transposable elements Flashcards
Transposons
“jumping genes”
- segments of DNA that integrate into new locations on the chromosome
- make up about 40% of human genome
- major source of mutations
- bacterial transposons can carry antibiotic-resistant genes
Cut and paste transposons
- element that is physically cut out of one site in a chromosome and pasted into a new site, “switch”
- catalyzed by transposase
- DNA transposon
- both prokaryotes and eukaryotes
Replicative transposons
- replicated with one copy inserted at a new site and one remains original
- catalyzed by transposase
- DNA transposon
- only in prokaryotes
Retrotransposons
- DNA copy of element made by reverse transcription from its RNA and then inserted into a new chromosomal site
- catalyzed by reverse transcriptase
- 2 kinds: retrovirus-like elements and retroposons
bacterial transposons
- move within and between chromosomes and plasmids
- cut and paste types: insertion sequences (IS elements) and composite transposons
- replicative transposons: Tn3 elements
General structure of Cut-and-Paste DNA transposons
- gene that encodes a transposes: protein required for transposition
- terminal inverted repeats: identical or nearly identical inverted sequences at both ends of element
- target site duplication: short directly repeated sequences at both ends, result from staggered cleavage of the double-stranded DNA
IS elements
- simplest bacterial transposons
- compactly organized and contain only genes whose products are involved in transposition
- inverted terminal repeats are found at the ends
- some IS elements encode transposase
How does insertion of an IS element cause target site duplication
- staggered cuts are made in the target DNA
- a transposable element inserts itself into the DNA
- the staggered cuts leave short, single-stranded pieces of DNA
- replication of this single-stranded DNA creates the flanking direct repeats
Multiple IS elements
- bacterial chromosome may contain many copies of an IS element
- when a particular IS element is found in both a plasmid and chromosome, homologous recombination may occur inserting the plasmid into the chromosome
conjugative and non-conjugative plasmid
conjugative: donor of DNA
non-conjugative: acceptor of DNA
Homologous recombination of IS elements
- involve a non-conjugative resistant (R) plasmid and a conjugative plasmid
- mobilize antibiotic resistance genes
- needs an RTF component to promote conjugation between bacteria
composite transposons
- created when 2 IS elements insert near each other “capturing” a DNA sequence
- IS element excision through transposase cleavage at each end of the transposons can mobilize the entire captured DNA, which may mobilize antibiotic-resistant genes
Tn3 elements
- Tn3 elements are larger than the simple IS elements
- often contain additional genes not involved in transposition
- have simple inverted repeats at each end
replicative transposons
- transposase encoded by Tn3 catalyzes the formation of cointegrate between the donor and recipient plasmids
- Tn3 is replicated so there is an element at each junction
- recombination separates the co-integrate into 2 DNAs each with a copy of Tn3
the discovery of transposons
- found in eukaryotes (corn)
- found by Barbra McClintock
Cut-and-paste transposons in eukaryotes
dependent on Ac and Ds elements
Ac = fully active transposon (activator)
Ds = have elements they can’t transpose without help from Ac (dissociation)
- 2 versions of the same transposon
The Ac/Ds system in corn
- Ds is located at a site on chromosome 9 in mosaic kernels where chromosome breakage occurs
- Ds cannot induce breakage by itself so Ac stimulates chromosome breakage at the site of Ds
Activities of the Ac/Ds elements
- similar to cut-and-paste in prokaryotes
- the Ac element encodes a transposase that is responsible for excision, transposition, mutation and chromosome breakage
- the Ac transposase interacts with sequences at the ends of Ac and Ds elements and catalyzes their movement
- deletions or mutations in the Ac gene abolish its catalytic function
similarities between 3 types of transposons
differences between 3 types of transposons
retroviruses
- use reverse transcriptase to copy the retroviral-like RNA into DNA
- unique to eukaryotes
retrotransposons
- use reverse transcriptase to copy the retroviral-like RNA into DNA
- do not have terminal inverted repeats
- resemble the reverse transcripts of poly A+ mRNA
general structure of retroviruses
- genes that encode reverse transcriptase and integrase
- terminal inverted repeats
- target site duplication
general structure of retroposons
- genes that encode reverse transcriptase and endonuclease activities
- 5’ and 3’ untranslated regions and poly A tail
- create a target site duplication but have no terminal inverted repeats
long interspersed nuclear elements (LINES)
example L1
- LINES are the most abundant class of transposable elements in humans
- retroposon
- have an internal promoter, 2 open reading frames that encode a nucleic acid binding protein and a protein with endonuclease and reverse transcriptase activities
short interspersed nuclear elements (SINES)
- second most abundant class of transposable elements
- only Alu elements are transpositional-active
- less than 400 base pairs long
- reverse transcriptase required for SINE is provided by LINE-type element
genetic evolutionary significance or transposons
- they are mutagens
- can mobilize foreign genes
- change in genome organization
transposons and chromosome rearrangements
- pairing by looping and crossing over between 2 transposons oriented in the same direction leads to deletion
- pairing by bending and crossing over between 2 transposons oriented in opposite directions leads to an inversion
- misalignment and unequal exchange between transposons on sister chromatids leads to one chromosome with a deletion and one with a duplication
