Primary transcript: Difference between revisions

Revision as of 16:12, 24 July 2024

Revision as of 16:33, 24 July 2024

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In eukaryotes, three kinds of RNA—[[rRNA]], [[tRNA]], and mRNA—are produced based on the activity of three distinct RNA polymerases, whereas, in [[prokaryotes]], only one RNA polymerase exists to create all kinds of RNA molecules.<ref>{{cite web| vauthors = Griffiths AJ |title=An Introduction to Genetic Analysis |url= https://www.ncbi.nlm.nih.gov/books/NBK21853/|work=NCBI|publisher=New York: W.H. Freeman}}</ref> RNA polymerase II of eukaryotes transcribes the primary transcript, a transcript destined to be processed into mRNA, from the [[antisense]] DNA template in the 5' to 3' direction, and this newly synthesized primary transcript is complementary to the antisense strand of DNA.<ref name="StrachanRead2004" /> RNA polymerase II constructs the primary transcript using a set of four specific [[ribonucleoside]] monophosphate residues ([[adenosine monophosphate]] (AMP), [[cytidine monophosphate]] (CMP), [[guanosine monophosphate]] (GMP), and [[uridine monophosphate]] (UMP)) that are added continuously to the 3' hydroxyl group on the 3' end of the growing mRNA.<ref name="StrachanRead2004" />

Studies of primary transcripts produced by RNA polymerase II reveal that an average primary transcript is 7,000 [[nucleotide]]s in length, with some growing as long as 20,000 nucleotides in length.<ref name="Alberts3rd"/> The inclusion of both [[exon]] and [[intron]] sequences within primary transcripts explains the size difference between larger primary transcripts and smaller, mature mRNA ready for translation into protein.{{cn|date=July 2024}}

===Regulation===

A number of factors contribute to the activation and inhibition of transcription and therefore regulate the production of primary transcripts from a given DNA template.{{cn|date=July 2024}}

'''Activation''' of RNA polymerase activity to produce primary transcripts is often controlled by sequences of DNA called [[enhancers]]. [[Transcription factors]], proteins that bind to DNA elements to either activate or repress transcription, bind to enhancers and recruit enzymes that alter [[nucleosome]] components, causing DNA to be either more or less accessible to RNA polymerase. The unique combinations of either activating or inhibiting transcription factors that bind to enhancer DNA regions determine whether or not the gene that enhancer interacts with is activated for transcription or not.<ref name="Gilbert2013">{{cite book|author=Scott F. Gilbert|title=Developmental Biology|url=https://books.google.com/books?id=-e_bmgEACAAJ|date=15 July 2013|publisher=Sinauer Associates, Incorporated|isbn=978-1-60535-173-5|pages=38–39, 50}}</ref> Activation of transcription depends on whether or not the transcription elongation complex, itself consisting of a variety of transcription factors, can induce RNA polymerase to dissociate from the [[Mediator (coactivator)|Mediator]] complex that connects an enhancer region to the promoter.<ref name="Gilbert2013" /> [[File:Role of transcription factor in gene expression regulation.svg|thumb|Role of transcription factors and enhancers in gene expression regulation]]

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==RNA processing==

{{Main|Post-transcriptional modification}}

Transcription, a highly regulated phase in gene expression, produces primary transcripts. However, transcription is only the first step which should be followed by many modifications that yield functional forms of RNAs.<ref name="Cooper GM">{{cite book| vauthors = Cooper GM |title=The Cell: A Molecular Approach |chapter=RNA Processing and Turnover |date=2000 | edition = 2nd |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK9864/ |publisher= Sunderland (MA): Sinauer Associates; 2000}}</ref> Otherwise stated, the newly synthesized primary transcripts are modified in several ways to be converted to their mature, functional forms to produce different proteins and RNAs such as mRNA, tRNA, and rRNA.{{cn|date=July 2024}}

===Processing===

The basic primary transcript modification process is similar for tRNA and rRNA in both eukaryotic and prokaryotic cells. On the other hand, primary transcript processing varies in mRNAs of prokaryotic and eukaryotic cells.<ref name="Cooper GM"/> For example, some prokaryotic bacterial mRNAs serve as templates for synthesis of proteins at the same time they are being produced via transcription. Alternatively, pre-mRNA of eukaryotic cells undergo a wide range of modifications prior to their transport from the nucleus to cytoplasm where their mature forms are translated.<ref name="Cooper GM"/> These modifications are responsible for the different types of encoded messages that lead to translation of various types of products. Furthermore, primary transcript processing provides a control for gene expression as well as a regulatory mechanism for the degradation rates of mRNAs. The processing of pre-mRNA in eukaryotic cells includes [[5' cap]]ping, [[polyadenylation|3' polyadenylation]], and [[alternative splicing]].{{cn|date=July 2024}}

===5' capping===