Lecture 23 - Lecture23

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Lecture 23 The end of DNA transposons goes through a number of families in corn, which form- ally is called maize, they are the first transposons that have been discovered. two things that are significant about the maize transposons they are transposed by conspaze mechanism they have a large number of non- autonomous elements that are activated by autonomous elements. They were discovered based on the genes that they land near the either activate or inactiv- ate genes in their vicinity. this can either be done by disruption or by introducing an enhancer that they carry for their own transcription near a promoter for a real cellular gene and then that gene product goes on. Variegation - Barbara Clintock - She was looking at plant genes that af- fected color or the texture of the kernel -- she was looking at traits that were not essential to the development of the corn -- what she would see is that during the life cycle of the corn, genes would be turned on in clusters, in clonal patches, or not turned on due to the action of these transposons. The other thing that is not clearly understood about maize transpo- sons is there activation and inactivation goes through cycles which are due to the methylated state of the DNA. Can have an element that is normal but the DNA that it is within gets methylated, which causes transcription to be terminated or not to occur. So the element will appear to be inactive; after many generation the methyl groups are removed and this element is active again. The drosophila P elements are unusual for two reasons they are the only element active in germ cells and not somatic cells be- cause one of the introns can only be spliced in the germ line In all of these cases they started with tn10, tna, maize elements and P elements, the transpositions frequency is always down regulated by the host so that the transposition frequency doesn’t kill off the host If you have hundreds of elements that integrate randomly every gen- eration into genes that might be assembled into genes that might be essential. Your basically doing a mutagenesis experiment on the or- ganism. So organisms cannot survive this, all organisms have mech- anisms to down regulate either the production or the activity of trans- posase. So P elements, as part of the mechanism of transposition, they generate a lot of truncated, internally deleted non-autonomous elements and the transposase that is synthesized from such ele- ments often is truncated, called a repressor, and antagonizes the activity of transposase. In flies there is an asymmetric activation of transposition de- pending on whether the female or male fly either has P ele- ments or doesn’t.
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Some of these transposons have been introduced relat- ively recently. Margaret Kidwell - Hybridous genesis - if the male had P elements and the female did-
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Lecture 23 - Lecture23

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