Transcription

Francis Crick's central dogma of molecular biology summarises the normal direction genetic information flows within a cell: DNA is transcribed into RNA (transcription), and that RNA is in turn translated into protein (translation); the one well-known exception is a group of RNA-genome viruses called retroviruses, such as HIV, whose reverse transcriptase enzyme runs transcription backwards, synthesising DNA from an RNA template, before the usual DNA-to-RNA-to-protein flow resumes. Transcription itself is the process of copying genetic information from one strand of a DNA molecule into a complementary RNA strand, carried out by the enzyme DNA-dependent RNA polymerase; because a gene's DNA double helix must first be prised apart, even if only temporarily and locally, RNA polymerase begins by binding to DNA at the very start of a gene and opening up the double helix there, after which it synthesises an RNA molecule whose nucleotide sequence is complementary to the single DNA strand it is reading, called the template strand. Only one of the gene's two DNA strands is ever used as a template, for two compelling reasons. First, if both strands were transcribed simultaneously, they would each generate an RNA with a different sequence, which would in turn direct the synthesis of two entirely different proteins from what is meant to be a single stretch of DNA -- an outcome that would hopelessly complicate how genetic information is organised and read. Second, if two RNA molecules were made si …