13.11: Transcription

Transcription is the process of synthesizing 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 the proper synthesis of messenger RNA (mRNA). Regulation of transcription is responsible for the differentiation of all the 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 a primary RNA, or pre-mRNA, that can be further processed into a mature mRNA to serve as a template for the synthesis of proteins. In prokaryotes such as bacteria, however, translation of RNA into polypeptides can begin while the transcription is still ongoing, as RNA can be quickly degraded. Transcription can also produce different kinds of RNA molecules that do not code for protein, such as microRNAs, transfer RNA (tRNA), and ribosomal RNA (rRNA)—all of which contribute to protein synthesis.

Regulation of Transcription Is Central to Development

With few exceptions, all of the cells in the human body have the same genetic information in them, from neurons in the brain to muscle cells in the heart. So how do cells assume such diverse forms and functions? To a large extent, the answer lies in the regulation of transcription during the development of the organism. 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 that can determine cellular fate. For example, early during 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 on or off a host of genes. In this way, transcriptional regulation determines whether ectoderm cells become skin cells or cells of the nervous system.

Responding to the Environment Often Requires Transcriptional Changes

Environments are rarely stable for long periods. Consider, for example, the changes in temperature, precipitation, and food availability that a plant is exposed to from day to day and sometimes from hour to hour. In order to function properly, individual organisms must respond to such environmental changes by adjusting key traits, such as their growth rates, immunity, or behavior. These adjustments often require increasing or decreasing the level of transcription of large numbers of genes. For instance, when exposed to drought conditions, Arabidopsis thaliana plants quickly adjust the transcription of hundreds of genes in order to increase root growth and therefore scavenge as much water from the soil as possible.

Transcription is the process of synthesizing RNA from a DNA template.

In the nucleus, the transcription preinitiation complex assembles around the core promoter of a gene, which includes a general transcription factor binding site, here a TATA box, a binding site for the RNA polymerase, and the transcription start site.

Once the necessary components are bound, the preinitiation complex unwinds a short stretch of the DNA upstream of the transcription start site, and RNA polymerase begins producing a new strand of mRNA.

Nucleotides are added one by one, and synthesis of the mRNA occurs in a five prime to three prime direction, reading from the template strand. This newly-created strand of mRNA represents a copy of the information in the coding strand, except the thymidines are replaced by uracils.

Synthesis will continue until a termination sequence is encountered, which will release the newly-made mRNA and allow for further processing.