# Transcription --- **Transcription** describes the process of getting the information that is coded in [[DNA]] onto [[mRNA]]. It is something that happens in every organism, as a part of the [[central dogma of biology]]. ## Process First, [[RNA polymerase]] reads the "template strand". The other strand is the non-template strand, or "coding strand". Like DNA polymerase, this goes in 5' -> 3' direction, but unlike the DNA version, it does not require a primer. ![[transcription.png]] ## Regions of DNA ### Nucleotide 0 **Nucleotide 0** is the nucleotide that marks the end of the [[promoter]] region and after which begins the [[gene]] itself. ![[upstream downstream.png]] ### Promoter A **[[promoter]]** region is the site on the DNA that tells [[RNA polymerase]] where to start transcription. In eukaryotes there is one promoter per gene, but in prokaryotes they may have one promoter per [[prokaryotic DNA#Operons|operon]]. ### Gene The **[[gene]]** is the segment of DNA that actually codes for the protein or whatever else it wants to make. In eukaryotes there are sections of unneeded sections called [[RNA splicing#Introns and Exons|exons]] that are spliced out during RNA processing. ### Terminator The **[[terminator]]** region signals for the RNA polymerase to end transcription. It is a non-coding sequence for a segment of RNA that will form a [[palindromic sequence|hairpin loop]] onto itself, forcing the transcription to end. ### (Eukaryotes Only) Enhancers **[[promoter#Enhancers|enhancer]]** regions may be quite far from their associated gene, but they can help promoters work. ## Transcription in Eukaryotes Each strand of mRNA codes for one gene, making it [[mRNA#Polycistronic vs Monocistronic|monocistronic]]. Additionally, as the DNA lives in the nucleus and the ribosomes live in the cytoplasm, mRNA needs to undergo [[mRNA#mRNA Processing|mRNA]] before they can make the journey. During this step, [[RNA splicing#Introns and Exons|unneeded sections]] of DNA are spliced out as well. ## Transcription in Prokaryotes Prokaryotes have to be very efficient with their DNA. Since some mRNA codes for entire [[prokaryotic DNA#Operons|operons]] at a time, some prokaryotic mRNA is [[mRNA#Polycistronic vs Monocistronic|polycistronic]]. The genome is also very compact in terms of having no "junk" DNA that needs to be spliced out, like with eukaryotes. Additionally, since there are no organelles to move about out of and in to, not only does the mRNA not need to be processed, but [[translation]] can and does start pretty much right away. ___