4 key features of transpoons
• Able to move from one site in the genome to another
independent of host recombination system – “jumping
genes”
• Cause large fraction of spontaneous mutations and
chromosome rearrangements (deletions, inversions,
replicon fusions)
• Enable the rapid acquisition of multiple antibiotic
resistances and facilitate the flow of such genes between
species
• Provide powerful tools for molecular genetic studie
Transposon definition
Transposable genetic elements
Types of Bacterial Transposons:
• Insertion sequences
• Composite transposons
• Non-composite transposons*
• Conjugative transposon*
*not covered
Insertion sequence (IS) elements
- Simplest type of transposon - ultimate in selfish DNA
- Encode only the protein needed for own transposition
(transposase) - Several present in multiple copies on E. coli chromosome
e.g. IS1, 4-19 copies depending on strain - Ends of all known IS elements show inverted terminal
repeats (ITRs) of 20 - 50 base pairs.
Many DNA transpoons in eukaryotes have a very simulat structure to IS elements:
• P elements - key tools for genetic analysis of Drosophila
• Tc1/mariner family elements e.g. Minos has a low
insertion bias and transposes with high frequency in
eukaryotes. Mariner Tns also used in bacteria.
• Ac and Ds in maize
Composite transposons features
• Consist of two IS elements flanking a central region carrying
other genes (e.g. antibiotic resistance genes)
• IS elements supply transposase and ITR recognition signals.
• Often only one IS module makes active transposase
What happens when one IS can no longer move by itself in a composite transpoons.
- two parts can not move individually anymore\
- recognises the two outside IRs
Conservative transposition - Is and composite Tns - how do they jump
“ Cut-and-paste”
Transposable recognises sequences on either end
Transposase makes:
1. dsDNA cuts in donor
2. Staggered cuts in target (will interact with almost any DNA)
Non-replicative: donor DNA lost or repaired
Small direct repeat of target DNA either side of transposon
Effects transposons have on genomes
Major contributors to genetic variation on the chromosome!
- Insertions - transposition or homologous recombination
- Replicon fusions - E.g. Hfr formation
- Deletions - Homologous recombination between two copies of a transposon present in direct orientation
- Inversions - Homologous recombination between two copies of a transposon present in inverted orientation
Uses of transpoons
How does Hfr strain form
- two identical IS regions result in the function of the plasmid into T he genome
What effects can transposons have on genomes
- Dna rearrangements mediated by transposons
- they are major contributors to genetic variation on the chromosome
Insertions: transposition or homologous recombination
Replicon fusions: e.g Hfr formation
Deletions: homologous recombination between two copies of a transposons present in direct orientation
Inversions: homologous recombination between two copies of a transposons present in inverted orientation
Rewatch the video from 30 minutes lecture 11
What effects can transposons have on genomes
- dna rearrangements mediated by transposons
- major contributors to genetic variation on the chromosome
Insertions: transposition or homologous recombination
Replication fusions: e.g Hfr formation
Deletions: homologous recombination between two copies of a transposon present in direct orientation
Inversions: homologous recombination between two copies of a transposon present in inverted orientation
Use of transposons
- generate mutations: antibiotic resistance gene allows easy selection (Tn5) each mutant is likely to have only single insertion
- ‘tag genes’ for subsequent isolation/identification
Powerful genetic tools
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