Gene expression is the most fundamental process by which the genotype gives rise to the phenotype. Step by step, information from the genes is used to create functional products through 1) the generation of RNA copies of the DNA via a process called transcription, which results in pre-mRNA (pre-messenger RNA) molecules; 2) posttranscriptional RNA modifications that produce different forms of protein-coding mRNAs (messenger RNA) from the same template pre-mRNA molecule, through the action of RNA splicing; 3) protein synthesis, called translation; and 4) posttranslational modifications of the synthesized proteins. Each of these steps is a complex set of interrelated and highly regulated events. In this article we focus on the first step—transcription. At any given moment, a cell is producing a set of RNA molecules (or transcripts), including protein-coding mRNA, ribosomal RNA (rRNA), transfer RNA (tRNA), and non-coding RNA (ncRNA). The specific subset of transcripts present in particular cell types and involved in the cell’s life support, development and specific functioning, is called the transcriptome. The term transcriptome might be applied to the total set of transcripts or to a special type of RNA (such as mRNA) in a given cell population, tissue or organism.
