Transcription initiation is controlled by proteins that interact with DNA and with each other. Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more available or less available for transcription. A cluster of proteins called a transcription initiation complex assembles on the promoter sequence at the upstream end of the gene. One component, RNA polymerase II, transcribes the gene, synthesizing a primary RNA transcript or pre-mRNA. RNA processing includes enzymatic addition of a 5 ′ cap and a poly-A tail, as well as splicing out of introns to yield a mature mRNA. Multiple control elements are associated with most eukaryotic genes. ○ Control elements are noncoding DNA segments that regulate transcription by binding certain proteins.
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