Chapter 16 Transcrip1on How does informa on from DNA produce cellular proteins Learning objec ves By the end of this class you should be able to Understand how RNA is processed aDer transcrip on eukaryotes Describe how the cell splices pre mRNA into mature mRNA Describe the func on of the 5 cap and the poly A tail of an mRNA Describe how prokaryo c transcrip on di ers from eukaryo c transcrip on Central Dogma Transcrip on What are these for First Step Ini a on Di erent bind to di erent promoters and determine which genes will be transcribed Once sigma binds the helix opens up and the template strand is threaded through a channel that leads to the ac ve site of RNA pol Sigma is released and the reac ons is spontaneous because NTPs have so much energy Elonga on Phase In the interior of the enzyme a group of amino acids called a rudder help steer the strands through the channels Another group of amino acids called the zipper help to separate the newly synthesized RNA from the DNA Enzymes ac ve site catalyzes the addi on of nucleo des at 50 nucleo des per second Termina on Phase In most cases transcrip on stops when RNA polymerase reaches a DNA sequences that func ons as a transcrip on termina on signal This codes for RNA that forms a structure The forma on of this hairpin is thought to disrupt the interac on between the transcript and RNA pol RNA Processing In bacteria the informa on in DNA is converted to mRNA directly and are ready to be translated In eukaryotes transcrip on results in a pre mRNA Before it can be used to make proteins it needs to be to into the mRNA transcript RNA Processing in Eukaryotes Eukaryote genes are not but are interrupted by hundreds or thousands of bases Why are these long stretches of DNA not part of the mRNA RNA Processing in Eukaryotes The nal mRNA does not match up with the DNA sequence It is than the DNA sequence Pieces of the gene make up the nal mature mRNA that codes for a The pre mRNA needs to be spliced down to the mature mRNA RNA Processing in Eukaryotes This suggested that the gene encoding a protein were in pieces Those to be in the nal mRNA were called Those sec ons not in the mRNA were called introns make up our mature mRNA RNA Splicing As transcrip on proceeds are removed from the growing RNA strand by a process known as splicing This is catalyzed by specialized RNAs and called small nuclear ribonucleoproteins or snRNPs The RNAs and proteins assemble into a complex called a spliceosome Transcript Processing In addi on to splicing mRNAs in eukaryotes have a 5 cap added consis ng of 7 methyl guanylate As soon as the 5 end of the mRNA emerges from polymerase enzyme add a structure called the 5 cap Transcript Processing Also have a long row of 100 250 adenines called a poly A tail An cleaves part of the 3 end of most RNAs aDer comple on of transcrip on A second enzyme than adds 1 250 adenine nucleo des crea ng a poly A tail Final transcript Our nal mRNA has a 5 cap and a poly A tail Note that the mRNA created contains regions that are not translated into proteins These are called our 5 and 3 untranslated regions Why do you think is the func on of the 5 cap and the poly A tail Di erences between Prokaryotes and Eukaryotes
View Full Document
Unlocking...