Bacterial Genetics Flashcards
Phenotype
Identifiable characteristics of a cell that can be altered by mutation
- results from gene expression
Genotype
Sequence of nucleotides in the DNA of a cell (genes)
- bacteria are haploid, so every mutation is automatically expressed
Mutant
Bacteria with altered genetic sequence
- may or may not have altered phenotype
Isogenic mutant
Bacteria with only a single gene difference compared to parent strain
- could lead to antibiotic resistance
Bacterial chromosome
Usually a single chromosome, double stranded DNA, circular, covalently closed, and extensively coiled
- has an origin of replication
- not associated with histones, no nuclear membrane
- appears as an amorphous, less densely packed region in the cell (nucleoid)
Gene organization
- genes have no introns
- several genes can be expressed on a single mRNA molecule –> operon
- gene expression is regulated by specific sequences and gene products
- sets of genes and operons are coordinately regulated in response to the environment
Plasmids
Smaller DNA molecules that are separate and replicate independently from the chromosome
- closed circular, supercoiled, double stranded DNA molecules, similar to chromosome except smaller
- each has its own origin of replication and replicates independently of the chromosome
- plasmids typically carry genes that ensure copies are maintained in each daughter cell as the parent divides
- do not carry the genes needed for survival!*
How many plasmids are found?
Depeding on plasmid, each cell can contain one copy, a few, or many
- a bacterial strain can carry more than one type of plasmid
What type of genes do plasmids carry?
Genes that offer a selective advantage for the host bacteria under certain environmental conditions (virulence genes, antibiotic resistance)
Bacteriophage
Bacterial virus
- similar to eukaryotic viruses, can replicate in lytic phase or enter quiescent lysogenic phase
Lytic phase
Bacteriophage directs the host cell to manufacture copies of the phage, and the host cell is lysed
Lysogenic phase
Bacteriophage inserts in host DNA
- can be subsequently induced to enter lytic phase
- virus lives in bacteria until the right conditions occur for lysis
Transposons
Jumping genes, genes that can move from one location in bacterial DNA and insert randomly in the same or a different DNA molecule
- can be carried in chromosome or plasmid (can jump from one to the other) –> becomes permanent once in the chromosome
- when transposition occurs, the gene removes itself and randomly inserts in either the same DNA molecule or a different one
Insertion sequence
Simple form of a transposon
- can cause an insertion mutation if it inserts in a gene
Composite transposons
Carry genes that offer a selective advantage
- ex: antibiotic resistance
- typically 2 transposons that flank a larger sequence
Integrons
Mobile DNA elements with the ability to capture genes, notably those encoding antibiotic resistance, by site-specific recombination
- a single integron can capture many antimicrobial resistance genes
- integrons can be carried in the chromosome (permanent) or plasmid
- will only capture genes with a certain sequence
Features of bacterial reproduction
- asexual reproduction: progeny are clones
- short generation time
- haploid: mutations have immediate effect
Mutations
- some are silent
- some are lethal
- some cause a detectable phenotypic change: nutrient requirement (auxotrophy), failure to grow under certain environmental conditions, adaptation to new environment, resistance
How do bacteria adapt when they reproduce asexually?
Have the ability to control gene expression in response to their environment
- genes are located close together and do not have interrupting sequences (introns)
- no nuclear membrane (transcription and translation occur simultaneously)
- adapt to their environment (salvage nutrients when possible)
- random mutations and short generation time
- exchange of DNA between bacteria
Evidence for DNA exchange between bacteria
- genomic sequencing of bacteria has revealed large differences in gene makeup even within bacterial species
- genomic islands: clusters of genes, located near phage remnants or other evidence of lateral exchange that have different G+C content or codon usage than the rest of the chromosome
Transformation
Uptake of naked DNA
- natural transformation occurs only in a few bacterial species
Transduction
Transfer of DNA by bacteriophage
- sometimes bacteriophage can carry chromosomal DNA from an infected bacterium and transfer it into an uninfected bacterium
Conjugation
Transfer of plasmid DNA (can sometimes chromosomal DNA) by mating of 2 bacterial cells
- DNA is transferred from a donor bacteria to a recipient
- host range: some plasmids are narrow (1 bacterial species or family), some are broad (large number of species)
How is exchanged DNA maintained?
Exchanged DNA must have its own origin of replication that is recognized by the host strain (plasmid) OR, it must be inserted into the chromosome
- transposition (transposons only)
- lysogeny (phage only)
- integron integration
- homologous recombination (allelic exchange)
