Editing Biomolecule (section)

===DNA and RNA structure===
{{Main|DNA|Nucleic acid structure}}

DNA structure is dominated by the well-known [[double helix]] formed by Watson-Crick [[base-pair]]ing of C with G and A with T. This is known as [[B-DNA|B-form]] DNA, and is overwhelmingly the most favorable and common state of DNA; its highly specific and stable base-pairing is the basis of reliable genetic information storage. DNA can sometimes occur as single strands (often needing to be stabilized by single-strand binding proteins) or as [[A-DNA|A-form]] or [[Z-DNA|Z-form]] helices, and occasionally in more complex 3D structures such as the crossover at [[Holliday junction]]s during DNA replication.<ref name=Alberts/>

[[Image:Twort groupI intron RNAribbon stereo.jpg|thumb|right|Stereo 3D image of a group I intron ribozyme (PDB file 1Y0Q); gray lines show base pairs; ribbon arrows show double-helix regions, blue to red from 5' to 3'{{definition|date=September 2020}} end; white ribbon is an RNA product.]] 
RNA, in contrast, forms large and complex 3D tertiary structures reminiscent of proteins, as well as the loose single strands with locally folded regions that constitute [[messenger RNA]] molecules. Those RNA structures contain many stretches of A-form double helix, connected into definite 3D arrangements by single-stranded loops, bulges, and junctions.<ref>{{cite book |author=Saenger W |year=1984 |title=Principles of Nucleic Acid Structure |publisher=[[Springer-Verlag]] |isbn=0387907629}}</ref> Examples are tRNA, ribosomes, [[ribozyme]]s, and [[riboswitch]]es. These complex structures are facilitated by the fact that RNA backbone has less local flexibility than DNA but a large set of distinct conformations, apparently because of both positive and negative interactions of the extra OH on the ribose.<ref>{{cite journal |vauthors=Richardson JS, Schneider B, Murray LW, Kapral GJ, Immormino RM, Headd JJ, Richardson DC, Ham D, Hershkovits E, Williams LD, Keating KS, Pyle AM, Micallef D, Westbrook J, Berman HM |year=2008 |title=RNA Backbone: Consensus all-angle conformers and modular string nomenclature |journal=RNA |volume=14 |issue=3 |pages=465–481 |pmc=2248255 |doi=10.1261/rna.657708 |pmid=18192612}}</ref> Structured RNA molecules can do highly specific binding of other molecules and can themselves be recognized specifically; in addition, they can perform enzymatic catalysis (when they are known as "[[ribozyme]]s", as initially discovered by Tom Cech and colleagues).<ref>{{cite journal |vauthors=Kruger K, Grabowski PJ, Zaug AJ, Sands J, Gottschling DE, Cech TR |year=1982 |title=Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena |journal=Cell |volume=31 |issue=1 |pages=147–157 |doi=10.1016/0092-8674(82)90414-7 |pmid=6297745|s2cid=14787080 }}</ref>