Study Guide: Genetics of Bacteria – Key Concepts and Review Questions

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Q1. What is a mutation? Distinguish between selectable and non-selectable mutations.

Background

Topic: Mutations in Bacterial Genetics

This question tests your understanding of what a mutation is and how different types of mutations can be identified or selected for in the laboratory.

Key Terms:

Mutation: A heritable change in the DNA sequence of an organism.
Selectable mutation: A mutation that gives the mutant a growth advantage under certain conditions, making it easy to identify.
Non-selectable mutation: A mutation that does not confer an obvious advantage, so special screening methods are needed to detect it.

Step-by-Step Guidance

Start by defining what a mutation is in your own words, focusing on its genetic basis.
Think about how selectable mutations can be identified in a population (for example, antibiotic resistance).
Consider why non-selectable mutations require screening methods rather than simple selection.
List examples of each type to help clarify the distinction.

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Q2. What is a mutant? How does it differ from wild type? What are a few examples of mutants?

Background

Topic: Mutants and Wild-Type Strains

This question is about understanding the definitions of mutant and wild-type, and recognizing examples of mutants in bacterial genetics.

Key Terms:

Mutant: An organism that carries a mutation.
Wild type: The standard or reference strain with the typical genotype found in nature.

Step-by-Step Guidance

Define 'mutant' and 'wild type' clearly, focusing on their genetic differences.
Think of examples of mutants, such as antibiotic-resistant bacteria or auxotrophs.
Explain how a mutant can be identified compared to the wild type.

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Q3. Distinguish between screening and selection.

Background

Topic: Methods for Identifying Mutants

This question tests your understanding of laboratory techniques used to find mutants among a population of bacteria.

Key Terms:

Screening: Examining many organisms to find those with a particular phenotype, often when the mutation is non-selectable.
Selection: Growing organisms under conditions where only those with a specific mutation can survive.

Step-by-Step Guidance

Define both screening and selection, focusing on the differences in approach.
Think of examples where each method would be used.
Explain why selection is generally easier when possible.

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Q4. What is an auxotrophic mutant? How would you screen for it?

Background

Topic: Auxotrophy and Screening Methods

This question focuses on understanding what an auxotroph is and how to identify such mutants in the lab.

Key Terms:

Auxotroph: A mutant organism that requires a specific additional nutrient that the wild type does not.
Screening: The process of identifying mutants by testing their growth on different media.

Step-by-Step Guidance

Define what an auxotrophic mutant is, using an example (e.g., a bacterium that cannot synthesize an amino acid).
Describe how you would use replica plating or other methods to screen for auxotrophs.
Explain the importance of using minimal and supplemented media in your screening process.

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Q5. How are missense, nonsense, and silent mutations similar and different?

Background

Topic: Types of Point Mutations

This question tests your understanding of how different point mutations affect protein coding and function.

Key Terms:

Missense mutation: A base substitution that changes one amino acid in a protein.
Nonsense mutation: A base substitution that creates a stop codon, truncating the protein.
Silent mutation: A base substitution that does not change the amino acid sequence.

Step-by-Step Guidance

Define each type of mutation and how it affects the resulting protein.
Compare and contrast their effects on protein function.
Think of examples for each type to illustrate the differences.

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Q6. Write a one-sentence definition of the term “genotype.” Do the same for “phenotype.” Does the phenotype of an organism automatically change when a change in the genotype occurs? Why or why not? Can phenotype change without a change in genotype? In both cases, give examples to support your answer.

Background

Topic: Genotype vs. Phenotype

This question explores the relationship between an organism's genetic makeup and its observable traits.

Key Terms:

Genotype: The genetic constitution of an organism.
Phenotype: The observable characteristics or traits of an organism.

Step-by-Step Guidance

Write concise definitions for genotype and phenotype.
Consider whether every change in genotype leads to a change in phenotype, and explain why or why not.
Think of examples where phenotype changes without genotype change (e.g., environmental effects).
Provide examples for both scenarios as requested.

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Q7. Why do frameshift mutations generally have more serious consequences than missense mutations?

Background

Topic: Mutation Effects on Proteins

This question asks you to compare the impact of different types of mutations on protein structure and function.

Key Terms:

Frameshift mutation: Insertion or deletion of nucleotides that shifts the reading frame.
Missense mutation: Substitution of one base that changes a single amino acid.

Step-by-Step Guidance

Define both frameshift and missense mutations.
Explain how a frameshift affects the reading frame and the resulting protein.
Discuss why this usually leads to more severe consequences than a missense mutation.

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Q8. What is reversion? What is a revertant? Explain same site vs second site reversion and outcome of each.

Background

Topic: Mutation Reversal

This question focuses on how mutations can be reversed and the different types of reversion events.

Key Terms:

Reversion: A mutation that restores the original phenotype.
Revertant: An organism in which reversion has occurred.
Same site reversion: The original mutation is corrected at the same location.
Second site reversion: A compensatory mutation occurs at a different site.

Step-by-Step Guidance

Define reversion and revertant.
Describe the difference between same site and second site reversion.
Explain the outcomes of each type of reversion.

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Q9. How do mutagens cause mutations?

Background

Topic: Mutagenesis

This question is about the mechanisms by which mutagens induce changes in DNA.

Key Terms:

Mutagen: An agent that increases the frequency of mutations.
Mutation: A heritable change in DNA sequence.

Step-by-Step Guidance

List common types of mutagens (chemical, physical, biological).
Explain how each type can alter DNA structure or sequence.
Give examples of specific mutagens and their effects.

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Q10. Explain mutagenesis by ionizing radiation and non-ionizing radiation.

Background

Topic: Radiation-Induced Mutagenesis

This question focuses on how different types of radiation can cause mutations in DNA.

Key Terms:

Ionizing radiation: High-energy radiation that can break DNA strands (e.g., X-rays, gamma rays).
Non-ionizing radiation: Lower energy radiation that causes DNA damage without breaking strands (e.g., UV light).

Step-by-Step Guidance

Describe how ionizing radiation causes mutations at the molecular level.
Explain the effects of non-ionizing radiation, such as thymine dimer formation.
Compare the types of DNA damage caused by each.

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Q11. Explain the SOS repair system.

Background

Topic: DNA Repair Mechanisms

This question is about the bacterial SOS response to extensive DNA damage.

Key Terms:

SOS repair system: An inducible DNA repair system activated by significant DNA damage.

Step-by-Step Guidance

Describe what triggers the SOS repair system in bacteria.
Explain the roles of key proteins (e.g., RecA, LexA) in the process.
Discuss the consequences of SOS repair, including increased mutation rates.

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Q12. Explain how transformation works. What is meant by competence in genetic transformation experiments?

Background

Topic: Horizontal Gene Transfer – Transformation

This question tests your understanding of how bacteria can take up foreign DNA from their environment.

Key Terms:

Transformation: Uptake of free DNA by a bacterial cell.
Competence: The physiological state that allows a cell to take up DNA.

Step-by-Step Guidance

Describe the process of transformation in bacteria.
Explain what it means for a cell to be competent.
Discuss how competence can be natural or induced in the lab.

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Q13. How does a transducing particle differ from an infectious bacteriophage?

Background

Topic: Transduction in Bacteria

This question is about the differences between particles that transfer bacterial DNA and those that infect bacteria to reproduce.

Key Terms:

Transducing particle: A phage particle that carries bacterial DNA instead of phage DNA.
Bacteriophage: A virus that infects bacteria.

Step-by-Step Guidance

Define both transducing particle and infectious bacteriophage.
Explain how their genetic contents differ.
Discuss the implications for gene transfer and infection.

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Q14. What is the major difference between generalized and specialized transduction and transformation?

Background

Topic: Mechanisms of Horizontal Gene Transfer

This question compares different methods by which bacteria exchange genetic material.

Key Terms:

Generalized transduction: Any part of the bacterial genome can be transferred by a phage.
Specialized transduction: Only specific bacterial genes are transferred by a phage.
Transformation: Uptake of free DNA from the environment.

Step-by-Step Guidance

Define each process (generalized transduction, specialized transduction, transformation).
Identify the key differences in the DNA transferred by each method.
Explain the biological significance of these differences.

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Q15. In conjugation, how are donor and recipient cells brought into contact with each other?

Background

Topic: Bacterial Conjugation

This question is about the physical mechanisms that allow DNA transfer between bacterial cells.

Key Terms:

Conjugation: Direct transfer of DNA from one bacterium to another via cell-to-cell contact.
Pilus (plural: pili): Surface appendage used to connect cells.

Step-by-Step Guidance

Describe the role of the pilus in bringing cells together.
Explain how the pilus is formed and what genes are involved.
Discuss what happens after the cells are connected.

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Q16. Explain how rolling circle DNA replication allows both donor and recipient to end up with a complete copy of a plasmid transferred by conjugation.

Background

Topic: Plasmid Transfer and Replication

This question focuses on the mechanism of DNA replication during conjugation.

Key Terms:

Rolling circle replication: A process of DNA replication that produces single-stranded DNA for transfer.
Plasmid: A small, circular DNA molecule separate from the bacterial chromosome.

Step-by-Step Guidance

Describe the initiation of rolling circle replication in the donor cell.
Explain how a single DNA strand is transferred to the recipient.
Discuss how both cells synthesize the complementary strand to complete the plasmid.

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Q17. Compare and contrast F+ conjugation and Hfr conjugation.

Background

Topic: Types of Bacterial Conjugation

This question is about the differences between two forms of conjugation involving the F plasmid.

Key Terms:

F+ cell: A bacterium with a free F plasmid.
Hfr cell: A bacterium with the F plasmid integrated into its chromosome.

Step-by-Step Guidance

Define F+ and Hfr cells and their genetic differences.
Explain what DNA is transferred during each type of conjugation.
Discuss the outcomes for the recipient cell in each case.

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Q18. What features do insertion sequences and transposons have in common?

Background

Topic: Mobile Genetic Elements

This question is about the similarities between two types of DNA elements that can move within the genome.

Key Terms:

Insertion sequence (IS): A simple transposable element containing only genes for transposition.
Transposon: A larger transposable element that may carry additional genes.

Step-by-Step Guidance

Define both insertion sequences and transposons.
Identify the structural features they share (e.g., inverted repeats, transposase gene).
Discuss their roles in genome evolution.

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Q19. Why is the CRISPR system considered a prokaryotic “adaptive immune system”? Explain the CRISPR process.

Background

Topic: Prokaryotic Immunity – CRISPR

This question is about how bacteria defend themselves against foreign genetic elements using the CRISPR system.

Key Terms:

CRISPR: Clustered Regularly Interspaced Short Palindromic Repeats, a system for adaptive immunity in bacteria.
Cas proteins: Enzymes that use CRISPR sequences to recognize and cut foreign DNA.

Step-by-Step Guidance

Describe how the CRISPR system acquires new spacers from invading DNA.
Explain how CRISPR RNA guides Cas proteins to target matching sequences.
Discuss why this system is considered adaptive immunity.