Functions: protects mRNa from nuclease attack, longer tail is more stable, recognized by proteins involved in exporting mRNA from the nucleus to the cytoplasm, help ribosomes recognize mRNA as a molecule to be translated b. The Discovery of Introns: sequences within the primary transcript that do not appear in the mature, functional RNA exons: sequences that are destined to appear in the final mRNA (expressed) converting pre-mRNA into mRNA requires specific mechanisms for removing introns IV. Spliceosomes Remove Introns from Pre-mRNA (begins before pre-mRNA transcription is complete) RNA splicing: process of removing introns and rejoining the exons Spliceosomes: catalyzes intron removal, large, molecular complexes of 5 kinds of RNA combined with proteins (assembly well-outlined in last paragraph on page 671) snRNPs(small nuclear ribonucleoproteins) small RNA-protein complex, being process of splicesome assembly, each contains 1-2 small nuclear RNA (snRNA) Figure 21.24 Intron Removal by Spliceosomes (branch-point sequence, U1, U2 and U4/U5, U6, lariat) Exon junction complex (EJC): deposited near the boundary of each exon-exon junction, required for efficient export of mRNA from the nucleus The snRNA molecules present in spiceosomes are directly involved in splicesite recognition, spliceosome assembly, and the catalytic mechanism of splicing V. Some Introns Are Self-Splicing Ribozymes: RNA molecules that function as catalysts in the absence of protein 2 classes of introns in which the intron RNA can function as a ribozyme to catalyze its own removal VI. The Existence of Introns Permits Alternative Splicing and Exon Shuffling Exon shuffling: genetic recombination between introns of different genes produces genes containing new combinations of exons Alternative splicing: certain splice sites are either activated or skipped VII. RNA Editing Allows mRNA Coding Sequences(codons) to Be Altered (insert, remove, chemical later coding sequence of an mRNA) ? can create new start/stop codons or alter the reading frame deamination: C?s converted to U?s (reverse is amination) (leads to G to A conversions in complementary strand) F. Key Aspects of mRNA Metabolism I. Most mRNA Molecules Have a Relatively Short Life Span II. The Existence of mRNA Allows Amplification of Genetic Information
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