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Chapter 15. MECHANISM OF GENE TRANSCRIPTION AND PROTEIN SYNTHESIS Genomic DNA (gDNA) consists largely of non-coding stretches, the function of much of which is still poorly understood. Genes, per se , are interspersed throughout the genome and each is associated with controlling domains ( promoter regions ) that serve to regulate the expression of each and every gene. There is no need for excessive description for the present purposes. The gene itself usually consists of coding sequences (these will directly specify the order by which amino acids are assembled into the gene product) called exons and non- coding portions ( introns ) that often "break" the overall coding region into a few to many separated parts (Figure 15-1). The whole gene (introns plus exons) is initially transcribed into a "primary" RNA molecule, usually (not exclusively) from the coding strand of the DNA. While the initial RNA molecule is being synthesized from the DNA template, the 5' end of the transcript is first slightly modified, to form a "cap" structure, then intron-based portions are excised. The remainder, corresponding only to the exons, is then spliced together. At the completion of transcription, a string of A s (usually a few hundred) is added to the 3' end as a poly(A) tail. This mature form of the transcript (Figure 15-1) passes from the nucleus to the cytoplasm and is in suitable form for use in protein synthesis. Figure 15-1. The “maturation” of messenger RNA. This summary includes many aspects outlined in the text. At the top, the primary mRNA product is generated off the coding strand of DNA. The blue segments correspond to introns, while purple segments correspond to exons. The 5’ end is modified to form the cap structure (brown) then the poly(A) tail is added at the 3’ end (green). Finally, the intronic portions (blue) are eliminated by splicing, resulting in the “mature” mRNA. 2007 version – page 106
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Figure 15-2. (Provisional) Interactions between the transcriptional machinery, transcriptional regulators and segments of DNA upstream of the gene. The gene to be transcribed is shown in dark yellow and the transcriptional machinery is shown closely associated with the promoter for that gene. Further regulatory control is achieved by proteins binding to pieces of the DNA and to specific components of the complex that actually generates the RNA copy. From: Alberts et. al. (loc. cit). Control of precisely which genes are transcribed in particular tissues at any instant is determined by a host of transcription controls that operate on the promoter region for that gene. Details of transcriptional control are way beyond the scope of this text but some aspects need to be mentioned. The promoter regions can be huge, often larger than the genes themselves. Within these regions, specific segments of
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