BCHM 307 1st Edition Lecture 23 Outline of Last Lecture I RNA and DNA II RNA Transcription III Three Classes of RNA A Messenger RNA B Transfer RNA C Ribosomal RNA IV Transfer RNA tRNA Structure V Ribosomal RNA Subunits Outline of Current Lecture I Messenger RNA mRNA II RNA Polymerase III Promoters IV Post Transcriptional Modifications A tRNA Modifications B mRNA Modifications Current Lecture This lecture will pick up talking about the different classes of RNA Messenger RNA mRNA as mentioned before makes up only 5 10 percent of cellular RNA The function of mRNA is to act as a template that is transcribed into proteins mRNA is copied from a DNA template and carries its information mRNA is needed for more reasons than this though In order for genetic information to be transferred from the nucleus to the cell s cytoplasm it must be in mRNA form mRNA is how genetic information is amplified and expressed Since it is passed on to daughter cells special care must be taken to make sure it isn t adulterated When mRNA is synthesized it is because the cells have a need for it mRNA normally does not last long and is degraded rapidly Now we will look at the transcription process The main enzyme that does the transcription process is called RNA polymerase This enzyme synthesizes an RNA strand from the DNA template In prokaryotic cells there is only a single type of RNA polymerase but in eukaryotic cells there are multiple RNA polymerases RNA polymerase I transcribes rRNAs RNA polymerase II transcribes mRNAs RNA polymerase III transcribes tRNAs RNA polymerases need to know where to start transcribing on the DNA strand Promoters are used to show a RNA polymerase where to start transcribing Promotors are at the 5 end of a strand They are small 200 1000 base pairs long strands of DNA The transcription process begins downstream of the promoter Promoters also tell how much RNA is to be synthesized RNA polymerase still needs a way to be guided to the promoter The transcription factor complex helps to achieve this The subunit binds to an RNA polymerase and guides it to the promoter The subunit will then dissociate from the complex The transcription factor complex will bind to a consensus or average sequence These are a motif that have the most common bases from various strands in common RNA polymerase synthesizes from 5 to 3 There are other transcription factors that can bind upstream These factors enhance the binding of RNA polymerase II To prove transcription factors and promoters are important a study was done with flies Scientists were able to pinpoint the gene in flies that determines whether or not they have eyes When this gene was turned on at the wrong time or in the wrong place eyes develop in odd places on the fly s body Once RNA has been transcribed it will be modified to complete the process tRNA is first modified by trimming the 5 end This is done by ribonuclease P Ribonuclease P is a ribozyme which is a type of enzyme that has RNA in the catalytic area Next the 3 end of the RNA is also modified The RNA base motif CCA is added via template independent RNA synthesis This CCA is important as it is necessary in order for the tRNA to be properly charged with an amino acid for translation mRNA is also modified on its end in eukaryotes A 7 methylguanosine cap is linked by 5 5 triphosphate to the 5 end of mRNA This cap helps to prevent mRNA from being degraded by exonucleases mRNA is eukaryotes is also modified at the 3 end A poly A tail is added through the process called polyadenylation This is a long stretch of A residues added to the 3 end This modification also helps to prevent exonuclease degradation The consensus sequence that allows this to happen is AAUAAA All of these mRNA modifications happen in the nucleus The final modification that happens with mRNA in eukaryotes is spicing Introns are regions that aren t translated into a protein sequence and are cut out of the mRNA strand Exons are the actual coding regions of DNA that are then sewn back together The complex that performs this is called the splicesosome These modifications also occur in the nucleus
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