Gene expression (Transcription)

Gene expression encompasses transcription and translation and the regulation of these processes. RNA Polymerase I Transcription produces the large preribosomal RNA transcript (45S pre-rRNA) that is processed to yield 18S rRNA, 28S rRNA, and 5.8S rRNA, accounting for about half the RNA in a cell. RNA Polymerase II transcription produces messenger RNAs (mRNA) as well as a subset of non-coding RNAs including many small nucleolar RNAs (snRNA) and microRNAs (miRNA). RNA Polymerase III Transcription produces transfer RNAs (tRNA), 5S RNA, 7SL RNA, and U6 snRNA. Transcription from mitochondrial promoters is performed by the mitochondrial RNA polymerase, POLRMT, to yield long transcripts from each DNA strand that are processed to yield 12S rRNA, 16S rRNA, tRNAs, and a few RNAs encoding components of the electron transport chain. Regulation of gene expression can be divided into epigenetic regulation, transcriptional regulation, and post-transcription regulation (comprising translational efficiency and RNA stability). Epigenetic regulation of gene expression is the result of heritable chemical modifications to DNA and DNA-binding proteins such as histones. Epigenetic changes result in altered chromatin complexes that influence transcription. Gene Silencing by RNA mostly occurs post-transcriptionally but can also affect transcription. Small RNAs originating from the genome (miRNAs) or from exogenous RNA (siRNAs) are processed and transferred to the RNA-induced silencing complex (RISC), which interacts with complementary RNA to cause cleavage, translational inhibition, or transcriptional inhibition.

基因表达涵盖了转录、翻译及其调控过程。RNA聚合酶I转录产生大型前核糖体RNA转录本（45S前rRNA），经加工处理得到18S rRNA、28S rRNA和5.8S rRNA，约占细胞中RNA总量的一半。RNA聚合酶II转录产生信使RNA（mRNA）以及包括许多小核仁RNA（snRNA）和微小RNA（miRNA）在内的非编码RNA子集。RNA聚合酶III转录产生转运RNA（tRNA）、5S RNA、7SL RNA和U6 snRNA。线粒体启动子上的转录由线粒体RNA聚合酶POLRMT执行，从每条DNA链产生长转录本，经加工处理得到12S rRNA、16S rRNA、tRNA以及编码电子传递链组成部分的少量RNA。基因表达的调控可分为表观遗传调控、转录调控和转录后调控（包括翻译效率和RNA稳定性）。表观遗传调控的基因表达是DNA及其结合蛋白（如组蛋白）的可遗传化学修饰的结果。表观遗传变化导致染色质复合物的改变，从而影响转录。RNA介导的基因沉默主要发生在转录后，但也可影响转录。源自基因组（miRNA）或外源RNA（siRNA）的小RNA经过加工转移至RNA诱导的沉默复合物（RISC），该复合物与互补RNA相互作用，导致切割、翻译抑制或转录抑制。