Exam 1: Bacterial Genetics Flashcards
What is a phenotype?
The identifiable characteristics of a cell that can be altered by mutation
What is a genotype?
They sequence of nucleotides in the DNA of a cell
What does phenotype result from?
Gene expression
What is a mutant?
Bacteria with altered genetic sequence; may or may not have altered phenotype
What is an isogenic mutant?
Bacteria with only a single gene difference compared to parent strain
What is the gene organization of bacterial chromosomes?
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
What are the types of extrachromosomal DNA?
Plasmids
Bacteriophage
Transposable elements/transposons
Integrons
What are plasmids?
Smaller DNA molecules that are separate and replicate independently from the chromosome
Close circular, supercoiled, double stranded DNA molecules, similar to chromosome except smaller
What do plasmids typically carry?
Genes that ensure copies are maintained in each daughter cell as the parent divides
What do the genes that plasmids carry offer?
A selective advantage for the host bacteria under certain environmental conditions
What is bacteriophage similar to?
Eukaryotic viruses. It can replicate lytic phase or enter quiescent lysogenic phase
What is the lytic phase?
Bacteriophage directs the host cell to manufaccture copies of the phage and the host cell is lysed
What is the lysogenic phase?
Bacteriophage inserts in host DNA
Can be subsequently induced to enter lytic phase
What are transposable elements/transposons?
Genes that can move from one location in bacterial DNA and insert randomly in the same or a different DNA molecule
What can transposable elements be carried in?
Chromosome of plasmid
What happens when transposition of transposable elements occurs?
The gene removes itself and randomly inserts in either the same DNA molecule or a different one
What can transposable elements cause?
An insertion mutation if it inserts a gene
What are composite transposons?
They carry genes that offers a selective advantage, such as antibiotic resistance
What are integrons?
Mobile DNA elements with the ability to capture genes, notably those encoding antibiotic resistance, by site-specific recombination
What can a single integron capture?
Many antimicrobial resistance genes
What can integrons be carried in?
Chromosome or on plasmids
What are features of bacterial reproduction?
Asexual reproduction
Short generation time
What is the generation time of E. coli?
20 minutes during log phase growth
What does a generation time of 20 minutes mean?
1 cell can produce more than 2000 progeny in less than 4 hours
What is the normal mutation rate in E. coli?
1/2000 progeny will have a mistake in DNA
What are the possible mutations like in bacteria?
Some are silent
Some are lethal
Some cause a detectable phenotypic change
What leads to mutations that can cause a detectable phenotypic change?
Nutrient requirement (auxotrophy)
Failure to grow under certain environmental conditions (conditional lethal)
Adaptation to new environment
Resistance
How is adaptation possible for bacterial progeny?
Bacteria still have the ability to control gene expression to their environment
Random mutations and short generation type
Exchange of DNA between bacteria
How can bacteria control gene expression?
Genes are located close together and do not have interrupting sequences (introns)
No nuclear membrane (transcription and translation occur simultaneously)
Adapt to their environment
What is some evidence for DNA exchange between bacteria?
Genomic sequencing of bacteria has revealed large differences in gene makeup within bacterial species
Genomic islands
What are genomic islands?
Clusters of genes, often located near phage remnants or other evidence of lateral exchange, that have different G+C content of codon usage than the rest of the chromosome
What are mechanisms of DNA exchange?
Transformation
Transduction
Conjugation
What is transformation?
Uptake of naked DNA
Natural transformation occurs in only a few species
What is transduction?
Transfer of DNA by bacteriophage
What can bacteriophages sometimes carry during transduction?
Chromosomal DNA from an infected bacterium and transfer it into an uninfected bacterium
What is conjugation?
Transfer of plasmid DNA by mating of 2 bacterial cells
What is DNA transferred from and to in conjugation?
From a donor bacteria to a recipient
What is conjugation pili?
Specialized surface structures on a donor cell that attach to a recipient cell and transfer DNA
What are genes for conjugation pili usually carried by?
Plasmid
What are host ranges like in conjugation?
Some plasmids are narrow (1 bacterial species or family)
Some are broad
Why are broad host ranges dangerous?
They can move around even more than narrow ranges
How is exchanges DNA maintained?
Exchanged DNA must have its own origin of replication that is recognized by the host strain OR
It must be inserted into the chromosome
How can exchanged DNA be inserted into the chromosome?
Transposition (transposons only)
Lysogeny (phage only)
Integron integration
Homologous recombination (allelic exchange)
What mediates homologous recombination?
Bacteria that have recombinases (RecA)
What does homologous recombination require?
A similar sequence already in the chromosome
