tailieunhanh - Biochemistry, 4th Edition P99

Biochemistry, 4th Edition P99. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Eukaryotic Transcripts Processed and Delivered to the Ribosomes for Translation 943 5 -splice site Branch site 3 -splice site FIGURE Splicing of mRNA precursors. A representative precursor mRNA is depicted. Exon 1 and Exon 2 indicate two exons separated by an intervening sequence or intron with consensus 5 3 and branch fate of the phosphates at the 5 - and 3 -splice sites can be followed by tracing the fate of the respective ps. The products of the splicing reaction the lariat form of the excised intron and the united exons are shown at the bottom of the figure. put for example as ATP is needed. The lariat product is unstable the 2 -5 phosphodiester branch is quickly cleaved to give a linear excised intron that is rapidly degraded in the nucleus. Splicing Depends on snRNPs The hnRNA pre-mRNA substrate is not the only RNP complex involved in the splicing process. Splicing also depends on a unique set of small nuclear ribonucleoprotein particles so-called snRNPs pronounced snurps . In higher eukaryotes each snRNP consists of a small RNA molecule 100 to 200 nucleotides long and a set of about 10 different proteins. Some of the different proteins form a core set common to all snRNPs whereas others are unique to a specific snRNP. The major snRNP species are very abundant present at greater than 100 000 copies per nucleus. The RNAs of snRNPs are typically rich in uridine hence the classification of particular snRNPs as U1 U2 and so on. The prominent snRNPs are given in Table . U1 snRNA folds into a secondary structure that leaves the 11 nucleotides at its 5 -end single-stranded. The 5 -end of U1 snRNA is complementary to the consensus sequence at the 5 -splice junction of the pre-mRNA Figure as is a region at the 5 -end of U6 snRNA. U2 snRNA is complementary to the consensus branch site sequence. The snRNPs Found in Spliceosomes snRNP Length nt Splicing Target U1 165 5 splice U2 189 Branch U4 145 -I f 5 splice recruitment U5 115 1 .