Eukaryotic Variations Transcription, Processing and Translation in Eukaryotes Eukaryotic Promoters eukaryotes use TFIID (a protein) to bind the TATA box (TATAAATA) at -25 bases from the start of transcription eukaryotes have other sequences upstream from the TATA box theat are also important, such as CCAAT, etc. Eukaryotic mRNA is heavily processed the initial transcript is called hnRNA (heterogenous RNA) because it was originally this unknown RNA found with a large range of sizes only in the nucleus the hnRNa is has sequences removed from each end and then a 5' cap (7methyl guanosine added 5' to 5') and a poly A tail (a string of about 300 As added to the 3' end of the RNA) are added introns are then spliced out - various stretches of RNA sequence are cut out of the transcript and then the remaining exons are spliced back together, the introns are presumably degraded Here is a more complete description of the discovery of introns, from the "DNA from the Beginning" site Introns (four types) Cech (1982) discovered that some introns can self-splice (ribozymes) - group I mitochondrial and chloroplast mRNA's also self-splice - group II using a lariat formation most mRNA's use a spliceosome (a 60S particle with snRNAs) - group III tRNAs have an intron that is removed enzymatically - group IV The mature mRNA is then exprted to the cytoplasm for translation Try answering these quesions about eukaryotic transcription from the Biology Project: question1, question2, question3, question4, question5, question6, question7 Eukaryotic ribosomes require the cap (the 7methyl guanosine) and start at the first AUG (monocistronic genes) Eukaryotes start at the first AUG from the beginning of the message and add methionine. Some proteins have the initial methionine removed (hemoglobin beta is an example of this). Origin of Life Conserved code argues strongly for one origin DNA replication, transcription and translation are too complicated to have been used by the first organisms Current hypothesis: an RNA origin RNA is present and required for replication (the primer, telomeres), transcription, and translation (mRNA, rRNA and tRNA) RNA can both code for information (RNA viruses) and act enzymatically (self-splicing, peptidyl transferase, ribozymes, etc.) For more on the origins of life check out this animation of the Urey-Miller experiments and Cech's discovery of ribozymes, from "DNA from the Beginning" and this article from the The Scientist Try this question from the Biology Project This document is copyright of Jeff Bell Last Update: Wednesday, April 11, 2001