9.4: Transcription
Transcription is the synthesis of RNA from a DNA sequence by RNA polymerase. It is the first step in producing a protein from a gene sequence. Additionally, many other proteins and regulatory sequences are involved in correctly synthesizing messenger RNA (mRNA). Transcriptional regulation is responsible for the differentiation of different types of cells and often for the proper cellular response to environmental signals.
Transcription Can Produce Different Kinds of RNA Molecules
In eukaryotes, the DNA is first transcribed into pre-mRNA, which can be further processed into a mature mRNA to serve as a template for the synthesis of proteins. However, in prokaryotes, RNA translation into polypeptides can begin while the transcription is still ongoing. Transcription can also produce different kinds of RNA molecules that do not code for protein, such as microRNAs (miRNAs), transfer RNA (tRNA), and ribosomal RNA (rRNA)—all of which affect protein synthesis.
Regulation of Transcription Is Central to Development
With few exceptions, all the cells in the human body have the same genetic information, but different cell types differentially regulate transcription during development. Specifically, transcriptional regulation plays a central role in cellular differentiation—the process of producing specialized cells, such as muscle cells, from the less specialized precursor cells. Some genes in the precursor cells must be turned on and others turned off to produce the specialized cells.
This process of cellular differentiation is orchestrated by DNA-binding proteins called transcription factors that control the level of transcription of genes. For example, during early vertebrate development, cells in the ectoderm layer of the developing embryo receive several induction signals from proteins such as BMP, WNT, and SHH. These signals activate transcription factors that turn a host of genes on or off. In this way, transcriptional regulation determines whether ectoderm cells become skin cells or cells of the nervous system.
