Lecture 13 Flashcards
What are transposons and what are some key facts?
Transposons are parts of the genome which can move around the genome, catalysed by an enzyme known as a transposase. These are the most abundant gene in nature, 50% of the human genome is made up of these. They cause a 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 and are powerful tools for molecular genetic studies.
What is another term for bacterial transposons and how many classes are there?
Bacterial transposons are transposable genetic elements, they have 4 classes: Insertion sequence elements, composite transposons, non-composite transposons, and conjugative transposons
What are insertion sequence elements?
Insertion sequence elements are the simplest transposon type, they encode only the protein needed for their own transposition (transposase), ends of all known of this type show inverted terminal repeats (ITRs) of 20-50 base pairs. Many DNA transposons in eukaryotes have a very similar structure (also has introns).
What are composite transposons?
Composite transposons consist of two insertion sequence elements flanking a central region which carrys other genes, the IS elements supply transposase (often only one IS module makes an active transposase) and ITR recognition signals.
How do insertion sequences and composite transposons move location?
Insertion sequences and composite transposons transpose via a conservative mechanism, this works by the transposase enzyme recognising the inverted repeat at each end and blunt cuts it out of the DNA and pastes the insertion sequence elsewhere in the DNA (sometimes by recognition of a particular base sequence) by cutting the DNA diagonally and inserting between the DNA segments, this leads to a small direct repeat of the Host DNA either side of the transposon.
How does non-composite transposition occur?
Non composite transposition occurs via copy and paste between plasmids (transposase makes two ddDNA cuts in donor and staggered cuts in target, resolution of cointegrate requires the resolvase(catalyses a recombination-like event), resulting in a small direct repeat of target DNA either side of the transposon).
What are conjugative transposons and how do they spread?
Conjugative transposons excise from genome and transpose from one cell to another via a conjugative intermediate, they insert randomly into the recipient and also reinsert into donor genome and are important in dissemination of antibiotic resistance and virulance (especially in gram positive bacteria). It occurs through excision from genome, transfer to cell membrane, replication and moving into other cell and integration, leading to donor and recipient both having a copy.
What are some DNA rearrangements mediated by transposons?
DNA rearrangements are mediated by transposons as they act as “portable” regions of homology to facilitate: replicon fusions (Hfr strain formation),
Deletions: homologous recombination between two copies of a transposon present in direct orientation (forming a circle between two same genes and removing inbetween segments, repeats in same direction.)
Inversions: homologous recombination between two copies of a transposon present in inverted orientation (repeats in opposite direction).
Insertions: by transposition or homologous recomination
What are some uses of transposons in the lab?
Transposons can be used to generate mutations (each mutant likely to only have one insertion and antibiotic resistance gene allows easy selection), to tag genes for subsequent isolation(due to knowing sequence) and many other uses.
