Transcript Modification

  The regulation of mRNA synthesis, processing and transport to the cytoplasm all represent crucial control points in the flow of genetic information (Figure 2). Once a gene has been selected to be transcribed into an RNA copy in the nucleus, there are many steps still which regulate its eventual expression as protein in the cytoplasm. First, the single-stranded RNA is modified at both ends. At the 5' end, a special nucleotide structure, called a cap, is added to increase translation efficiency by helping ribosomes to bind to the RNA. At the 3' end, a nuclease recognizes an A/T-rich sequence in the non-coding portion of the transcript, and trims the transcript about 20 bases downstream. The new cleaved 3' end of the transcript is modified by an enzyme that adds a stretch of adenine bases to form a polyA tail, which is thought to stabilize the transcript.