R-loop
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Background
An R-loop is a loop of single-stranded DNA resulting from a lack of complementary mRNA in a DNA-mature mRNA hybrid. An R-loop forms when mature mRNA is hybridized with double-stranded DNA under conditions favoring the formation of a DNA-RNA hybrid. The DNA regions coding introns form single-stranded loops because they cannot hybridize with the mRNA. This occurs because the intron regions have been excised from the mRNA through splicing. Once the hybrid DNA-mRNA is formed, it can be visualized through electron microscopy.
History
R-looping was first described in 1976.[1] Independent R-looping studies from the laboratories of Richard J. Roberts and Phillip A. Sharp showed that protein coding adenovirus genes contained DNA sequences that were not present in the mature mRNA.[2][3] Roberts and Sharp were awarded the Nobel Prize in 1993 for independently discovering introns. After their discovery in adenovirus, introns were found in a number of eukaryotic genes such as the eukaryotic ovalbumin gene (reference for Pierre Chambon’s paper)[4], hexon DNA[5], and extrachromosomal rRNA genes of Tetrahymena thermophila.[6]
R-loop Mapping
R-loop mapping is a laboratory technique used to distinguish introns from exons in double-stranded DNA.[7]. These R-loops are visualized by electron microscopy and reveal the intron regions of DNA because they do not match the spliced mRNA product.[8]
References
- ^ Thomas M, White RL, Davis RW. “Hybridization of RNA to double-stranded DNA: formation of R-loops”. Proc. Natl Acad. Sci. USA. 1976; 73:2294–2298.
- ^ Berget SM, Moore C, Sharp PA. “Spliced segments at the 5′ terminus of adenovirus 2 late mRNA”. Proc Natl Acad Sci USA. 1977; 8:3171–3175. doi: 10.1073/pnas.74.8.3171.
- ^ Chow LT, Gelinas RE, Broker TR, Roberts RJ. “An amazing sequence arrangement at the 5' ends of adenovirus 2 messenger RNA”. Cell. 1977; 12(1):1-8.
- ^ O’Hare K, Breathnach R, Benoist C, Chambon P (1979). “No more than seven interruptions in the ovalbumin gene: comparison of genomic and double-stranded cDNA sequences.” Oxford Journals, 7(2): 321-334.
- ^ Berget SM, Moore C, Sharp PA. “Spliced segments at the 5′ terminus of adenovirus 2 late mRNA”. Proc Natl Acad Sci USA. 1977; 8:3171–3175. doi: 10.1073/pnas.74.8.3171.
- ^ Cech TR and Rio DC (1979). “Localization of transcribed regions on extrachromosomal ribosomal RNA genes of Tetrahymena thermophila by R-loop mapping.” Proc Natl Acad Sci USA. 76, (10): 5051-5055.
- ^ Woolford, John L., Jr.; Rosbash, Michael (1979). (The use of R-looping for structural gene identification and mRNA purification (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC327867/pdf/nar00448-0111.pdf)”. Nucleic Acids Research. 6 (7):2483-97. PMC327867
- ^ King RC, Stansfield WD, Mulligan PK (2007). “A dictionary of genetics.” Oxford University Press 7.