Transposable Elements Flashcards
TE
They are called “Jumping genes”
discrete segments of DNA that can mode from one location to another in a genome. Independant to homologous recombination.
Found in all organisms
60% of our DNA is transposable element
Class 1 elements
They encode a gene for transposase, which allows insertion sequences or transposons to jump between genes
Class 2 elements
Tn3, encodes for ampicillin resistance.
It has 2 genes, transposase and resolvase.
Bacterial TE
2 Types: Insertion sequences - cryptic nomenclature: IS\_\_\_ e.g IS1,IS2 transposons - carry phenotypic markers such as antibiotic resistance. nomenclature Tn\_\_\_ e.g Tn1
How TE’s jump
cut and paste model
Transposase recognises and binds to the inverted repeats which forms a complex. Excise the IS and generate a double strand break which is staggered (sticky ends).
It is ligated into another genome.
Everytime IS10 jumps, it createds a 9bp target repeat
How TE’s move: Class 2 elements
Transposase binds to both repeats on their ends, it is a double strand break.
The donor and recipient will “fuse” together, but have single strand regions. They will then be completed by DNA pol I. They are convalently bonded together (cointegrate).
Resolvase is then used to cleave the fused complex to return the donor and recipient, but now both with TE’s.
They can increase copy number at the expense of their host.
Transposition regulation
When it moves it will disrupt the gene. If it occurs at frequent rate, it becomes deleterious to their host.
TE’s offer a potential advantage to an organism/population in that way they can facilitate rapid evolution and spread new genetic traits.
transposition is a rare event due to mechanisms that limit the synthesis of transposase.
IS10/Tn10 is regulated by DNA methylation:
E-coli has an enzyme DAM methylase, which methylates adenine residues in sequence GATC
DAM sites are hemimethylated as only the old strand is methylated.
If DNA is fully methylated, then RNA polymerase does not undero transcription
Regulation 2 - anti-sense RNA
2 promoters are used for transposase synthesis; Pin, Pout
They both do complimentary base pairing, if Pout overlaps Pin’s sequence for transposase synthesis, then it is stopped as RNA polymerase cannot bind.
Tn3 regulation: Repressor regulation
binds to an operator sequence and effectively prevents RNA polymerase binding to the promoter.
Class 2 Transposable element
Not regulated as it is a virus.
It replicates itself via transposition.
Intergrates itself into a host (e.g ecoli)
It can detect if the host is “sick”, and it is a trigger for replication.
Exponential increase in bacteriophage.
TE and antibiotic resistance
Each TE in a plasmid carries antibiotic resistance gene .
Transposons as genetic tools
used to cause mutation in organism
mutated gene is tagged with the Tn, enabling subssequenct identification of the disrupted gene by cloning or sequencing.
Suicide vectors
Plasmid that have mutations and carries TE’s
i.e temp sensitive mutation
Once DNA is infected, bacteria will be incubated at specific temperature to denature plasmid.
