L19: Microbial Genetics

Question 1

2 types of bacterial genetic material

Answer 1

1. Chromosome – large circular DNA molecule

2. Plasmid – small circular DNA molecule that is transferred to daughter cells

Question 2

Episomes

Answer 2

Plasmids that can insert into the bacterial chromosome

Question 3

Homologous recombination

Answer 3

Exchange of DNA between two DNA molecules based on homology (have to be somewhat similar in sequence) – RecA binds and mediates this between linear DNA and chromosome

Question 4

Why is homologous recombination in bacteria important?

Answer 4

For horizontal gene transfer (transformation, conjugation, transduction)

Question 5

Transformation

Answer 5

Uptake of free DNA from the environment to allow bacteria to gain new traits quickly (homologous recombination happens when DNA is released into the environment and imported into cell)

Question 6

Conjugation

Answer 6

DNA transfer directly from one bacterial cell to another through cell contact to acquire new DNA from another bacterium; F-factor is important

Question 7

F-factor (fertility factor)

Answer 7

Carries genes in tra operon that encode components of the sex pilus and also oriT

Question 8

Bacterial mating types for conjugation

Answer 8

F- = no F factor (always recipient)
F+ = contains F factor
Hfr (high-frequency recombination) = F factor integrated into bacterial chromosome

Question 9

F+ x F-

Answer 9

Unidirectional transfer where F- undergoes sex change to F+; only single strand is transferred (like an arrow being shot)

Question 10

Hfr x F-

Answer 10

• -Sex pilus bridge forms
• -DNA is transferred from oriT in (including chromosomal DNA) – tra genes come in last
• -Normally sex change does not occur (remains F-, just has new DNA)

Question 11

Transduction

Answer 11

Transfer of bacterial DNA via a bacteriophage (virulent/lytic phages or temperate phages)

Question 12

Process of generalized transduction

Answer 12

• -Done with a lytic phage
• -Phage DNA is injected into bacteria
• -Phage produces enzymes that degrade bacterial chromosome
• -When progeny are packed up, some bacterial DNA gets included in phage head
• -When phage infects another bacteria, DNA can be incorporated into its chromosome via homologous recombination

Question 13

Process of specialized transduction

Answer 13

• -Done with lysogenic phage
• -Phase incorporates into chromosome as prophage
• -Conversion to lytic life cycle occurs
• -Upon excision from the genome, a small portion of chromosome adjacent to where phage was integrated is packaged into phage head

Question 14

Importance of lysogenic phages

Answer 14

Many prophages are important in virulence of certain bacteria

Question 15

Only method of DNA transfer in bacteria that doesn’t require homologous recombination

Answer 15

F+ x F- conjugation

Question 16

3 categories of antibiotic resistance

Answer 16

1. Intrinsic
2. Chromosome-mediated
3. Plasmid-mediated

Question 17

Intrinsic antibiotic resistance

Answer 17

Not transferable between bacteria, not increasing among bacterial populations

Question 18

Chromosome-mediated antibiotic resistance

Answer 18

Chromosomal genes encoding antibiotic resistance that arise through random mutation or acquisition of DNA through horizontal gene transfer and incorporation into genome via homologous recombination

Question 19

Plasmid-mediated antibiotic resistance

Answer 19

Genes on a plasmid that encode antibiotic resistance; have resistance factors (R-factor) that are made up of a resistance transfer factor and a resistance determinant

Question 20

Transposons

Answer 20

Mobile DNA elements that can transfer themselves or a copy from one molecule of DNA to another; contain indirect repeat sequences on each end so they can jump around

Question 21

Why are transposons dangerous for antibiotic resistance?

Answer 21

Multi-drug resistant plasmids can be constructed and passed between bacterial species due to transposons

Question 22

Integrons

Answer 22

Diverse group of genetic elements that encode a site-specific recombination system that can capture gene cassettes (often antibiotic resistant)

Question 23

Resistance gene cassettes

Answer 23

New genes that are expressed in coordinated manner under control of integron promoter

Question 24

How do bacteria regulate gene expression at different levels?

Answer 24

RNA polymerase binds to a promoter to promote transcription; repressors can inhibit transcription in negative gene regulation and activators can begin transcription in positive gene regulation