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Bacterial Genetic Variation and Manipulation (Horizontal gene transfer…
Bacterial Genetic Variation and Manipulation
Types of mutation
Single base changes
SNP (Single nucleotide polymorphisms)
Transition or transversion
Can cause:
Amino acid change (missense)
Premature stop codon (nonsense)
Nothing (due to redundancy in triplet code)
Removal of stop codon (sense)
Nucleotide deletions and insertions
Ribosome reads nucleotide in triplet codon
Insertion/deletion causes frame shift (triplet reading frame altered)
Usually renders protein non-funcional & premature stop codon
Variation
Sources
Prophages
Pathogenicity islands
Encode clusters of virulence genes
Adhesins/toxins/secretion systems
Encode genes for insertion into DNA ("integrases")
Not self mobile
Plasmids
Small independently replicating circular DNA
Cannot replicate outside host
Require host encoded enzymes
Non-essential but beneficial gene functions
e.g. Antibiotic resistance or virulence factors
Strains mostly differentiated by regions of variability (clinically important phenotypes)
Horizontal gene transfer
Conjugation
Genetic material between cells by cell-to-cell contact
Plasmid transfered from donor to recipient
Foreign DNA recombines into host genome
Larger fragments of DNA
(Whole gene(s)/parts of genes)
Acquisition/loss of phenotypes
Unexpected, inheritable change to host
e.g antibiotic resistance
Transformation
DNA can be from
Nearby lysed cells
Plasmids deliberately added to environment
Not all bacterial competent
Transfer of DNA from environment into cell
Component cells bind DNA and transport it into cell. DNA then recombines into host chromosome
Transduction
Bacteriophage
Bacterial viruses (virus with bacterial host)
Encode genetic information for own replication and proliferation
Specificity: Specific phages often only infect certain bacteria
Accidental phage mediated transfer of non-phage DNA
Host DNA (chromosomal or plasmid) mistakenly packaged into capsids
Very low frequency as needs successful recombination
Bacterial genetic engineering
Basic molecular tools
Restriction enzymes
: Cut DNA specific locations
Ligase
: Repairs broken sugarphosphate backbones
Vectors
: Artificially engineered plasmid, small, selective marker
Clinical/industrial applications or for research
Basic molecular cloning
Vector and insert cut with same restriction enzyme
Insert and vector stuck together with DNA ligase
Recombinant vector transformed into competent bacterial cells
Screen for antibiotic resistant (testing to see worked)
Why do?
East/cheap/fast grow
Easily manipulated in lab
Mutation
Any change in nucleotide sequence
Due to mistakes in replication or mutagen
Most no effect as in non-coding regions
Most mutations causing effects are lethal
Tiny proportion are observable and non-lethal, even smaller considered 'beneficial'