Transcription

  The first step in the expression of genetic information is transcription, which serves to carry the information out of the nucleus, since synthesis of the corresponding protein occurs in the cytoplasm. In this process, a template of the gene to be expressed, called messenger RNA (mRNA), is produced in the nucleus by a specialized RNA polymerase enzyme which copies, base by base, a discrete stretch of sequence complementary to one of the two DNA strands (the "antisense" strand), thereby generating an exact copy of the other strand (the "sense" strand, Figure 1, bottom). RNA structure differs only slightly from that of DNA, in one of its bases (uracil replaces thymine), and in the composition of the sugar-phosphate component (ribose replaces deoxyribose). The ribose component makes mRNA molecules inherently more susceptible to degradation than the deoxyribose-based backbone in DNA. This allows for rapid shifts in the population of specific RNA molecules sent to the cytoplasm for protein production, to direct different cellular functions or to respond to changing extracellular signals.