Chapter 11 - Transcription and Translation

Segment eukaryotic mrna have modifitcations at their

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Unformatted text preview: ations at their 5 (guanosone cap) and a 3 polyA tail not found in prokaryotes or tRNA and rRNA mRNA Split Genes o o o o o o o o o o o o o recall hnRNAs are several times larger than mRNAs hnRNAs were immediately thought to be precursor of mRNA but why would they be so much larger from rRNA studies, we knew that mature RNAs could be made from larger precursors large segments from 5 to 3 end are removed from precursor rRNA until 1977, it was thought this was how hnRNAs were processed into mRNA Studied sequence of adenovirus mRNAs number of different mRNAs had the same 5 terminus 5 terminus is not complementary to a continuous stretch of template DNA instead 5 terminus is transcribed from 3 distinct segments of DNA regions of DNA between these segments is called intervening sequences Presence of intervening sequences in nonviral cellular genes were confirmed Jeffrey and Flavell discovered intervening sequence of about 600 bases within part of globin gene intervening sequences soon found in other genes too genes with intervening sequences splitgenes, became the rule not the exception 4 o o o o o The parts of the split gene that contribute to the mature mRNA are called exons The intervening sequences are called introns Cells produce a primary transcript that includes introns introns are then removed which explains why hnRNA are so much larger Hybridization experiments supported the concept of mRNA precursors (pre-mRNAs) Later studies revealed that exons average about 150 nuc...
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