Lecture 14-15

Lecture 14-15 - Lecture14...

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Lecture 14 End of chapter 25 --> structure and binding (helix-loop helix, etc; Active/inactive compon- ents) Lecture 15 4 types of RNA splicing in cells 1. Nuclear RNA splicing - transesterification, RNA mediated 2. Group II autocatalytic - transesterification, RNA mediated 3. Group I autocatalytic - transesterification, RNA mediated 4. tRNA splicing - cleavage and ligation, protein mediate Splicing occurs in the nucleus and involves the spliceosome and it is believe that the catalysis is mediated by RNA because it was discovered in nuclear splicing organelles, such as chloroplast and mitochondria those genes also have introns but have either group I or II autocatalytic introns. These are introns that take up very specific 3D structures, (just like a protein enzyme have binding sites for substrates and catalytic residues) these RNAs take a 3D structure that juxtapose the splice junctions near cata- lytic residues, the functional groups are on RNA and not on amino acids. Organelles on lower eukaryotes have these classes of the autosplicing introns. The mechanism of Group II autocatalytic and nuclear splicing are almost identical. It is believed that nuclear splicing evolved from group II autocatalytic introns The first three types of RNA splicing involve transesterification transesterification - exchange of one phosphodiester bond with another without re- quiring energy. all believed to be RNA mediated reactions tRNA splicing - requires cleavage followed by ligation in the other three forms of splicing only see the exchange of phosphodiester bonds. Reason that splicing is so accurate is because there are no bonds being broken until all of the proteins and RNAs that are recognized in the junction are in place. Then finally all of the exchanges of bonds occur simultaneously, so never get free exons or introns floating around (very regulated) Group II introns - usually use nuclear splicing Nuclear splicing have eukaryotic mRNA which is transcribed in the nucleus and most genes have an average of 8 introns. primary transcript is much more complex and larger than the mRNA. That does not include the 5’ and 3’ UTR which don’t code for proteins and are part of the mRNA, but the primary transcript (sometimes called pre-mRNA, heterogeneous nuclear RNA) is much larger because it usually has a much longer 3’ end before the poly-A tail is added there is a cleavage of the 3’ end and all the introns have to be relieved. What was the feature recognized? as soon as you start transcription because one of the auxiliary factors for the basal apparatus binds guanyl transferase, the cap of the 5’ end is added almost as soon as transcription starts. Have to wait till transcription ends then termina- tion of transcription, usually, is kilo-bases downstream of where the mature 3’ end terminates. Then there is a endonuclear cleavage, then a poly-A poly-
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merase adds ~200 non-template directed A residues to the 3’ region. This all oc-
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This note was uploaded on 01/26/2009 for the course BIOLOGY 244 taught by Professor Palter during the Fall '05 term at Temple.

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Lecture 14-15 - Lecture14...

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