Primary transcript: Difference between revisions

Revision as of 03:12, 1 November 2013

Revision as of 03:13, 1 November 2013

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The steps contributing to the production of the primary transcript involves a series of interactions to initiate the [[transcription]] of DNA in 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 initiate the transcription process in a cell's nucleus, DNA double helices are unwound and hydrogen bonds connecting compatible nucleic acids of DNA are broken to produce two unconnected single strands.<ref>{{cite web|last=Strachan|first=T.|title=Human Molecular Genetics 2nd Edition|url=http://www.ncbi.nlm.nih.gov/books/NBK7585/|work=NCBI|publisher=New York: Wiley-Liss}}</ref> The DNA template to be used for transcription of the single-stranded primary transcript mRNA is then bound by an [[RNA polymerase]] at the [[promoter]] region.<ref>{{cite web|last=Alberts|first=B.|title=Molecular Biology of the Cell 3rd Edition|url=http://www.ncbi.nlm.nih.gov/books/NBK28319/|work=NCBI|publisher=New York: Garland Science}}</ref>

In eukaryotes, three kinds of RNA—[[rRNA]], [[tRNA]], and mRNA—may bemRNA—are produced based on the activity of three distinct RNA polymerases, whereas, in prokaryotes, only one RNA polymerase exists to create any and all kinds of RNA molecules.<ref>{{cite web|last=Griffiths|first=AJF|title=An Introduction to Genetic Analysis|url=http://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 mRNA from the [[antisense]] DNA template in the 5' to 3' direction, and the newly synthesized mRNA is complimentary to this template strand of DNA.<ref>{{cite web|last=Strachan|first=T.|title=Human Molecular Genetics 2nd Edition|url=http://www.ncbi.nlm.nih.gov/books/NBK7585/|work=NCBI|publisher=New York: Wiley-Liss}}</ref> RNA polymerase II constructs the primary transcript using a set of four specific ribonucleoside monophosphate residues (AMP, CMP, GMP, and UMP) that are added continuously to the 3' hydroxyl group on the 3' end of the growing mRNA.<ref>{{cite web|last=Strachan|first=T.|title=Human Molecular Genetics 2nd Edition|url=http://www.ncbi.nlm.nih.gov/books/NBK7585/|work=NCBI|publisher=New York: Wiley-Liss}}</ref>

Studies of primary transcripts produced by RNA polymerase II reveal that a given primary transcript averages 7000 nucleotides in length, some growing as long as 20,000 nucleotides in length.<ref>{{cite web|last=Alberts|first=B.|title=Molecular Biology of the Cell 3rd Edition|url=http://www.ncbi.nlm.nih.gov/books/NBK28319/|work=NCBI|publisher=New York: Garland Science}}</ref> Inclusion of the [[exon]] ''and'' [[intron]] components within primary transcript sequences explain for the extreme size disparity between larger primary transcripts and smaller, modified mRNA ready for translation into protein.