Exam #1: Microbial Genetics

Question 1

Describe bacterial DNA.

Answer 1

Single, large circular DNA molecule

Question 2

What is a plasmid?

Answer 2

A small circular DNA molecule separate from the bacterial chromosome

Question 3

What is an episome?

Answer 3

Bacterial chromosome w/ plasmid integrated into it

Question 4

Polycystroic mRNA

Answer 4

Polycistronic mRNA carries several open reading frames (ORFs), each of which is translated into a polypeptide

Question 5

Describe the mechanism of repression/ negative gene regulation.

Answer 5

Normally, a repressor binds the operator–something must alter the repressor & remove it for RNA polymerase to bind & transcribe the gene

Question 6

Describe the mechanism of activation/ positive gene regulation.

Answer 6

Activator binds the activator region, recruiting RNA polymerase & leading to gene transcription

Question 7

Vertical Gene Transfer vs. Horizontal Gene Transfer

Answer 7

• Vertical Gene Transfer= genes passed via replication to progeny
• Horizontal= Genes passed via one of three mechanisms:
  1) Transformation
  2) Conjugation
  3) Transduction

Question 8

How does bacterial evolution occur via vertical gene transfer? Is this type of evolution fast or slow?

Answer 8

• Very slowly b/c it require errors in DNA polymerase that confers a selective advantage

Question 9

By what mechanism do bacterial traits evolve quickly?

Answer 9

Horizontal gene transfer

Question 10

What is homologous recombination? What enzyme mediates homologous recombination?

Answer 10

Homologous recombination is the exchange of DNA between two similar DNA sequences, which requires the enzyme, RecA

Question 11

What is transformation?

Answer 11

Uptake of free DNA from the environment

• DNA released into environment e.g. through bacterial death
• DNA then incorporated into bacterial cell
• IF similar to some portion of chromosome then it will be incorporated via homolgous recombination

Question 12

What is conjugation?

Answer 12

DNA transfer directly from one bacterial cell to another via cell contact

• Requires F-Factor
• Only from donor to recipient (F+–>F-)

Question 13

What is F-Factor?

Answer 13

Fertility Factor that encodes the sex pilus; contains two important genes, the Tra Operon & the OriT

Question 14

What is the Tra Operon?

Answer 14

Gene of the F-factor that encodes the components of the sex pilus

Question 15

What is the OriT?

Answer 15

• Gene in the F-factor known as the “Origin of Transfer”

- Where a single strand break occurs for transfer

Question 16

What is a Hfr?

Answer 16

High Frequency Recombination, when the F-factor is integrated into bacterial chromosome

Question 17

Describe the mechanism of F+ x F- conjugation.

Answer 17

1) Sex pilus bridge forms
2) 1 strand of F-plasmid beginning of OriT is “shot” into recipient w/ Tra region behind
3) Recipient undergoes a “sex change” & is now F+

• Unidirectional
• Single strand transferred
• No bacterial genes transferred (chromosomal)

Question 18

What is transduction?

Answer 18

Transfer of bacterial DNA by bacteriophage

Question 19

What is bacteriophage?

Answer 19

Bacterial viruses

- Bind cell membrane & inject DNA into bacterial cell

Question 20

What is the difference between Lytic & Lysogenic bacteriophages?

Answer 20

• Lytic= actively infectious

- Lysogenic= integration of phage DNA followed by quiescence until demise of bacteria–>lytic

Question 21

What is generalized transduction?

Answer 21

Process by which a lytic phage incorporates bacterial chromosomal DNA into its phage head. When the phage infects another bacteria, transduced DNA can be incorporated into the chromosome of the new cell via homologous recombination.

• Phage nucleases that degrade the host chromosome
• Packages phage DNA & some degraded bacterial DNA into phage head
• Infects new cells
• Homologous recombination can occur with the new cell

Question 22

In contrast to generalized transduction, which is specialized transduction?

Answer 22

A lysogenic phage incorporates into the chromosome as a prophage. Something (e.g. UV damage) triggers conversion to lytic cycle. Upon excision, phage cuts out some adjacent host DNA & then goes on to infect other cells.

Question 23

What is Intrinsic Antibiotic Resistance?

Answer 23

Lack of an antibiotic target e.g. mycoplasma does NOT have peptidoglycan thus, no target
- NOT transferable & NOT increasing among bacterial populations

Question 24

What is Chromosome-Mediated Antibiotic Resistance?

Answer 24

Chromosomal genes encoding antibiotic resistance

• Arises through random mutation
• Acquisition of DNA via horizontal transfer & incorporation via homologous recombination

Question 25

What is Plasmid-Mediated Antibiotic Resistance?

Answer 25

Genes encoding antibiotic resistance located on a plasmid

- E.g. genes that encode proteins that degrade or modify antibiotics & genes encoding efflux pumps

Question 26

What is R-Factor? What is the difference between Resistance Transfer Factor & Resistance Determinant?

Answer 26

R-Factor= Resistance Factor, a conjugative plasmid made of:

• Resistance Transfer Factor= encodes all proteins needed for conjugation
• Resistance Determinant= encodes the genes for drug resistance

Question 27

What is a transposon?

Answer 27

A mobile genetic element that can excised out of DNA & then inserted back into the same DNA or other DNA

• TnpA encodes the enzyme required for “jumping”
• Can contain a single or multiple antibiotic resistance genes

Question 28

What is an Integron?

Answer 28

Diverse group of genetic elements that:

• Encode a site specific recombination system
• Can capture antibiotic resistance gene cassettes
• Coordinately expresses these cassettes under control of the integron promoter

Question 29

What is a pathogenicity island?

Answer 29

A region of bacterial chromosome acquired through horizontal gene transfer that carries regulated virulence genes surrounded by insertion sequences
- Acts like a very large transposon

Question 30

What gene encodes the enzyme necessary for transposon “jumping?”

Answer 30

TnpA