9.4:
Transcription
Transcription is the process of synthesizing RNA from a DNA template.
On the template DNA, the transcription pre-initiation complex assembles around the core promoter of a gene. The promoter includes the TATA box and an Initiator sequence. The general transcription factors bind at the TATA box, while the initiator sequence contains 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. After that, the general transcription factors get dissociated from the strand and the 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, except the thymidines are replaced by uridines.
This newly-created mRNA strand represents a copy of the information in the coding strand. The synthesis will continue until a termination sequence is encountered, which will release the newly-made mRNA.
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.