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. McClintocks 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
Common Features Are
2. Terminal Inverted repeats
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Mechanisms of Transposition

• Movement through DNA intermediates
• Movement through RNA intermediates

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DNA Transposition
Cut and Paste
Copy and Paste

• Movement through DNA intermediates

Replicative
Non-Replicative
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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 TRANSPOSON IS any DNA segment
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Dr. Barbara McClintock 1902-1992
Nobel Prize, 1983 Physiology or Medicine
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I dont 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
transpositionThe P Element
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The P element Codes for both

• Transposase
• Repressor of transposition

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Female P- and male P
Female P and male P-
Offspring sterile
Offspring normal
Non-hybrid dysgenesis
Hybrid dysgenesis
Why the difference?
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Remember the P element Codes for

• A Transposase
• AND
• A Repressor of transposition

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Science. 1991 Sep 6253(5024)1125-8Possible
horizontal transfer of Drosophila genes by the
mite Proctolaelaps regalis.Houck MA, Clark JB,
Peterson KR, Kidwell MG.
D. willistoni
The P element took a ride on a
50 yrs ago
That moved it onto
Mite
D. melanogaster
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Retrotransposons
mRNA
DNA
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Examples of retrotransposons elements
Yeast

• Ty
• Copia
• Alu

Drosophila
Humans
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General Mechanism Of Retrotransposons Note
retrotransposons are not found in prokaryotes
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Types of Retrotransposons
Humans
Non-Viral

• Alu

Yeast

• Ty
• Copia

Viral
Drosophila
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Non-viral Retrotransposons
Humans
Non-Viral
SINES
LINES
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 dominant Growth of
nodules mainly affecting nerves in the skin
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Viral retrotransposons look a little like
retrovirusesbut 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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Heres the Copia element of Drosophila
Similar gag and pol sequences
Compare to the retrovirus
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Heres the Ty element, again notice similarities
to retroviruses
Delta sequences function like LTRs in viruses
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Summary Table
Ty and Copia viral retrotransposons
Viral and non-viral
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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HUMAN IMMUNODEFICIENCY VIRUS
Human Helper T Lymphocyte