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Transcribe the following strand of DNA into mRNA AP Lesson #50 EQ: After transcription has occurred, does mRNA need to be modified? RNA polymerase TGGTACAGCTACTCATCGTACCGA 3’ DNA Template Strand 5’ After transcription, do prokaryotes need to modify their mRNA? • NO! – Protein coding gene is colinear with the mRNA – mRNA is ready to be translated into a protein Why would we need to modify mRNA? • Eukaryotic genes are not continuous – exons = the real gene • expressed / coding DNA – introns = the junk (noncoding) • inbetween sequence What about eukaryotes? • Need to be modified – Eukaryotic genes have junk DNA! – Average gene contains 27,000 bases, of which about 1,200 bases actually code for proteins How do we get rid of that junk? • mRNA splicing – Turn 27,000 bases into 1,200 bases – Create a primary transcript = pre-mRNA – mRNA splicing - edit out introns – make mature mRNA transcript intron = noncoding (inbetween) sequence eukaryotic DNA intron = noncoding (inbetween) sequence ~10,000 bases eukaryotic DNA exon = coding (expressed) sequence exon = coding (expressed) sequence primary mRNA transcript mature mRNA transcript ~1,000 bases spliced mRNA What molecules help to cut out the junk? • snRNPs – small nuclear RNA proteins – Recognize inton/exon and attach exon • Spliceosome snRNPs snRNA intron exon 5' 3' – several snRNPs join – splice site sequence • cut & paste gene • Remove introns and paste together exons spliceosome 5' mature mRNA lariat exon 5' exon excised intron 3' • Need to protect mRNA on its trip from nucleus to cytoplasm – Enzymes in cytoplasm attack mRNA (nucleases nucleases) • protect the ends of the molecule • add 5′ GTP cap (ribosomal recognition site) • add poly-A tail –longer tail, mRNA lasts longer: produces more protein because it won’t get broken down as il quickly A ta 3' oly3' p 5 5' mRNA – Alternative mRNA splicing – Allows a single gene to possibly create more than one protein 3' What is left to do after splicing? p ' ca • A single gene can code for more than one protein • It depends on which segment is treated as an exon • Exon Shuffling (creates variation) 3' • Remaining exons form a 5' continuous coding sequence If we just cut them out anyways, why are introns important? A A A 50-2 A What does a cell do with the mature mRNA? • Mature mRNA exits the nucleus through the nuclear pores • mRNA enters the cytoplasm where it will go through translation Nuclear envelope DNA TRANSCRIPTION RNA PROCESSING Pre-mRNA mRNA TRANSLATION Ribosome A Polypeptide ’s 50 A P P G P What happens if snRNPs screw up? • A single base added or lost throws off the reading frame – Section of mRNA (codons) that will be translated • One change in the mature mRNA could change the protein AUGCGGCTATGGGUCCGAUAAGGGCCAU AUGCGGUCCGAUAAGGGCCAU AUG|CGG|UCC|GAU|AAG|GGC|CAU Met|Arg|Ser|Asp|Lys|Gly|His AUGCGGCTATGGGUCCGAUAAGGGCCAU AUGCGGGUCCGAUAAGGGCCAU AUG|CGG|GUC|CGA|UAA|GGG|CCA|U Met|Arg|Val|Arg|STOP| Summarizing Strategy • Quick Write: – Explain the difference between prokaryotic and eukaryotic mRNA modification and why it must occur. Assessment • HW: Chapter 17.1 thru 17.3 Wkst & Read Section 17.4 (Pgs. 337 – 344)