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Chapter 8-2.docx

4 Pages

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BIOL 2000
Joseph Rasmussen

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Chapter 8-2 Steps in Transcription  Rule o For most genes, there will be a single transcription start point and a single end point  5’ start point will always be the same  3’ end point will always be the same  Initiation o Start making the RNA chain (slow step) o Governs rate of transcription  Bind the template  Positioned to select where the 5’ will be  First phoshodiester bonds formed  Elongation o Continuous synthesis of the RNA chain  RNA polymerase movement along the template  Termination o Stop synthesis  Phosphodiester bond formation ceases  RNA transcript and DNA template release Bacteria  Have one main RNA polymerase  Multi subunit complex o Two forms  Core enzyme- three different protein subunits  Beta prime  Beta subunit  Alpha subunit  Holoenzyme- four protein subunits  Additional sigma protein subunit o Core enzyme is elongation and termination steps o Holoenzyme is initiation step  “Whole enzyme” all protein subunits involve  Sigma subunit allows for initiation How does RNA polymerases know where to start?  Approach o Compare many genes and start sites  What is common? o Conserved DNA sequences?  Consensus sequences  Where at the same positons (-10) has all the same nucleotides which is T in E.coli  Coding strand of DNA is the same as RNA but with U for T  E. Coli promoters o Set of DNA sequences that are required to initiate transcription o Recognized by RNA polymerase and other protein factors o Numbering of the nucleotides  Start site = +1 and gets more positive as the polymerase moves  Gets -1 as moves upstream  No 0… only -1 and +1  Summary of E.Coli o Promoter contains two minimal elements  -10 box and -35 box  Upstream of transcriptional start site  Binding sites for RNA polymerase  Conserved sequences (consensus)  Strong promoters (high level RNA synthesis) are closer matches to the consensus  Allows for some variations in the sequence boxes Transcription initiation in prokaryotes o Sigma factor directly recognizes the conserved sequence elements o Holoenzyme complex o Allows RNA polymerases to bind to the promoter o Initiation o Requires separation of two strands o Done by the Beta prime o Allow sigma factor to leave
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