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Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of pyrimidine, so those three bases are called the pyrimidine bases. [ 6 ] Each of the base pairs in a typical double- helix DNA comprises a purine and a pyrimidine: either an A paired with a T or a C paired with a G.
Nucleic acid secondary structure is generally divided into helices (contiguous base pairs), and various kinds of loops (unpaired nucleotides surrounded by helices). Frequently these elements, or combinations of them, are further classified into additional categories including, for example, tetraloops , pseudoknots , and stem-loops .
The chemical structure of DNA base-pairs . A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA.
One hydrogen bond from the Watson-Crick base pair is maintained (guanine O6 and cytosine N4) and the other occurs between guanine N7 and a protonated cytosine N3 (note that the Hoogsteen G-C base pair has two hydrogen bonds, while the Watson-Crick G-C base pair has three). [65] Figure 6: Four examples of wobble base pairs.
In order to create, i.e., design, RNA for any given secondary structure, two or three bases would not be enough, but four bases are enough. [16] This is likely why nature has "chosen" a four base alphabet: fewer than four would not allow the creation of all structures, while more than four bases are not necessary to do so.
The angle between the two glycosidic bonds (ca. 80° in the A• T pair) is larger and the C1 ′ –C1 ′ distance (ca. 860 pm or 8.6 Å) is smaller than in the regular geometry. In some cases, called reversed Hoogsteen base pairs, one base is rotated 180° with respect to the other.
A tetraloop is a four-base pairs hairpin RNA structure. There are three common families of tetraloop in ribosomal RNA: UNCG, GNRA, and CUUG (N is one of the four nucleotides and R is a purine). UNCG is the most stable tetraloop. [9] Pseudoknot is an RNA secondary structure first identified in turnip yellow mosaic virus. [10] It is minimally ...
In RNA, adenine-uracil pairings featuring two hydrogen bonds are equal to the adenine-thymine bond of DNA. Base stacking interactions, which align the pi bonds of the bases' aromatic rings in a favorable orientation, also promote helix formation. The stability of the loop also influences the formation of the stem-loop structure.