Viruses

Non-cellular particles of nucleic acid, protein, and in some cases lipids that can reproduce only by infecting living cells Differ widely in terms of size and structure They enter living cells and, once inside, use the machinery of the infected cell to produce more viruses

Structure Typical virus is composed of a core of either DNA or RNA surrounded by a protein coat Outer protein coat is called its capsid Includes proteins that enable a virus to enter a host cell Bind to the surface of a cell and “trick” the cell into allowing it inside

Viral Infection Most viruses are highly specific to the cells they infect Plant viruses do not infect animal cells; most animal viruses infect only certain species of animals; and bacterial viruses infect only certain types of bacteria Bacteriophages are viruses that infect bacteria

2 Modes of Infection Lytic Infection –Virus enters a cell, makes copies of itself, and causes the cell to burst –Host cell is lysed and destroyed Lysogenic Infection –Host cell makes copies of the virus indefinitely –Virus embeds its DNA into the DNA of the host cell and is replicated along with the host cell’s DNA –Does not lyse the host cell right away

Viruses and Disease Cause human diseases such as polio, measles, AIDS, mumps, influenza, yellow fever, rabies, and the common cold Oncogenic viruses cause cancer in animals Those that contain RNA as their genetic information are called retroviruses Prions contain only the protein portion and can cause mad cow disease and CJD

Prevention Best way to protect against most viral diseases lies in prevention via vaccines Vaccine made up of weakened or killed virus or viral proteins Injected into the body, a vaccine stimulates the immune system, sometimes producing permanent immunity to the disease Only effective before an infection begins

Are Viruses Alive? Living things are made up of units called cells. Living things reproduce. Living things are based on a universal genetic code. Living things grow and develop. Living things obtain and use materials and energy. Living things respond to their environment. Living things maintain a stable internal environment. Taken as a group, living things change over time.