Editing Nucleic acid (section)

==Topology==
Double-stranded nucleic acids are made up of complementary sequences, in which extensive [[base pair|Watson-Crick base pairing]] results in a highly repeated and quite uniform nucleic acid [[double helix|double-helical]] three-dimensional structure.<ref>{{cite journal |vauthors = Watson JD, Crick FH |title = Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid |journal = Nature |volume = 171 |issue = 4356 |pages = 737–8 |date = April 1953 |pmid = 13054692 |doi = 10.1038/171737a0 |bibcode = 1953Natur.171..737W |s2cid = 4253007 }}</ref> In contrast, single-stranded RNA and DNA molecules are not constrained to a regular double helix, and can adopt [[nucleic acid tertiary structure|highly complex three-dimensional structures]] that are based on short stretches of intramolecular base-paired sequences including both Watson-Crick and noncanonical base pairs, and a wide range of complex tertiary interactions.<ref>{{cite journal |vauthors = Ferré-D'Amaré AR, Doudna JA |title = RNA folds: insights from recent crystal structures |journal = [[Annual Review of Biophysics and Biomolecular Structure]] |volume=28 |pages=57–73 |year=1999 |pmid=10410795 |doi=10.1146/annurev.biophys.28.1.57 }}</ref>

Nucleic acid molecules are usually unbranched and may occur as linear and circular molecules. For example, bacterial chromosomes, [[plasmid]]s, [[mitochondrial DNA]], and chloroplast DNA are usually circular double-stranded DNA molecules, while [[Chromosome|chromosomes]] of the eukaryotic nucleus are usually linear double-stranded DNA molecules.<ref name="Brock, Thomas D.; Madigan, Michael T. 2009"/> Most RNA molecules are linear, single-stranded molecules, but both circular and branched molecules can result from [[RNA splicing]] reactions.<ref name="Alberts">{{cite book |author=Alberts, Bruce |title=Molecular biology of the cell |publisher=Garland Science |location=New York |year=2008 |isbn=978-0-8153-4105-5 }}</ref> The total amount of pyrimidines in a double-stranded DNA molecule is equal to the total amount of purines. The diameter of the helix is about 20 [[angstrom|Å]].