Chapter 18
The Genetics of Viruses and Bacteria
Teaching Objectives
The Genetics of Viruses
1.Recount the history leading up to the discovery of viruses. Include the contributions of Adolf Mayer, Dimitri Ivanowsky, Martinus Beijerinck, and Wendell Stanley.
2.List and describe the structural components of viruses.
3.Explain why viruses are obligate intracellular parasites.
4.Explain how a virus identifies its host cell.
5.Describe bacterial defenses against phages.
6.Distinguish between the lytic and lysogenic reproductive cycles, using phage lambda as an example.
7.Describe the reproductive cycle of an enveloped virus. Explain the reproductive cycle of the herpesvirus.
8.Describe the reproductive cycle of retroviruses.
9.List some characteristics that viruses share with living organisms and explain why viruses do not fit our usual definition of life.
10.Describe the evidence that viruses probably evolved from fragments of cellular nucleic acids.
11.Define and describe mobile genetic elements.
12.Explain how viral infections in animals cause disease.
13.Describe the best current medical defenses against viruses. Explain how AZT helps to fight HIV infections.
14.Describe the mechanisms by which new viral diseases emerge.
15.Distinguish between the horizontal and vertical routes of viral transmission in plants.
16.Describe viroids and prions.
17.Explain how a non-replicating protein can act as a transmissible pathogen.
The Genetics of Bacteria
18.Describe the structure of a bacterial chromosome.
19.Compare the sources of genetic variation in bacteria and humans.
20.Compare the processes of transformation, transduction, and conjugation.
21.Distinguish between generalized and specialized transduction.
22.Define an episome. Explain why a plasmid can be an episome.
23.Explain how the F plasmid controls conjugation in bacteria.
24.Describe the significance of R plasmids. Explain how the widespread use of antibiotics contributes to R plasmid-related disease.
25.Explain how transposable elementsmay cause recombination of bacterial DNA.
26.Distinguish between an insertion sequence and a transposon.
27.Describe the role of transposase in the process of transposition.
28.Briefly describe two main strategies that cells use to control metabolism.
29.Explain the adaptive advantage of genes grouped into an operon.
30.Using the trp operon as an example, explain the concept of an operon and the function of the operator, repressor, and corepressor.
31.Distinguish between structural and regulatory genes.
32.Describe how the lac operon functions and explain the role of the inducer, allolactose.
33.Explain how repressible and inducible enzymes differ and how those differences reflect differences in the pathways they control.
34.Distinguish between positive and negative control and give examples of each from the lac operon.
35.Explain how cyclic AMP and catabolite activator proteinare affected by glucose concentration.
Student Misconceptions
1.Students may find it difficult to understand how viroids and prions can act as infectious agents to spread disease. Clearly explain the mechanisms by which these molecules cause infection and disease, and discuss the possible implications of these pathogens to our understanding of life.
2.Teaching about the genetics of viruses and bacteria provides an opportunity to familiarize students with some important health treatment issues. Explain to your class the futility—and the danger—of prescribing antibiotics to treat viral disease. Discuss with them the science of vaccines, and ask them to assess the legitimacy of the concerns of some parents about vaccination.
3.Students may find it difficult to understand how conjugation can be considered “bacterial sex.” Explain that, in biological terms, sex is a process that produces a genetically novel individual with genetic contributions from two “parents.”
4.Many students have difficulty mastering the distinction between repressible and inducible operons. When teaching this material, remind students of the role of each type of operon in cell metabolism. This will help students to understand the significance of these forms of negative gene regulation, rather than focusing on memorization of detail.
Chapter Guide to Teaching Resources
Overview: Microbial model systems
Concept 18.1A virus has a genome but can reproduce only within a host cell
Transparencies
Figure 18.2Comparing the size of a virus, a bacterium, and an animal cell
Figure 18.4Viral structure
Figure 18.5A simplified viral reproductive cycle
Figure 18.6The lytic cycle of phage T4, a virulent phage
Figure 18.7The lytic and lysogenic cycles of phage l, a temperate phage
Table 18.1Classes of animal viruses
Figure 18.8The reproductive cycle of an enveloped RNA virus
Figure 18.9The structure of HIV, the retrovirus that causes AIDS
Figure 18.10The reproductive cycle of HIV, a retrovirus
Student Media Resources
Activity: Simplified viral reproductive cycle
Activity: Phage lytic cycle
Activity: Phage lysogenic and lytic cycles
Activity: Retrovirus (HIV) reproductive cycle
Concept 18.2Viruses, viroids, and prions are formidable pathogens in animals and plants
Transparency
Figure 18.13Model for how prions propagate
Student Media Resources
Investigation: What causes infections in AIDS patients?
Investigation: Why do AIDS rates differ across the U.S.?
Concept 18.3Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria
Transparencies
Figure 18.14Replication of a bacterial chromosome
Figure 18.15Can a bacterial cell acquire genes from another bacterial cell?
Figure 18.16Generalized transduction (layer 1)
Figure 18.16Generalized transduction (layer 2)
Figure 18.18Conjugation and recombination in E. coli (layer 1)
Figure 18.18Conjugation and recombination in E. coli (layer 2)
Figure 18.18Conjugation and recombination in E. coli (layer 3)
Figure 18.18Conjugation and recombination in E. coli (layer 4)
Figure 18.19Transposable genetic elements in bacteria
Instructor and Student Media Resources
Video: Prokaryotic flagella
Investigation: What are the patterns of antibiotic resistance?
Concept 18.4Individual bacteria respond to environmental change by regulating their gene expression
Transparencies
Figure 18.20Regulation of a metabolic pathway
Figure 18.21aThe trp operon: regulated synthesis of repressible enzymes
Figure 18.21bThe trp operon: regulated synthesis of repressible enzymes (layer 1)
Figure 18.21bThe trp operon: regulated synthesis of repressible enzymes (layer 2)
Figure 18.22aThe lac operon: regulated synthesis of inducible enzymes
Figure 18.22bThe lac operon: regulated synthesis of inducible enzymes
Figure 18.23Positive control of the lac operon by catabolite activator protein (CAP)
Student Media Resource
Activity: The lac operon in E. coli
Review
Transparency
Page 358Bacterial and viral growth curves
For additional resources such as digital images and lecture outlines, goto the Campbell Media Manager or the Instructor Resources section of
Key Terms
AIDS (acquired immunodeficiency syndrome)
bacteriophage
capsid
conjugation
corepressor
cyclic AMP (cAMP)
episome
F factor
F plasmid
HIV (human immunodeficiency virus)
host range
inducer
insertion sequence
lysogenic cycle
lytic cycle
nucleoid
operator
operon
phage
plasmid
prion
prophage
provirus
R plasmid
regulatory gene
repressor
restriction enzyme
retrovirus
reverse transcriptase
temperate phage
transduction
transformation
transposable element
transposon
vaccine
viral envelope
viroid
virulent phage
Chapter 18The Genetics of Viruses and BacteriaInstructor’s Guide for Campbell/Reece Biology, Seventh Edition
Word Roots
capsa-5 a box (capsid: the protein shell that encloses the viral genome)
conjug-5 together (conjugation: in bacteria, the transfer of DNA between two cells that are temporarily joined)
lyto-5 loosen (lytic cycle: a type of viral replication cycle resulting in the release of new phages by death or lysis of the host cell)
-oid5 like, form (nucleoid: a dense region of DNA in a prokaryotic cell)
-phage5 to eat (bacteriophages: viruses that infect bacteria)
pro-5 before (provirus: viral DNA that inserts into a host genome)
retro-5 backward (retrovirus: an RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome)
trans-5 across (transformation: a phenomenon in which external DNA is assimilated by a cell)
virul-5 poisonous (virulent virus: a virus that reproduces only by a lytic cycle)
Instructor’s Guide for Campbell/Reece Biology, Seventh EditionChapter 18The Genetics of Viruses and BacteriaInstructor’s Guide for Campbell/Reece Biology, Seventh Editionactivator