The Nature of Transposons Chapter 11 pp 300-315
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Transcript of The Nature of Transposons Chapter 11 pp 300-315
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The Nature of TransposonsChapter 11pp 300-315
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Outline• Nature of Transposons
• Transposons – Prokaryotic– Eukaryotic: Dr. McClintock’s research
• Retrotransposons vs Retroviruses
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Transposons
• Are mobile DNA sometimes referred to as “jumping genes”.
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Why are transposons in our genome?
• Cellular function hypothesis– Antibody diversity of VDJ
• Genetic variation hypothesis
• Self-DNA hypothesis
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Common features associated with transposons
1. Direct repeats
2. Terminal Inverted repeats
CommonFeatures
Are:
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Mechanisms of Transposition
• Movement through DNA intermediates
• Movement through RNA intermediates
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• Movement through DNA intermediates
DNA TranspositionC
ut and Paste
Replicative Non-Replicative
Copy and Paste
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Non-Replicative Movement by Transposition
• 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
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Staggered cut
Direct repeats are generated
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Staggered cut
Direct repeats are generated
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Examples of: Transposable Elements in Bacteria
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1. Simple Bacterial Transposon
INSERTION SEQUENCE
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2. COMPOSITE TRANSPOSONIS + any DNA segment
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Dr. Barbara McClintock1902-1992
Nobel Prize, 1983Physiology or Medicine
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“I don’t understand a word that she said, but if she says it is so, it must
be so!”
Alfred Sturtevant
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The Ds element often results in chromosomal breakage
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Drosophilia melanogaster&
DNA transposition
“The P Element”
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The P element Codes for both
• Transposase
• Repressor of transposition
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Offspring sterile
Offspring norm
al
Why the difference?
Female P- and male P+ Female P+ and male P-
Hybrid dysgenesis Non-hybrid dysgenesis
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Remember the P element Codes for
• A Transposase
• AND
• A Repressor of transposition
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Mite
Science. 1991 Sep 6;253(5024):1125-8Possible horizontal transfer of Drosophila genes by the
mite Proctolaelaps regalis.
Houck MA, Clark JB, Peterson KR, Kidwell MG.
D. melanogaster
D. willistoni
The P element took a ride on a
That moved it onto
~50 yrs ago
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Retrotransposons
mRNADNA
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Examples of retrotransposons elements
• Ty
• Copia
• Alu
Yeast
Drosophila
Humans
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General MechanismOf Retrotransposons
Note: retrotransposons are not found in prokaryotes
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Types of Retrotransposons
• Ty
• Copia
Yeast
Drosophila
Humans
Viral
Non-Viral• Alu
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Non-viral Retrotransposons
Humans
Non-Viral
LINES SINES
Alu Example: Line 1 and Line 2
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ALU
• 300 bp
• Endonuclease cutting site: site5' AG/CT 3‘
• Retrotransposon
• 1 million in genome
• Linked to variety of disorders
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Alu has inserted in NF-1 gene
Neurofibromatosis: autosomal dominantGrowth of nodules mainly affecting nerves in the skin
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Viral retrotransposons look a little like retroviruses—but are NOT the same !
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Now we must compare a Viral RNA with a Viral Retrotransposon
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VIRAL RNA
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Now a viral retrotransposon
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Here’s the Copia element of Drosophila
Compare to the retrovirus
Similar gag and pol sequences
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Here’s the Ty element, again notice similarities to retroviruses
Delta sequences function like LTR’s in viruses
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Summary Table
Viral and non-viralTy and Copia viral retrotransposons
Alu non viral retrotransposons
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A topic of scientific discussion is what is a “prion”?
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Blunt cut from donor
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HUMANIMMUNODEFICIENCYVIRUS
Human Helper T Lymphocyte