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Chapter 20 Section 1 Viruses. 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. The protein coat surrounding a virus is called a capsid.
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Chapter 20 Section 1 Viruses
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 • 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 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.
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 has a DNA core inside a protein capsid that binds to the surface of a host cell.
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.
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 • 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.
