Bacteriophage enzymes destroy bacterium’s cell wall and release the viruses. This is called a lytic infection because the host cell is lysed and destroyed. **Lysogenic Infections** Lysogenic infections occur when a virus injects its DNA into the DNA of the host cell. When the host cell replicates the bacteriophage DNA will also replicate with it. After the DNA is injected into the bacteria, it inserts itself into bacterial chromosome. The bacteriophage DNA is called prophage. The prophage may replicate with the bacteria for many generations. The prophage can exit the bacterial chromosome. After the prophage has exited the bacterial chromosome, it makes proteins and nucleic acids to assemble into complete bacteriophage particles. At this point, it is no longer in the lysogenic cycle, it is now in the lytic cycle and after the bacteriophages are assembled, they will lyse the bacterium cell wall. **Viruses and Diseases** Viruses cause many human diseases. The best way to protect against viral diseases is by the use of vaccine. A vaccine is a preparation of weakened or killed virus or viral protein. When injected into the body, the vaccine stimulates the immune system; sometimes producing immunity to the disease. Vaccines provide protection only if they are used before the infection begins. Once the virus infects the body, it may be too late to control.
 * Viruses**

Certain viruses called oncogenic viruses cause cancer in animals. Oncogenic viruses generally carry genes that disrupt the normal controls over cell growth and division. Viruses that contain RNA as their genetic information are called retroviruses. When infecting a cell, retroviruses produce a DNA copy of their RNA. They get their name from the genetic information being copied backwards from RNA to DNA. Prions are particles that contain no DNA or RNA, only protein.

Viruses are not considered to be alive because they do not have all the characteristics of life. For example, viruses cannot reproduce independently, but when they infect living cells, they can make copies of themselves, regulate gene expression, and even evolve.