Exam #1: Microbial Genetics Flashcards
Describe bacterial DNA.
Single, large circular DNA molecule
What is a plasmid?
A small circular DNA molecule separate from the bacterial chromosome
What is an episome?
Bacterial chromosome w/ plasmid integrated into it
Polycystroic mRNA
Polycistronic mRNA carries several open reading frames (ORFs), each of which is translated into a polypeptide
Describe the mechanism of repression/ negative gene regulation.
Normally, a repressor binds the operator–something must alter the repressor & remove it for RNA polymerase to bind & transcribe the gene
Describe the mechanism of activation/ positive gene regulation.
Activator binds the activator region, recruiting RNA polymerase & leading to gene transcription
Vertical Gene Transfer vs. Horizontal Gene Transfer
- Vertical Gene Transfer= genes passed via replication to progeny
- Horizontal= Genes passed via one of three mechanisms:
1) Transformation
2) Conjugation
3) Transduction
How does bacterial evolution occur via vertical gene transfer? Is this type of evolution fast or slow?
- Very slowly b/c it require errors in DNA polymerase that confers a selective advantage
By what mechanism do bacterial traits evolve quickly?
Horizontal gene transfer
What is homologous recombination? What enzyme mediates homologous recombination?
Homologous recombination is the exchange of DNA between two similar DNA sequences, which requires the enzyme, RecA
What is transformation?
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
What is conjugation?
DNA transfer directly from one bacterial cell to another via cell contact
- Requires F-Factor
- Only from donor to recipient (F+–>F-)
What is F-Factor?
Fertility Factor that encodes the sex pilus; contains two important genes, the Tra Operon & the OriT
What is the Tra Operon?
Gene of the F-factor that encodes the components of the sex pilus
What is the OriT?
- Gene in the F-factor known as the “Origin of Transfer”
- Where a single strand break occurs for transfer
What is a Hfr?
High Frequency Recombination, when the F-factor is integrated into bacterial chromosome
Describe the mechanism of F+ x F- conjugation.
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)
What is transduction?
Transfer of bacterial DNA by bacteriophage
What is bacteriophage?
Bacterial viruses
- Bind cell membrane & inject DNA into bacterial cell
What is the difference between Lytic & Lysogenic bacteriophages?
- Lytic= actively infectious
- Lysogenic= integration of phage DNA followed by quiescence until demise of bacteria–>lytic
What is generalized transduction?
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
In contrast to generalized transduction, which is specialized transduction?
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.
What is Intrinsic Antibiotic Resistance?
Lack of an antibiotic target e.g. mycoplasma does NOT have peptidoglycan thus, no target
- NOT transferable & NOT increasing among bacterial populations
What is Chromosome-Mediated Antibiotic Resistance?
Chromosomal genes encoding antibiotic resistance
- Arises through random mutation
- Acquisition of DNA via horizontal transfer & incorporation via homologous recombination
What is Plasmid-Mediated Antibiotic Resistance?
Genes encoding antibiotic resistance located on a plasmid
- E.g. genes that encode proteins that degrade or modify antibiotics & genes encoding efflux pumps
What is R-Factor? What is the difference between Resistance Transfer Factor & Resistance Determinant?
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
What is a transposon?
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
What is an Integron?
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
What is a pathogenicity island?
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
What gene encodes the enzyme necessary for transposon “jumping?”
TnpA
