Lesson Overview 20.1 Viruses

Discovery of Viruses In 1892, Dmitri Ivanovski demonstrated that the cause of tobacco mosaic disease was found in the liquid extracted from infected plants. In 1897, Martinus Beijerinck suggested that tiny particles in the juice caused the disease, and he named these particles viruses, after the Latin word for “poison.” In 1935, Wendell Stanley isolated crystals of tobacco mosaic virus. Since living organisms do not crystallize, Stanley inferred that viruses were not truly alive.

Discovery of Viruses A virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids. Viruses can reproduce only by infecting living cells.

Structure and Composition Viruses differ widely in terms of size and structure. Most viruses are so small they can be seen only with the aid of a powerful electron microscope.

Structure and Composition The protein coat surrounding a virus is called a capsid. Some viruses, such as the influenza virus, have an additional membrane that surrounds the capsid. The simplest viruses contain only a few genes, whereas the most complex may have more than a hundred genes.

Structure and Composition Most viruses have proteins on their surface membrane or capsid that bind to receptor proteins on the host cell. The proteins “trick” the cell to take the virus, or in some cases just its genetic material, into the cell. Once inside, the viral genes are eventually expressed and may destroy the cell.

Structure and Composition Most viruses infect only a very specific kind of cell. Plant viruses infect plant cells; most animal viruses infect only certain related species of animals; viruses that infect bacteria are called bacteriophages.

Viral Infections What happens after a virus infects a cell?

Viral Infections What happens after a virus infects a cell? Inside living cells, viruses use their genetic information to make multiple copies of themselves. Some viruses replicate immediately, while others initially persist in an inactive state within the host.

Lytic Infections In a lytic infection, a virus enters a bacterial cell, makes copies of itself, and causes the cell to burst, or lyse. Bacteriophage T4 is an example of a bacteriophage that causes such an infection.

Lytic Infections Bacteriophage T4 has a DNA core inside a protein capsid that binds to the surface of a host cell.

Lytic Infections The virus injects its DNA into the cell.

Lytic Infections The cell then begins to make messenger RNA (mRNA) from the viral genes.

Lytic Infections The viral mRNA is translated into viral proteins that chop up the cell’s DNA.

Lytic Infections Controlled by viral genes, the host cell’s metabolic system makes copies of viral nucleic acid.

Lytic Infections The host cell’s metabolic system also makes copies of capsid proteins.

Lytic Infections The viral nucleic acid and capsid proteins are then assembled into new virus particles.

Lytic Infections The host cell lyses, releasing hundreds of virus particles that go on to infect other cells.

Lytic Infections A lytic virus is similar to an outlaw in the Wild West of the American frontier in the demands the virus makes on its host. First, the outlaw eliminates the town’s existing authority. In a lytic infection, the host cell’s DNA is chopped up.

Lytic Infections Next, the outlaw demands to be outfitted with new equipment from the local townspeople. In a lytic infection, the viruses use the host cell to make viral DNA and viral proteins.

Lytic Infections Finally, the outlaw forms a gang that leaves the town to attack new communities. In a lytic infection, the host cell bursts, releasing hundreds of virus particles.

Lysogenic Infection Some bacterial viruses cause a lysogenic infection.

Lysogenic Infection In a lysogenic infection a host cell is not immediately taken over.

Lysogenic Infection The viral nucleic acid is inserted into the host cell’s DNA.

Lysogenic Infection The viral DNA is then copied along with the host DNA without damaging the host.

Lysogenic Infection Viral DNA multiplies as the host cells multiply.

Lysogenic Infection In this way, each generation of daughter cells derived from the original host cell is infected.

Lysogenic Infection Bacteriophage DNA that becomes embedded in the bacterial host’s DNA is called a prophage.

Lysogenic Infection The prophage may remain part of the DNA of the host cell for many generations.

Lysogenic Infection Influences from the environment—radiation, heat, etc—trigger the prophage to become active.

Lysogenic Infection It then removes itself from the host cell DNA, directs the synthesis of new virus particles, and now becomes an active lytic infection.

A Closer Look at Two RNA Viruses About 70 percent of viruses contain RNA rather than DNA. In humans, RNA viruses cause a wide range of infections, from relatively mild colds to severe cases of HIV. Certain kinds of cancer also begin with an infection by viral RNA.

The Common Cold Cold viruses attack with a very simple, fast-acting infection. A capsid settles on a cell, typically in the host’s nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.

The Common Cold The host cell’s ribosomes mistake the viral RNA for the host’s own mRNA and translate it into capsids and other viral proteins. The new capsids assemble around the viral RNA copies, and within 8 hours, the host cell releases hundreds of new virus particles to infect other cells.

HIV The deadly disease called acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV). HIV belongs to a group of RNA viruses that are called retroviruses. The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.

HIV When a retrovirus infects a cell, it makes a DNA copy of its RNA.

HIV The copy inserts itself into the DNA of the host cell.

HIV Retroviral infections are similar to lysogenic infections of bacteria. Much like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system.

Viruses and Cells All viruses are parasites. Parasites depend entirely upon other living organisms for their existence, harming these organisms in the process. Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts.

Viruses and Cells Viruses have many of the characteristics of living things. After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.

Viruses and Cells Some of the main differences between cells and viruses are summarized in this chart.

Viruses and Cells Although viruses are smaller and simpler than the smallest cells, it is unlikely that they were the first living organisms. Because viruses are dependent upon living organisms, it seems more likely that viruses developed after living cells. The first viruses may have evolved from the genetic material of living cells. Viruses have continued to evolve, along with the cells they infect, for billions of years.