Book cover for Nester's Microbiology: A Human Perspective

Nester's Microbiology: A Human Perspective

Denise Anderson, Sarah Salm, Deborah Allen

ISBN #9780073522593

8th Edition

301 Questions

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9,223 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter section on bacterial genetics provides a historical perspective on the mechanisms of genetic change in bacteria, including both spontaneous and induced mutations along with various DNA repair systems. It delves into horizontal gene transfer mechanisms like transformation, transduction, and conjugation, highlighting how mobile genetic elements such as plasmids, transposons, and genomic islands contribute to the spread of traits like antibiotic resistance. Additionally, the section underscores the relevance of biotechnological techniques in modern applications such as genetic engineering and forensic science.

Learning Objectives

1

Explain the fundamental mechanisms by which bacteria undergo genetic change.

2

Differentiate between spontaneous and induced mutations and describe various DNA repair mechanisms.

3

Identify and explain the modes of horizontal gene transfer including transformation, transduction, and conjugation.

4

Analyze the role of mobile genetic elements (plasmids, transposons, genomic islands) in spreading traits such as antibiotic resistance.

5

Understand the historical and modern applications of biotechnological techniques in genetic engineering and forensic science.

Key Concepts

CONCEPT

DEFINITION

Spontaneous Mutation

A mutation that occurs naturally without external factors, often due to errors in DNA replication or cellular processes.

Induced Mutation

A mutation that occurs as a result of exposure to external factors like chemicals, radiation, or biological agents.

DNA Repair Mechanisms

Processes by which cells recognize and correct errors in DNA, thereby ensuring genomic integrity.

Horizontal Gene Transfer (HGT)

The movement of genetic material between organisms, bypassing traditional parent-to-offspring inheritance.

Transformation

A mode of HGT where bacteria take up free DNA fragments from their environment.

Transduction

A form of HGT facilitated by bacteriophages, which transfer DNA from one bacterium to another.

Conjugation

A process of direct transfer of DNA between two bacteria through cell-to-cell contact, often mediated by plasmids.

Plasmids

Small, circular DNA molecules that can replicate independently of the bacterial chromosome and often carry genes beneficial for survival, like antibiotic resistance.

Transposons

Mobile genetic elements that can change positions within the genome, potentially disrupting or altering gene function.

Genomic Islands

Large segments of DNA acquired through horizontal gene transfer that often contain clusters of genes conferring advantageous traits.

Genetic Engineering

The deliberate manipulation of an organism’s genetic material using biotechnological techniques.

Forensic Science

The application of scientific methods to investigate crimes, which may include analyzing genetic material for identification purposes.

Example Problems

Example 1

A culture of $E$. coli is irradiated with ultraviolet (UV) light. Answer questions 1 and 2 based on this statement. The UV light specifically a) joins the two strands of DNA together by covalent bonds. b) joins the two strands of DNA together by hydrogen bonds. c) forms covalent bonds between thymine molecules on the same strand of DNA. d) forms covalent bonds between guanine and cytosine. e) deletes bases.

Example 2

The highest frequency of mutations would be obtained if, after irradiation, the cells were immediately a) placed in the dark. b) exposed to visible light. c) shaken vigorously. d) incubated at a temperature below their optimum for growth. e) The frequency would be the same no matter what the environmental conditions are after irradiation.

Example 3

Penicillin enrichment of mutants works on the principle that a) only Gram-positive cells are killed. b) cells are most sensitive to antimicrobial medications during the lag phase of growth. c) most Gram-negative cells are resistant to penicillin. d) penicillin kills only growing cells. e) penicillin inhibits formation of the lipopolysaccharide layer.

Example 4

Repair mechanisms that occur during DNA replication are 1. mismatch repair. 2. proofreading by DNA polymerase. 3. light repair. 4. SOS repair. 5. excision repair. a) 1,2 b) 2,3 c) 3,4 d) 4,5 e) 1,5

Example 5

You are trying to isolate a mutant of wild-type $E$, coli that requires histidine for growth. This can best be done using 1. direct selection. 2. replica plating. 3. penicillin enrichment. 4. a procedure for isolating conditional mutants. 5. reversion. a) 1,2 b) 2,3 c) 3,4 d) 4,5 e) 1,5
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Step-by-Step Explanations

QUESTION

How does the process of transformation contribute to bacterial genetic diversity?

STEP-BY-STEP ANSWER:

Step 1: A bacterium takes up free-floating DNA fragments from its surroundings.
Step 2: The acquired DNA may contain new genes that, once integrated, can be expressed in the bacterial cell.
Step 3: Successful integration of this DNA leads to genetic variation, potentially conferring new traits.
Final Answer: Transformation increases bacterial genetic diversity by incorporating new genetic information from the environment.

Transformation

QUESTION

Describe the steps by which bacteriophages mediate horizontal gene transfer in transduction.

STEP-BY-STEP ANSWER:

Step 1: A bacteriophage infects a donor bacterium, accidentally packaging fragments of the host's DNA into its viral particle.
Step 2: The phage carries the bacterial DNA to a recipient bacterium during a subsequent infection.
Step 3: The introduced DNA recombines with the recipient’s genome, leading to new genetic combinations.
Final Answer: Transduction facilitates horizontal gene transfer by transferring genetic material via bacteriophages between bacteria.

Transduction

QUESTION

Explain how conjugation can lead to the spread of antibiotic resistance among bacterial populations.

STEP-BY-STEP ANSWER:

Step 1: Two bacteria establish direct contact, typically via a pilus.
Step 2: A plasmid carrying antibiotic resistance genes is transferred from the donor to the recipient cell.
Step 3: The recipient bacterium incorporates the plasmid into its own genetic system, acquiring resistance.
Final Answer: Conjugation spreads antibiotic resistance by directly transferring resistance-bearing plasmids between bacteria.

Conjugation

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Common Mistakes

  • Confusing the processes of transformation, transduction, and conjugation.
  • Misunderstanding the difference between spontaneous and induced mutations.
  • Underestimating the importance of DNA repair mechanisms in maintaining genomic integrity.
  • Overlooking the role of mobile genetic elements in the spread of antibiotic resistance.
  • Assuming that historical biotechnological techniques are no longer relevant in modern applications.