Nucleobase
Nuclein, nucleic bases also, nucleobases or nucleobase, which is part of the nucleotides ( DNA building blocks ) that are responsible for the formation of base pairs in DNA and RNA are. Comparing the double-helix structure of DNA with a staircase, they form the steps. They are described as bases, because they can be protonated at the nitrogen atoms and react slightly basic in aqueous solution.
DNA in the four bases adenine (A ), guanine (G), cytosine (C) and thymine (T ) occur, they are therefore referred to as DNA bases. In RNA is uracil instead of thymine, using, refers to A, G, C and U in accordance with the RNA bases. Uracil is different from thymine, only by the lack of a methyl group. The basic structure of adenine and guanine is a purine, cytosine, thymine, and uracil are based on the basic structure of pyrimidine.
Occurrence
The nucleic bases, their abbreviations in brackets and their occurrence are listed in the table. They form together with ribose or deoxyribose nucleosides, more specifically ribonucleosides or deoxyribonucleosides. Virtually all important functions of the nucleic they meet as part of nucleosides. Nucleic bases are thus essential components of deoxyribonucleic acid ( DNA) and ribonucleic acid (RNA) but they are also included in other important biomolecules.
In adenosine in conjunction with a different number of phosphate groups adenine appears as adenosine monophosphate ( AMP), cyclic adenosine monophosphate (cAMP), adenosine diphosphate ( ADP) and adenosine triphosphate (ATP ), in conjunction with nicotinamide to NADPH and NADH, and in conjunction with flavin in flavin adenine dinucleotide (FAD ), and as part of coenzyme A. the same is true for guanine in guanosine triphosphate (GTP ) and cytosine in cytidine triphosphate.
Hypoxanthine and xanthine are not normal components of DNA and RNA, that is, not included in the genetic code. They are formed by the action of mutagens: through deamination (replacement of the amino group by a hydroxyl group ) is formed from adenine, hypoxanthine, xanthine from guanine and uracil from cytosine. They are also important intermediates in the synthesis of purines.
Structure
Purine bases
The basic structure of adenine, guanine, hypoxanthine and xanthine corresponds to the purine. However, these molecules are also referred to as a purine bases.
Guanine
Hypoxanthine
Xanthine
Pyrimidine bases
The basic structure of the bases cytosine, uracil and thymine is pyrimidine, which are therefore also called pyrimidine bases.
Uracil
Thymine
Base pairing
Adenine forms with thymine ( or uracil in RNA ) of two hydrogen bonds. Cytosine and guanine on the other hand are connected to each other by three hydrogen bonds.
This is called complementary bases.
This means that may face in DNA double-strand only A and T or C and G, however, never A and C or T and G.
Derived molecules
The bases in the nucleic acids only occur in conjunction with other molecules: First, the base is bound to a sugar molecule of five carbon atoms, this unit is referred to as a nucleoside. In DNA the sugar is 2' -deoxyribose (English deoxyribose ) ribose in RNA. The names of the nucleosides are derived from the names of the bases: adenosine, guanosine, cytidine, thymidine and uridine.