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10-29 - BIO 5099 Molecular Biology for Computer...

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BIO 5099: Molecular Biology for Computer Scientists (et al) Lecture 18: Eukaryotic genes http://compbio.uchsc.edu/hunter/bio5099 [email protected] Eukaryotic vs. Prokaryotic genes Like in prokaryotes, Eukaryotic genes are regions of DNA that act as templates for the production of RNA by RNA polymerases Recall Prokaryotic transcription: Transcription factors bind to specific DNA sequences upstream of the start of operons, or sets of related genes. Transcribed mRNA is directly translated by ribosomes. In Eukaryotes, Each gene has its own transcriptional control (no operons) mRNA is processed before translation Eukaryotic Genes Eukaryotic genes divided by long intergenic regions They are also interrupted by long regions of non-coding sequence called introns . Each contiguous portion of a coding sequence is called an exon . mnemonic: EXons are EXpressed, INtrons are INserts into genes.
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mRNA processing After transcription, but before translation, mRNAs are processed. Processing includes Splicing out introns A 5' 7-methylguanylate cap (m7Gppp) is added Polyadenylation adds a PolyA tail. Processed mRNA is called mature 5' cap prevents degradation until translation Poly-A tail plays a role in transport to ribosome 5' capping Almost as soon as the pre-mRNA starts coming off of the DNA, it's 5' end is capped Process is to phosphorylate (driven by DNA polymerase) Add guanine Methylate the guanine Methylate the methionine codons Needed for translation to start Polyadenylation Nearly all mRNAs have a signal after the stop codon containing the sequence AAUAAA This is the polyadenylation signal An enzymatic system recognizes this signal, and adds ~200 A's to the transcript Without a poly-A tail, transcripts are rapidly degraded in the nucleus.
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Untranslated regions Mature mRNAs can have untranslated regions at either end. Before start codon is 5' UTR After stop codon is 3' UTR Signals in these regions are sometimes used to direct active transport of these mRNAs to particular parts of the cell.
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