Cracking the AP Biology Exam
Diversity of Organisms
Now let’s talk about viruses. Viruses are the smallest organisms that you’ll have to deal with on the AP Biology Exam. They might be tiny, but they have an enormous impact on many organisms, including humans: Such diseases as hepatitis, the common cold, and AIDS are all caused by viruses.
Scientists believe that viruses don’t belong with the other organisms we’ve just discussed. This is because they are not true cells. In fact, they aren’t really “alive” in the classic sense. When we say that a virus is not “alive,” we mean that it doesn’t live or reproduce independently.
Viruses have only two basic components:
- A coat made of proteins.
- A viral chromosome, which can be either DNA or RNA.
Because they will most likely appear on the AP Biology Exam, let’s discuss how they reproduce in a process called lysis:
1. A virus attaches to receptors on a “host” cell. This can be any type of cell—a bacterium, a plant, or an animal cell.
2. Once attached, the virus injects its nucleic acid into the host cell.
3. The virus’s nucleic acid forces its host to use its metabolic machinery to synthesize viral particles.
4. Newly synthesized viral components are assembled.
5. Eventually, viruses produced in the cell are released and lytic enzymes kill the host cell.
Take a look at the lytic cycle of a typical bacterial virus known as a bacteriophage:
Sometimes a virus can integrate its genetic material into the host’s DNA. In this case, when the host’s DNA replicates, the viral DNA also replicates. However, this does not necessarily kill the host cell, as long as the viral genes that code for viral structural proteins are repressed. A virus that can coexist with its host cell is called a temperate virus, and the cycle in which viral DNA gets copied but new viruses are not made is called the lysogenic cycle. Certain conditions will cause a temperate virus to revert to a lytic cycle, in which new viruses do get made and eventually kill the host cell.
Retroviruses are RNA viruses that go through the lysogenic life cycle. Because in this case the viral genome is RNA, and all other organisms have a DNA genome, a DNA copy of the viral genome must first be made before integration can occur. This requires a special enzyme, because the host has no enzymes that make DNA by reading an RNA template. This enzyme is an RNA-dependent DNA-polymerase, and, as with the RNA-dependent RNA-polymerase discussed in Chapter 6, must either be carried with the virus or synthesized during translation. The common name for RNA-dependent DNA-polymerase is reverse transcriptase—normally transcription creates RNA from DNA, and this enzyme runs reverse transcription; it creates DNA from RNA. The virus that causes AIDS is a retrovirus (HIV), and many cancer-causing viruses are retroviruses.