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Download Ch 11 Transposable Elements
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The Nature of Transposons Chapter 11 pp 300-315 Outline • Nature of Transposons • Transposons – Prokaryotic – Eukaryotic: Dr. McClintock’s research • Retrotransposons vs Retroviruses Transposons • Are mobile DNA sometimes referred to as “jumping genes”. Why are transposons in our genome? • Cellular function hypothesis – Antibody diversity of VDJ • Genetic variation hypothesis • Self-DNA hypothesis Common features associated with transposons Common Features Are: 1. Direct repeats 2. Terminal Inverted repeats Mechanisms of Transposition • Movement through DNA intermediates • Movement through RNA intermediates DNA Transposition Replicative Cut and Paste Copy and Paste • Movement through DNA intermediates Non-Replicative 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 Staggered cut Direct repeats are generated Staggered cut Direct repeats are generated Examples of: Transposable Elements in Bacteria 1. Simple Bacterial Transposon INSERTION SEQUENCE 2. COMPOSITE TRANSPOSON IS + any DNA segment Dr. Barbara McClintock 1902-1992 Nobel Prize, 1983 Physiology or Medicine “I don’t understand a word that she said, but if she says it is so, it must be so!” Alfred Sturtevant The Ds element often results in chromosomal breakage Drosophilia melanogaster & DNA transposition “The P Element” The P element Codes for both • Transposase • Repressor of transposition Female P- and male P+ Offspring normal Offspring sterile Hybrid dysgenesis Female P+ and male P- Why the difference? Non-hybrid dysgenesis Remember the P element Codes for • A Transposase • AND • A Repressor of transposition D. willistoni Science. 1991 Sep 6;253(5024):1125-8 Possible horizontal transfer of Drosophila genes by the mite Proctolaelaps regalis. Houck MA, Clark JB, Peterson KR, Kidwell MG. ~50 yrs ago That moved it onto Mite D. melanogaster Retrotransposons DNA mRNA Examples of retrotransposons elements • Ty Yeast • Copia • Alu Drosophila Humans General Mechanism Of Retrotransposons Note: retrotransposons are not found in prokaryotes Types of Retrotransposons • Alu • Ty Humans Non-Viral Yeast Viral • Copia Drosophila Example: Non-viral Retrotransposons Humans Non-Viral LINES SINES Line 1 and Line 2 Alu ALU • 300 bp • Endonuclease cutting site: site5' AG/CT 3‘ • Retrotransposon • 1 million in genome • Linked to variety of disorders Alu has inserted in NF-1 gene Neurofibromatosis: autosomal dominant Growth of nodules mainly affecting nerves in the skin Viral retrotransposons look a little like retroviruses—but are NOT the same ! Now we must compare a Viral RNA with a Viral Retrotransposon VIRAL RNA Now a viral retrotransposon Here’s the Copia element of Drosophila Similar gag and pol sequences Compare to the retrovirus Here’s the Ty element, again notice similarities to retroviruses Delta sequences function like LTR’s in viruses Summary Table Viral and non-viral Ty and Copia viral retrotransposons Alu non viral retrotransposons A topic of scientific discussion is what is a “prion”? Blunt cut from donor HUMAN IMMUNODEFICIENCY VIRUS Human Helper T Lymphocyte
