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Transposable elements lecture notes
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Jumping Genes: Transposable ElementsWhat are te’s?
o Transposable elements can move and mutate genomic DNA
o A.K.A mobile genetic elements & transposons
o Make up a high % of genome, especially in eukaryotes – 45% of human genome
o Most can insert in different locations by mechanisms that are not homologous
recombination (nucleotide sequences exchanged between 2 similar/ identical DNA
molecules)
o May cause mutations:
- Insert into genes, disrupting them
- Promote DNA rearrangements – deletions, duplications, inversions and
translocations
o Many different types, varieties of structure and mechanisms of transposition
General characteristics:
o Short flanking direct repeats are present on both sides of most TEs. The sequence
of repeats varies, but the length is constant for each type of TE (3-312 bp long)
- Not part of the TE and don’t move with it. Produced by staggered cuts on
insertion of TE.
o Terminal inverted repeats are inverted complements of eachother, 9-40 bp in
length.
- Recognised by enzymes that catalyse transposition – transposases.
How do they work?
o Transposition = the movement of a TE from one
Location to another
1. Staggered breaks are made in the target DNA
(no recognition of the sequence – don’t confuse with
restriction sites)
2. TE is joined to single stranded ends of the target
DNA
3. DNA is replicated at the single-stranded gaps, creating
the flanking direct repeats
, Classes of te’s:
o Transpose as DNA:
- DNA transposons (Class II transposable elements)
- Found in prokaryotes and eukaryotes
- Transposition can be:
Replicative: copy-and-paste – a new copy of the TE is introduced at a
new site while the old copy remains behind at the original site, so the
number of copies of the TE increases due to transposition.
Non-replicative: cut-and-paste – TE deletes from the old site and inserts
at a new site without any increase in the number of its copies. Requires
the replication of only the few nucleotides that constitute the direct
repeats.
Replicative transposition
o Replicative transposition can be either between 2 different DNA molecules or
between 2 parts of the same molecule.
o Transposase, usually coded for by the TE itself, recognises the inverted repeats at
the ends of the TE and makes single strand breaks at the ends of these. Also
makes similar breaks at the ends of the target sequence.
o Free ends of both join and the normal cell replication and repair mechanisms
convert the single-stranded regions back to double strands.
o A cointegrate of the 2 molecules containing the 2 copies of the TE and the target
site is produced.
o The cointegrate then undergoes resolution through crossing over between sites
located within the TEs
o Resolution gives rise to 2 copies of the TE. The new one now has the new target
site duplication.
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