Viruses

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.