[基因Ⅸ(基因9).本杰

[基因Ⅸ(基因9).本杰

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The Interrupted Gene CHAPTER OUTLINE Introduction .. - An Interrupted Gene Consists of Exons and Introns • Introns are removed by the process of RNA splicing, which occurs only in cis on an individual RNA molecule. .. • Only mutations in exons can affect protein sequence; how- ever, mutations in introns can affect processing of the RNA and therefore prevent production of protein . Restriction Endonucleases Are a Key Tool in Mapping DNA Restriction endonucleases can be used to cleave DNA into defined fragments. • Amap can be generated by using the overlaps between the fragments generated by different restriction enzymes. Organization of Interrupted Genes May Be Conserved • Introns can be detected by the presence of additional regions when genes are compared with their RNA products by restriction mapping or electron microscopy. The ultimate definition, though, is based on comparison of sequences. The positions of introns are usually conserved when homol- ogous genes are compared betwe,en different organisms. The lengths of the corresponding introns may vary greatly. • Introns usually do not code for proteins. Exon Sequences Are but Introns Vary Comparisons of related genes in different species show that the sequences of the corresponding exons are usually con- served but the sequences of the introns are much less well related. • Introns evolve much more rapidly than exons because of the lack of selective pressure to produce a protei n with a useful sequence. Genes Show a Wide Distribution of Sizes Most genes are uninterrupted in yeasts, but are interrupted in higher eukaryotes. Exons are usually short, typically coding for <100 amino acids. • Introns are short in lower eukaryotes, but range up to sev- erall0s of kb in length in higher eukaryotes. The overall length of a gene is determi ned largely by its introns. .. Some DNA Sequences Code for More Than One Protein The use of alternative initiation or termination codons allows two protei ns to be generated where one is equivalent to a fragment of the other. Nonhomologous protein sequences can be produced from the same sequence of DNA when it is read in different read- ing frames by two (overlapping) genes. Homologous proteins that differ by the presence or absence of certain regions can be generated by differential (alterna- tive) splicing when certain exons are included or excluded. This may take the form of including or excluding individual exons or of choosing between alternative exons. How Did Interrupted Genes Evolve? The major evolutionary question is whether genes origi- nated as sequences interrupted by introns or whether they were originally uninterrupted. Most protein-coding genes probably originated in an inter- rupted form, but interrupted genes that code for RNA may have originally been uninterrupted .
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