genexppt(gene10基因十)--chapter(已修改)
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【正文】 Chapter 4 The Interrupted Gene Introduction ? interrupted gene – A gene in which the coding sequence is not continuous due to the presence of introns. ? primary (RNA) transcript – The original unmodified RNA product corresponding to a transcription unit. ? RNA splicing – The process of excising introns from RNA and connecting the exons into a continuous mRNA. Introduction ? intron – A segment of DNA that is transcribed, but later removed from within the transcript by splicing together the sequences (exons) on either side of it. ? mature transcript – A modified RNA transcript. Modification may include the removal of intron sequences and alterations to the 5′ and 3′ ends. FIGURE 01: Interrupted genes are expressed via a precursor RNA An Interrupted Gene Consists of Exons and Introns ? Introns are removed by RNA splicing, which occurs in cis in individual RNA molecules. ? Mutations in exons can affect polypeptide sequence。 mutations in introns can affect RNA processing and hence may influence the sequence and/or production of a polypeptide. FIGURE 02: Exons remain in the same order in mRNA as in DNA, but distances along the gene do not correspond Exon and Intron Base Compositions Differ ? The four “rules” for DNA base position are the first and second parity rules, the cluster rule, and the GC rule. ? The second parity rule suggests an extrusion of structured stemloop segments from duplex DNA, which would be greater in introns. ? The rules relate to genomic characteristics, or “pressures,” that constitute the genome phenoty
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