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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.
There are diverse structures of RNA base quadruplexes. Four consecutive guanine residues can form a quadruplex in RNA by Hoogsteen hydrogen bonds to form a “Hoogsteen ring” (See Figure). [12] G-C and A-U pairs can also form base quadruplex with a combination of Watson-Crick pairing and noncanonical pairing in the minor groove. [17]
An unnatural base pair (UBP) is a designed subunit (or nucleobase) of DNA which is created in a laboratory and does not occur in nature. DNA sequences have been described which use newly created nucleobases to form a third base pair, in addition to the two base pairs found in nature, A-T (adenine – thymine) and G-C (guanine – cytosine).
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.
RNA secondary structure is usually represented by the dot-bracket notation, with pairing round brackets indicating basepairs in a stem and dots representing loops. The interrupted stems of pseudoknots mean that such notation must be extended with extra brackets, or even letters, so that different sets of stems can be represented.
In molecular biology, two nucleotides on opposite complementary DNA or RNA strands that are connected via hydrogen bonds are called a base pair (often abbreviated bp). In the canonical Watson-Crick base pairing, adenine (A) forms a base pair with thymine (T) and guanine (G) forms one with cytosine (C) in DNA.
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.
The genomic RNA of retroviruses is linked non-covalently to the dimer linkage structure (DLS), a non-coding region in the 5' UTR. For the kissing loop interaction to occur, there is a triple interaction that involves a 5'-flanking purine and 2 centralized bases in the complementary strand.