CH 15 & 8: Gene and Chromosome Mutations

Who discovered bacterial conjugation?

Lederberg and Tatum.

What did the Davis U-tube experiment show?

Physical contact is required for conjugation.

What is the F factor?

A plasmid that enables conjugation in bacteria.

What is an F+ cell?

A bacterial cell containing the F plasmid.

What is an F− cell?

A cell lacking the F plasmid.

What is an Hfr strain?

A strain where the F plasmid is integrated into the chromosome.

What does Hfr stand for?

High-frequency recombination.

What happens when an Hfr cell conjugates with an F− cell?

Chromosomal genes transfer but F− usually remains F−.

What determines gene transfer order in Hfr mapping?

The location and orientation of F plasmid integration.

What is an F’ cell?

A cell with an F plasmid carrying part of the bacterial chromosome.

What is sexduction?

Transfer of chromosomal genes via an F’ plasmid.

What is transformation?

Uptake of free DNA from the environment.

What is a competent cell?

A bacterial cell able to take up DNA.

What is heteroduplex DNA?

A DNA molecule with one donor and one recipient strand after transformation.

How is gene distance estimated in transformation mapping?

By co-transformation frequency; closer genes transform together more often.

What is transduction?

Bacteriophage-mediated transfer of DNA.

What is a bacteriophage?

A virus that infects bacteria.

What is the lytic cycle?

Viral replication cycle causing host lysis.

What is the lysogenic cycle?

Viral DNA integrates into host genome as a prophage.

What is generalized transduction?

Random bacterial DNA is packaged into phage particles.

What is specialized transduction?

Only DNA adjacent to prophage integration site is transferred.

What is cotransduction frequency?

Probability that two genes are transferred together by a phage.

What does high cotransduction frequency indicate?

Genes are close together on the chromosome.

What is a plaque?

A clearing on a bacterial lawn caused by viral lysis.

What is multiplicity of infection (MOI)?

Ratio of phage particles to bacterial cells.

What is recombination in phage?

Exchange of DNA between phage genomes during co-infection.

What are rII mutants?

T4 phage mutants used by Seymour Benzer to study recombination.

What did Benzer’s experiments demonstrate?

That mutations can occur within a gene (fine structure mapping).

What is the significance of the F factor containing oriT?

It defines the starting point for DNA transfer during conjugation.

How long does it take to transfer the full bacterial chromosome?

Approximately 100 minutes in E. coli.

What is time-of-entry mapping?

Determining gene order based on how long they take to transfer from Hfr to F−.

Why don’t F− cells usually become F+ in Hfr × F− crosses?

The entire F plasmid rarely transfers because conjugation bridges break early.

What is a merozygote?

A partially diploid bacterial cell created by conjugation or transformation.

What is recombination frequency in bacteria?

Frequency that donor DNA recombines into the recipient chromosome.

What is the role of RecA protein?

Catalyzes homologous recombination in bacteria.

What is the function of nuclease during gene transfer?

Degrades one strand of DNA for single-strand transfer.

What is electroporation?

A method using electric shock to make cells competent for transformation.

What is a prophage?

Integrated viral DNA in the bacterial genome.

What induces the lysogenic→lytic switch?

DNA damage or stress causing prophage excision.

What is a temperate phage?

A phage capable of both lytic and lysogenic cycles.

What is a virulent phage?

A phage that performs only the lytic cycle.

What is abortive transduction?

Donor DNA enters recipient but only one daughter inherits it.

What is complete transduction?

Donor DNA recombines into recipient chromosome and is inherited by all progeny.

What is a bacterial operon?

A group of genes regulated together under one promoter.

What is the role of plasmids in antibiotic resistance?

They often carry resistance genes (R plasmids).

What is an R plasmid?

A plasmid containing antibiotic resistance genes and transfer genes.

What is a col plasmid?

Plasmid producing bacteriocins that kill other bacteria.

What are bacteriocins?

Toxic proteins produced by bacteria to inhibit competitors.

What is a fertility pilus?

A structure encoded by F plasmid enabling DNA transfer.

Why is recombination essential in Hfr crosses?

Only DNA that recombines is maintained; the rest is degraded.

What is the purpose of selection media in bacterial genetics?

Allows isolation of cells with desired genetic traits.

What is an auxotroph?

A mutant unable to synthesize a nutrient required for growth.

What is a prototroph?

A wild-type cell capable of synthesizing all required metabolites.

What is a complement test in bacteria?

A test to determine whether two mutations affect the same gene.

What is bacterial competence regulated by?

Quorum sensing and environmental conditions.

What are quantitative traits?

Traits that vary continuously and are influenced by multiple genes.

What are polygenic traits?

Traits controlled by two or more genes each contributing a small effect.

What are multifactorial traits?

Traits influenced by both genetic and environmental factors.

What type of distribution do quantitative traits follow?

A normal (bell-shaped) distribution.

What is a meristic trait?

A trait measured in whole numbers.

What is a threshold trait?

Trait expressed after a certain liability threshold is reached.

What is the multiple-gene hypothesis?

The idea that many genes with small additive effects determine quantitative traits.

What is additive genetic variance (VA)?

Variance due to the sum of average effects of individual alleles.

What is dominance variance (VD)?

Variance caused by dominance interactions between alleles.

What is epistatic variance (VI)?

Variance caused by interactions between genes.

What is environmental variance (VE)?

Variation caused by environmental factors.

What is phenotypic variance (VP)?

The total variance of a trait in a population.

What is the equation for phenotypic variance?

VP = VA + VD + VI + VE.

What is narrow-sense heritability (h²)?

The proportion of phenotypic variance due to additive genetic variance.

What is the formula for h²?

h² = VA / VP.

What does high h² indicate?

Variation is mostly due to genes rather than environment.

What does low h² indicate?

Variation is mostly environmental.

What is broad-sense heritability (H²)?

H² = VG / VP (VG includes all genetic variance components).

What is realized heritability?

Heritability estimated from response to selection.

What is selection differential (S)?

Difference between mean of selected parents and mean of population.

What is response to selection (R)?

Difference between mean of offspring and original population mean.

What is the equation R = h²S used for?

Predicting offspring response to artificial selection.

What is a QTL?

Quantitative Trait Locus — a chromosomal region contributing to a quantitative trait.

What is QTL mapping?

Statistical method linking genetic markers to variation in quantitative traits.