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Lecture 4

BIOSC 0160 Lecture 4: Biology 2 Lecture 1.4: Transcription

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Biological Sciences
BIOSC 0160

Biology Week 3 Lecture 1.4 How is DNA transcribed into mRNA? How does transcription differ in prokaryotes vs. eukaryotes? Transcription occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes o Transcription and translation can occur simultaneously in prokaryotes but not in eukaryotes RNA polymerases synthesize RNA 5 to 3 form a DNA template by forming a phosphodiester bond between the 3 hydroxy of one ribonucleoside triphosphate (NTP) and the 5 phosphate group of another NTP o Does not require a primer o Uses NTP not dNTP o Uses uracil not thymine o Only reads one strand (template not coding) o Coding is identical to the transcribed RNA (except T sub U), the strand depends on the gene (promotor regionsstart codons) o Contains a central active site and lots of channels and nubby structures that are important for directing transcription o Unwinds and separated a small stretch of DNA forming a transcription bubble o Rudder: helps guide the strands to the appropriate channels (the template strand and coding strand enter different channels) o NTPs enter the site through a different channel o Sigmas dissociate end of transcription, mRNA exits polymerase through a different channel o RNA polymerase transcribes a termination signal and then stops hairpin loop o In E. Coli, there are 7 different sigma proteins that bind to promoters with slightly different sequences bind in slightly different environmental situations) Promoter: the DNA sequence that is recognized by sigma and promotes transcription of the gene o Two boxes in prokaryotes that Sigma binds to: TTGACA (35 box meaning it is about 35 pairs upstream) and TATAAT (10 box) o Consensus sequences: 10 box and 35 box can vary slightly o Transcription initiation occurs when sigma binds and orients the DNA within RNA polymerase o TATA box (30) is eukaryotes o PolyA signal terminates o PremRNA, goes through processing Exons are expressed, introns are intervening sequences that are removed by RNA splicing, when exons are joined together o UTR = untranslated regions o snRNAs = small nuclear RNAs catalyze the excision of introns and the ligation of exons
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