Content deleted Content added
consistent citation formatting |
consistent citation formatting |
||
Line 11:
{{main|Transcription (genetics)}}
The steps contributing to the production of primary transcripts involve a series of molecular interactions that initiate transcription of DNA within a cell's nucleus. Based on the needs of a given cell, certain DNA sequences are transcribed to produce a variety of RNA products to be translated into functional proteins for cellular use. To initiate the transcription process in a cell's nucleus, DNA double helices are unwound and [[hydrogen bond]]s connecting compatible nucleic acids of DNA are broken to produce two unconnected single DNA strands.<ref name="StrachanRead2004">{{cite book|author1=T. Strachan|author2=Andrew P. Read|title=Human Molecular Genetics 3|url=https://books.google.com/books?id=g4hC63UrPbUC|date=January 2004|publisher=Garland Science|isbn=978-0-8153-4184-0|pages=16–17}}</ref> One strand of the DNA template is used for transcription of the single-stranded primary transcript mRNA. This DNA strand is bound by an [[RNA polymerase]] at the [[promoter (genetics)|promoter]] region of the DNA.<ref name="Alberts3rd">{{cite web|
[[File:Transcription.jpg|thumb|Transcription of DNA by RNA polymerase to produce primary transcript]]
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|
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.
Line 23:
'''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]]
'''Inhibition''' of RNA polymerase activity can also be regulated by DNA sequences called [[silencer (DNA)|silencer]]s. Like enhancers, silencers may be located at locations farther up or downstream from the genes they regulate. These DNA sequences bind to factors that contribute to the destabilization of the initiation complex required to activate RNA polymerase, and therefore inhibit transcription.<ref>{{cite web|
[[Histone]] modification by transcription factors is another key regulatory factor for transcription by RNA polymerase. In general, factors that lead to histone [[acetylation]] activate transcription while factors that lead to histone [[deacetylation]] inhibit transcription.<ref name="Lodish2008">{{cite book|author=Harvey Lodish|title=Molecular Cell Biology|url=https://books.google.com/books?id=K3JbjG1JiUMC|year=2008|publisher=W. H. Freeman|isbn=978-0-7167-7601-7|pages=303–306}}</ref> Acetylation of histones induces repulsion between negative components within nucleosomes, allowing for RNA polymerase access. Deacetylation of histones stabilizes tightly coiled nucleosomes, inhibiting RNA polymerase access. In addition to acetylation patterns of histones, methylation patterns at promoter regions of DNA can regulate RNA polymerase access to a given template. RNA polymerase is often incapable of synthesizing a primary transcript if the targeted gene's promoter region contains specific methylated cytosines— residues that hinder binding of transcription-activating factors and recruit other enzymes to stabilize a tightly bound nucleosome structure, excluding access to RNA polymerase and preventing the production of primary transcripts.<ref name="Gilbert2013" />
Line 32:
==RNA processing==
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 web|
===Processing===
| |||