Lecture 23Outline of Last Lecture I. RNA and DNAII. RNA TranscriptionIII. Three Classes of RNAA. Messenger RNAB. Transfer RNAC. Ribosomal RNAIV. Transfer RNA (tRNA) StructureV. Ribosomal RNA SubunitsOutline of Current Lecture I. Messenger RNA (mRNA)II. RNA PolymeraseIII. PromotersIV. Post-Transcriptional ModificationsA. tRNA ModificationsB. mRNA ModificationsCurrent LectureThis 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. BCHM 307 1st EditionRNA polymerase still needs a way to be guided to the promoter. The transcription factor complex helps to achieve this. The σ subunit binds to an RNA polymerase and guides it to the promoter. The σ 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 preventexonuclease 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’ttranslated 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
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