Microbial genetics Flashcards
What is a plasmid?
- Small circular DNA molecule separate from bacterial chromosome2. Replicated and transferred to daughter cells3. May integrate into chromosome (episome)
What types of genes can be carried on a plasmid?
- R factor2. Toxin3. Bacteriocins4. Virulence determinants
Vertical gene transfer
Genes passed via replication to progeny
Horizontal gene transfer
Genes passed via1. Transformation2. Conjugation3. Transduction
Is new DNA acquired during bacterial cell division?
No
How is evolution of new traits conferred in bacteria?
DNA polymerase error rate
What is the importance of horizontal gene transfer?
- A way for bacteria to get around slow evolution rate2. Bacteria can become antibiotic resistant and produce new virulence factors more quickly than compared to vertical
Homologous recombination
Exchange of DNA between two DNA molecules based on homology
What is the result of homologous recombination?
DNA incorporated into the chromosome are passed to daughter cells / become a permanent part of the genome
What are the steps of homologous recombination in bacteria?
- Linear DNA acquired somehow2. RecA-mediated gene exchange at a homologous region 3. Linear DNA degraded by exonucleases
What is transformation?
- Uptake of free DNA from environment2. DNA is released into environment3. DNA is actively imported into the bacterial cell4. Homologous recombination can occur if DNA sequence is similar to portion of chromosome
What is the purpose of transformation?
Allows bacteria to gain new traits quickly
What is conjugation?
DNA transfer directly from one bacterial cell to anther through cell contact
What is the purpose of conjugation?
Acquire new DNA from another viable bacterium
What is the F factor?
- Fertility factor2. Carries genes, tra operon (encodes sex pillus)3. OriT is where single strand break occurs for transfer
What is required to pass on newly acquired DNA to progeny during conjugation?
Homologous recombination
What are the characteristics of F+ x F- conjugation?
- Transfer is unidirectional2. F- undergoes a sex change to F+3. Only a single strand is transferred4. No bacterial genes are transferred
What are the characteristics of Hfr x F- conjugation?
- Sex pillus forms2. When DNA is transferred from oriT, it actually brings in chromosomal DNA3. Partial genetic material is transferred4. No sex change occurs
What is transduction?
Transfer of bacterial DNA via a bacteriophage
What is generalized transduction?
- Occurs with lytic phage2. A lytic phage incorporates bacterial chromosomal DNA into its phage head3. When the phage infects another bacterium the DNA can be incorporated into its chromosome via homologous recombination
Generalized transduction occurs with what type of bacteriophage?
Lytic phage
What is specialized transduction?
- Occurs with lysogenic phage2. Incorporates as a prophage next to a gene3. A damaging event leads to conversion to a lytic lifecycle4. Upon excision from the genome, a small portion of the chromosome directly adjacent to where the phage was integrated is packaged into a phage head
During what type of DNA transfer is homologous recombination NOT required?
F- / F+ conjugation
What are the three categories for antibiotic resistance?
- Intrinsic2. Chromosome mediated3. Plasmid mediated
What are characteristics of intrinsic antibiotic resistance?
- Non transferable (horizontally) between bacteria2. Not increasing among bacteria populations3. Example - lack of a bacterial target
What are characteristics of chromosome mediated antibiotic resistance?
- Chromosomal genes encode resistance2. Arise through random mutation during replication 3. Arise through acquisition of DNA via horizontal gene transfer and homologous recombination4. Examples - ribosomal proteins, PBPs, gyrase
What are characteristics of plasmid mediated antibiotic resistance?
- Genes encoding antibiotic resistance located on a plasmid2. Commonly plasmids carry genes that encode proteins that degrade or modify an antibiotic or an efflux pump to pump antibiotic out of cell
What is the R factor?
- Conjugated plasmid 2. Encodes a tra gene and a resistance determinant 3. Has an Rtf (resistance transfer factor) - resistance machinery
What are transposons?
- Mobile DNA elements that can transfer themselves or a copy from one DNA molecule to another 2. Present in both eukaryotic and bacterial cells, and viruses3. Contain:a) Indirect repeat sequences on each endb) A single gene for transposasec) A single or multiple antibiotic resistance genes
How can multi drug resistance plasmids be constructed and passed between bacterial species?
Transposons can jump between two chromosomes, between plasmids, or between one another
What are integrons?
- Diverse group of genetic elements that a) Encode a site specific recombination systemb) Capture antibiotic resistance gene cassettesc) Coordinately express them under control of integron promoter2. Associated with large mobile genetic elements like plasmids or transposons3. Very important in rapid multi-drug resistance
What are pathogenicity islands?
- Region of a bacterial chromosome acquired through horizontal gene transfer2. Carry coordinately regulated virulence genes surrounded by insertion sequences 3. Another way to gain different traits quickly
