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The Interrupted Gene © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT for SALE OR DISTRIBUTION NOT for SALE OR DISTRIBUTION 12658_CH04_093_111.indd Page 93 8/23/11 5:40 PM user f-403 F-403 4 © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC A self-splicing© Jones intron & Bartlett in an rRNA Learning, of the LLC NOT FOR SALE OR DISTRIBUTION largeNOT ribosomal FOR subunit. SALE © KennethOR DISTRIBUTION Eward/Photo Researchers, Inc. The Interrupted Gene © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION CHAPTER OUTLINE © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC 4.1 Introduction 4.6 Some DNA Sequences Encode More Than One NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 4.2 An Interrupted Gene Consists of Exons Polypeptide and Introns 4.7 Some Exons Can Be Equated with Protein 4.3 Organization of Interrupted Genes May Functional Domains Be Conserved 4.8 Members of a Gene Family Have a Common © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Organization MethodsNOT and Techniques: FOR SALE The OR Discovery DISTRIBUTION of NOT FOR SALE OR DISTRIBUTION Introns by DNA-RNA Hybridization 4.9 Summary 4.4 Exon Sequences Are Usually Conserved but, Introns Vary 4.5© Jones Genes & Show Bartlett a Wide Learning, Distribution LLC of Sizes © Jones & Bartlett Learning, LLC NOT DueFOR Primarily SALE toOR Intron DISTRIBUTION Size and Number NOT FOR SALE OR DISTRIBUTION Variation © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 93 © Jones and Bartlett Publishers. NOT FOR SALE OR DISTRIBUTION 12658_CH04_093_111.indd Page 94 8/16/11 5:09 PM user f-400 F-400 © Jones & Bartlett Learning, LLC4.1 Introduction© Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION The simplest form of a geneNOT is aFOR length SALE of DNA OR that DISTRIBUTION directly corresponds to its poly- peptide product. Bacterial genes are almost always of this type, in which a continuous coding sequence of 3 N base pairs encodes a polypeptide of N amino acids. In eukary- otes, however, a gene may include additional sequences that lie within the coding region and interrupt the sequence that encodes the polypeptide. These sequences © Jonesare &removed Bartlett from Learning, the RNA LLCproduct following transcription,© Jones generating & Bartlett an mRNA Learning, LLC NOT FORthat includes SALE aOR nucleotide DISTRIBUTION sequence exactly correspondingNOT to the FOR polypeptide SALE product OR DISTRIBUTION according to the rules of the genetic code. interrupted gene A gene in The sequences of DNA comprising an interrupted gene are divided into the two which the coding sequence categories depicted in FIGURE 4.1 . is not continuous due to the • Exons are the sequences retained in the mature RNA product. By definition, presence of introns. © Jones & Bartlett Learning,a gene LLC begins and ends with exons© Jonesthat correspond & Bartlett to the Learning, 59 and 39 endsLLC of exon Any segment of an the RNA. interruptedNOT gene FOR that SALEis rep- OR DISTRIBUTION • Introns are the intervening sequencesNOT FORthat are SALE removed OR whenDISTRIBUTION the primary resented in the mature RNA transcript is processed to give the mature RNA product. product. The exon sequences are in the same order in the gene and in the RNA, but an intron A segment of DNA interrupted gene is longer than its final RNA product because of the presence of the that is transcribed but later introns. © Jonesremoved & Bartlett from within Learning, the LLC © Jones & Bartlett Learning, LLC transcript by splicing together The processing of interrupted genes requires an additional step that is not needed NOT FORthe sequencesSALE OR (exons) DISTRIBUTION on for uninterrupted genes.NOT The DNA FOR of SALE an interrupted OR DISTRIBUTION gene is transcribed to an RNA either side of it. copy (a transcript) that is exactly complementary to the original gene sequence. This RNA is only a precursor, though; it cannot yet be used to produce a polypep- tide. First, the introns must be removed from the RNA to give a messenger RNA RNA splicing The process that consists only of the series of exons. This process is called RNA splicing (see of excising introns from RNA© Jones Section & Bartlett2.10, Several Learning, Processes Are LLC Required to Express the Product© Jones of a Gene & )Bartlett and involves Learning, LLC and connecting the exons into NOT FORprecisely SALE deleting OR DISTRIBUTIONthe introns from the primary transcriptNOT and thenFOR joining SALE the OR ends DISTRIBUTION a continuous mRNA. of the RNA on either side to form a covalently intact molecule (see Chapter 21, RNA Splicing and Processing). The original gene comprises the region in the genome between the points cor- responding to the 59 and 39 terminal bases of the mature mRNA. We know that tran- © Jones & Bartlett scriptionLearning, starts LLC at the DNA template corresponding© Jones &to Bartlettthe 59 end Learning, of the mRNA LLC and NOT FOR SALE ORusually DISTRIBUTION extends beyond the complementNOT to the FOR 39 end SALE of the OR mature DISTRIBUTION mRNA, which is generated by cleavage of the primary RNA transcript (see Section 21.14, The 39 Ends of mRNAs Are Generated by Cleavage and Polyadenylation ). The gene is also considered to include the regulatory regions on both sides of the gene that are required for initiating and (sometimes) terminating transcription. © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR FIGURE SALE4.1 Interrupted OR DISTRIBUTION genes are expressed via a precursor RNA. IntronsNOT FOR SALEEXON 1 OR DISTRIBUTIONEXON 2 EXON 3 are removed when the exons are spliced together. The mature mRNA has DNA INTRON 1 INTRON 2 only the sequences of the exons. RNA synthesis pre-mRNA © Jones & Bartlett Learning, LLC INTRON 1 © JonesINTRON & Bartlett 2 Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FORSplicing SALE removes OR DISTRIBUTION introns mRNA Protein synthesis © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION Protein NOT FOR SALE OR DISTRIBUTION Length of precursor RNA (not mRNA) defines region of gene © Jones & Bartlett Learning, LLC © Jones & BartlettIndividual coding Learning, regions are LLCseparated in gene NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 94 CHAPTER 4 The Interrupted Gene © Jones and Bartlett Publishers. NOT FOR SALE OR DISTRIBUTION 12658_CH04_093_111.indd Page 95 8/16/11 5:09 PM user f-400 F-400 © Jones &CONCEPT Bartlett AND REASONING Learning, CHECK LLC © Jones & Bartlett Learning, LLC Why might it be difficult to express an intact human gene inserted into a bacterial cell? NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 4.2 An Interrupted Gene Consists © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC of ExonsNOT and FOR Introns SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION How does the existence of introns change our view of the gene? During splicing, the exons are always joined together in the same order they are found in the orig- inal DNA, so the correspondence between the gene and polypeptide sequences is maintained. FIGURE 4.2 shows that the order of exons in the gene remains the same© as Jones the order & Bartlett of exons Learning, in the processed LLC mRNA, but the distances© Jones between & Bartlett Learning, LLC sites NOTin the FOR gene SALE (as determined OR DISTRIBUTION by recombination analysis) do notNOT correspond FOR SALE OR DISTRIBUTION to the distances between sites in the processed mRNA. The length of the gene is defined by the length of the primary RNA transcript instead of the length of the processed mRNA. All the exons are present in the primary RNA transcript, and their splicing © Jones &together Bartlett occurs Learning, only as anLLC intramolecular reaction.© There Jones is usually & Bartlett no joining Learning, of LLC NOT FORexons SALE carried OR DISTRIBUTIONby different RNA transcripts, so the splicingNOT mechanismFOR SALE excludes OR DISTRIBUTION any splicing together of sequences from different alleles. (However, in a phenomenon known as trans -splicing, sequences from different mRNAs are ligated together into a single molecule for translation.) Mutations located in different exons of a gene cannot complement one another, so they continue to be defined as members of the same complementation© Jones group. & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Mutations that directlyNOT FORaffect theSALE sequence OR DISTRIBUTION of a polypeptide must occur in exons. NOT FOR SALE OR DISTRIBUTION What are the effects of mutations in the introns? The introns are not part of the pro- cessed messenger RNA, so mutations in them cannot directly affect the polypeptide sequence. However, they may affect the processing of the messenger RNA by inhibit- ing the splicing together of exons. A mutation of this sort acts only on the allele that carries© it.Jones As a result, & Bartlett it fails Learning,to complement LLC any other mutation in that© Jones allele and & Bartlettis Learning, LLC part ofNOT the sameFOR complementation SALE OR DISTRIBUTION group as the exons. NOT FOR SALE OR DISTRIBUTION Mutations that affect splicing are usually deleterious. The majority are single-base substitutions at the junctions between introns and exons. They may cause an exon to be left out of the product, cause an intron to be included, or make splicing occur at a different site. The most common outcome is a termination codon that truncates the © Jones &polypeptide Bartlett sequence.Learning, About LLC 15% of the point mutations© Jones that cause & Bartlett human diseasesLearning, LLC NOT FORdisrupt SALE splicing.
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