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Chapter 18 Quizzes: Self Activities Chapter Cum.
Unit 3: Genetics The Genetics of Viruses and Bacteria Review
  1. Prokaryotes, such as the bacterium Escherichia coli that lives in the human colon, are organisms whose cells are much smaller and simpler than those of .
  2. are smaller and simpler still. Viruses that infect bacteria are called , or phages.
  3. A virus consists of a genome of nucleic acid (either or ) enclosed in a coat made of which form a .
  4. Viruses are obligate intracellular . and can reproduce only within a limited range of host cells. Host enzymes, ribosomes, and other molecules are used to synthesize progeny viruses.
  5. Phages go through two alternative mechanisms: the cycle and the cycle.
    • The lytic cycle of a phage such as culminates in the death of the host; such a phage is considered .
    • The lysogenic cycle of a phage such as (λ) does not kill the host; the viral genome into the host DNA and is along with the host as a . Temperate phages are also capable entering a cycle under certain conditions.
  6. Many animal viruses have a membranous envelope with that bind to specific molecules on the surface of a host cell.
    • Retroviruses use the enzyme transcriptase to copy their genome into , which can then be integrated into the host genome as a .
    • HIV is a that contains its own transcriptase.
  7. The prokaryotic genome is usually a circular ring of DNA located in a region; some also have smaller rings of DNA called . Bacterial cells divide by fission, an asexual process that produces two genetically identical cells.
  8. Genetic recombination in bacteria occurs in three processes: , , and .
    • Transformation is the alteration of a bacterial cell's by the uptake of naked, foreign DNA from the surrounding environment.
    • Transduction is the transfer of bacterial from one host cell to another through .
    • Conjugation is the direct transfer of genetic material between bacterial cells joined by sex .
  9. Bacterial genes are often clustered into , composed of an ("on-off" switch), a , and for enzymes.
  10. In the trp operon, the presence of the amino acid (a corepressor) activates a which binds to the to inhibit the of genes that code for enzymes for tryptophan synthesis. Repressible enzymes usually function in pathways.
  11. In the lac operon, the lactose isomer serves as an which binds to and inactivates the , turning on transcription and translation of enzymes for lactose digestion. Inducible enzymes usually function in pathways.
  12. The lac operon is also subject to control, where a catabolite activator protein (CAP) serves as an .
    • When glucose (a preferred food source), is scarce, the level of cAMP rises, activating , which binds to the to increase transcription.
    • When glucose levels increase, cAMP levels drop, and detaches from the lac operon, becoming an inactive activator.
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