Transposons
Transposons - mobile segments of DNA that cannot exist independent of a replicon (because they don't have an ori of their own)
3 types of transposable elements:
1) IS - insertion sequence. Inverted repeats + transposase gene
2) Composite transposon. Two IS elements + antibiotic resistance gene(s)
3) Noncomposite transposon. Inverted repeats + transposase gene + antibiotic resistance gene(s)
Transposition
Transposase must be able to: 1) recognize the inverted repeats and 2) recognize the target sequence (or a partial match of the target sequence)
Target sequence is 5-9 bp on the recipient DNA molecule
Requires transposase
1) Transposase cuts transposon out of donor DNA (blunt cut)
2) Transposase makes a staggered cut at the target sequence
3) Transposase ligates transposon into the target
4) Gaps are filled in with DNA pol I and ligase
Results:
Target DNA has been duplicated in process
Usually destroys donor DNA
Requires transposase, resolvase and an IRS (internal resolution site) recognized by resolvase
1) Transposase makes nicks at ends of transposon and at ends of target
2) Transposase ligates the donor and target together such that they're linked together via single strands of the transposon
3) Gaps are filled in with host DNA pol I and ligase (transposon is replicated)
4) Resolvase catalyzes recombination between the 2 IRSs
Results:
Transposon does not leave the donor DNA so donor is not destroyed
Transposon is replicated so 2 copies of the transposon are produced
Other Transposable Elements
Ac/Ds family of transposons in maize
First studied by Barbara McClintock in 1940s
Discovered unstable mutations in gene for purple kernel color - could get mottled kernel by original (white) cell dividing to produce a mixture of cells (white and purple)
Named it Ac for "activator" because activates Ds and Ds for "dissociation" because associated with chromosome breaks
Ac element - fully functional transposon that moves by conservative transposition
Ds elements - defective Ac, missing or inactive transposase gene, must maintain the inverted repeats to be mobile
Ds is mobile only if transposase activity is supplied by Ac; also moves by conservative transposition. Even though transposition is conservative, the donor molecule survives because the cell can repair the site from which the Ac/Ds transposed
Ds can insert into gene for purple kernel color. Is stable (kernel always white) if no Ac present to supply transposase activity. But unstable (kernel mottled) if Ac present so some cells have Ds inserted in gene and some do not.
Retrotransposons
Transposons that replicate via an RNA intermediate
Some are likely derived from retroviruses because have retained genes for reverse transcriptase (RT) and integrase (INT, functions like transposase) and are flanked by long direct repeats called long terminal repeats (LTRs)
Ty Elements in Yeast
Has 4 genes: 1) gag for a virion internal protein, 2) int for integrase, 3) env (may not be present) for virion envelope protein, and 4) pol for reverse transcriptase
Transposition of Ty elements:
1) Transcription to produce ssRNA
2) Translation to produce reverse transcriptase (RT) and integrase
3) RT produces a RNA/DNA hybrid from the ssRNA
4) RT degrades the RNA in the hybrid, leaving ssDNA
5) RT makes another strand of DNA using the ssDNA as the template
6) Integrase inserts dsDNA into chromosome at a different site
Moderately Repetitive DNA in Eukaryotes
Most is derived from defunct viruses and retrotransposons
Two classes:
SINES - short interspersed sequences
LINES - long interspersed sequences
Most LINES are defective retrotransposons
Humans have 50,000-100,000 copies of LINES-1 element (about 5% of total DNA!) but only 3,000 copies are full-length. Most not functional because of mutated RT gene. Some still functional. Rare cases of hemophilia are caused by an insertion of a LINES-1 element from chrom. 22 into the gene for clotting factor VIII located on the X chrom.
Retrons
Bacterial elements that may be remnants of a retrovirus because has gene for reverse transcriptase. Doesn't move, just uses its reverse transcriptase activity to make unusual DNA/RNA molecules whose function is unknown
Selfish DNA
Confers no selective advantage on host
Junk DNA is mostly mutated viruses and transposons that can no longer move and can't express their genes; pseudogenes are also considered junk DNA.
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Last updated: 9-April-98 / laa