转录

Transcription is the process of synthesizing RNA from a DNA template. In the nucleus, the transcription preinitiation complex assembles around the core promoter, which includes a TATA box, an RNA polymerase binding site, and the transcription start site. Once components bind, the preinitiation complex unwinds DNA upstream of the start site, and RNA polymerase begins producing mRNA.

RNA polymerase adds nucleotides one by one to synthesize mRNA in the five prime to three prime direction, reading from the DNA template strand. The newly created mRNA represents a copy of the coding strand information, except thymidines are replaced by uracils. Synthesis continues until a termination sequence is encountered, releasing the mRNA for further processing.

Transcription produces different kinds of RNA molecules beyond messenger RNA. In eukaryotes, DNA is transcribed into pre-mRNA that becomes mature mRNA for protein synthesis. Transcription also produces microRNAs, transfer RNA (tRNA), and ribosomal RNA (rRNA), all of which contribute to protein synthesis in cells.

Transcriptional regulation determines cellular differentiation by controlling which genes are turned on or off in specialized cells. DNA-binding proteins called transcription factors orchestrate this process, activating or repressing genes that determine cellular fate. For example, during vertebrate development, induction signals activate transcription factors that direct ectoderm cells to become either skin cells or nervous system cells.

Organisms must respond to environmental changes such as temperature, precipitation, and food availability by adjusting transcription levels of large numbers of genes. These adjustments modify key traits like growth rates, immunity, and behavior. For instance, plants exposed to drought quickly adjust transcription of hundreds of genes to increase root growth and scavenge water from soil.

Transcription is the first step in producing a protein from a gene sequence. RNA polymerase synthesizes mRNA from the DNA template, and this mRNA serves as the genetic instructions for protein synthesis. In prokaryotes, translation can begin while transcription is ongoing, but in eukaryotes, the pre-mRNA undergoes processing before translation occurs.

Although all cells contain the same genetic information, they assume diverse forms and functions through regulation of transcription during development. Transcriptional regulation controls which genes are expressed in each cell type, determining whether a precursor cell becomes a neuron, muscle cell, or other specialized cell. This selective gene expression orchestrated by transcription factors enables cellular differentiation.