ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Thirty Review Answers 1. Viruses are not considered to be living organisms because they cannot grow or replicate on their own. The most general structure of a virus is a nucleic acid encased in a protein coat. They act like living organisms when they infect a living host cell and use the cell's machinery to reproduce themselves. 2. Scientists in these fields discovered simultaneously that the agents of these diseases were not filterable out of solutions by the filters that typically caught bacteria. 3. Viruses are generally highly specific; therefore one can assume that there are as many viruses as there are kinds of living organisms. 4. A virulent virus is one that takes over the cell's machinery, makes more viruses, and ruptures or lyses the cell, releasing the new viral particles. Temperate viruses infect a host cell and become established in the cell's own genome. It is difficult to treat a viral infection of either type because the virus becomes so well integrated with the host genome that inhibiting the virus also inhibits the host cells. In bacterial infections, on the other hand, one can generally kill prokaryotic bacteria with little effect on the eukaryotic host cells. 5. Generally an electron microscope is required to see an actual viral particle, so the approximate size range is 17 to 1000 nanometers. The two types of structures are helical (rodlike shaped) and isometric (somewhat spherical shaped called an icosahedron). 6. A bacteriophage is a virus that infects a bacterium. A T4 phage infects a bacterium by landing on it "whisker" side down, allowing the tail to make contact with the bacterial cell wall. The tail retracts, and the tube within it penetrates the bacterial cell wall, permitting the T4 nucleic acid to enter the bacterium. 7. Viruses recognize the specific cell surface markers of the cells they infect. The HIV recognizes and infects the specific T-lymphocytes (T4 cells) in the immune system. This cell alone possesses the CD4+ marker on its surface (hence the name T4 cell). 8. This animal virus penetrates the host cell via endocytosis. Although the coat is shed, the protein RNA and the enzyme reverse transcriptase stay in the cytoplasm. A plant virus infects its host by entering at a point of injury, bacterial viruses inject their nucleic acid into the bacterium through the cell wall. 9. Most species of bacteria are identified by their metabolic actions, which are determined by how they respond when grown on different growth media. 10. Prokaryotes differ from eukaryotes by (1) multicellularity-bacteria are not multicellular; (2) cell size-bacteria are very small; (3) chromosomes-bacteria lack a nucleus and DNA is not complexed with proteins; (4) cell division and genetic recombination-bacteria do not have true sexual reproduction, but do have means to transfer genetic material; (5) internal compartmentalization-bacteria lack membrane-bound organelles; (6) flagella-bacterial flagella are composed of single fibers of flagellin; (7) autotrophic diversity-bacteria have several different kinds of aerobic and anaerobic photosynthesis with a variety of end products, including sulfur, sulfates, and oxygen; other bacteria are chemosynthesizers, metabolizing various inorganic and organic compounds. 11. The bacterial cell wall is a network of polysaccharide molecules cross-linked by polypeptides. Gram-positive bacteria have a plain polypeptide-linked polysaccharide wall, whereas gram-negative bacteria have an additional layer of large lipopolysaccharide molecules deposited over a plain layer. Gram-negative bacteria are generally more resistant to most antibiotics because of the nature of the cell wall. 12. Mutation is important because with rapid generation time, populations can double within several minutes, allowing a favorable mutation to be represented in large numbers quickly. Bacterial recombination results in genetic diversity because genes are transferred from one bacterium to another via viruses, plasmids, or other DNA fragments. An example of such variability is the development of antibiotic resistance. 13. STDs are sexually transmitted diseases. Viral STDs include: AIDS, herpes simplex, and genital warts (HPV); bacterial STDs include gonorrhea, syphilis, and chlamydia. Chlamydia is an unusual bacterium because although it is susceptible to antibiotics, it requires host genetic machinery to replicate its DNA. 14. Archaebacteria differ from the eubacteria in a number of ways, most notably in the rRNA base sequences, the absence of muramic acid in their cell walls, and their often extreme habitats (anaerobic, very salty, very hot, etc.). The methanogens are the most prominent group of the Archaebacteria. Their unique metabolism allows them to synthesize methane from carbon dioxide and hydrogen, generating energy along the way. They are obligate anaerobic organisms-oxygen is literally toxic to them. They can sustain their own anaerobic environment through the reduction of elemental sulfur to hydrogen sulfide (the "rotten egg" gas).