Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 50

If we think of DNA strands as chains, deoxyribonucleotides form the links. Figure 1.2 shows the basic structure of deoxyribonucleotides, called deoxynucleotides for short. Each is composed of a base, a sugar, and a phosphate group. The DNA bases are adenine (A), cytosine (C), guanine (G), and thymine (T), which have either one or two rings, as shown in Figure 1.2. The bases with two rings (A and G) are the purines, and those with only one ring (T and C) are pyrimidines. A third pyrimidine, uracil (U), replaces thymine in RNA. The carbons and nitrogens making up the rings of the bases are numbered sequentially, as shown in the figure. All four DNA bases are attached to the five-carbon sugar deoxyribose. This sugar is identical to ribose, which is found in RNA, except that it does not have an oxygen attached to the second carbon—hence the name deoxyribose. The carbons in the sugar of a nucleotide are also numbered 1, 2, 3, and so on, but they are labeled with “primes” to distinguish them from the carbons in the bases (Figure 1.2). The nucleotides also have one or more phosphate groups attached to a carbon of the deoxyribose sugar, as shown. The carbon to which the phosphate group is attached is indicated, although if the group is attached to the 5' carbon (the usual situation), the carbon to which it is attached is often not stipulated.

Figure 1.1 Schematic drawing of the Watson-Crick structure of DNA, showing the helical sugar-phosphate backbones of the two strands held together by hydrogen bonding between the bases. Also shown are the major and minor grooves and the dimensions of the helix.

The components of the deoxynucleotides have special names. A deoxynudeoside (rather than -tide) is a base attached to a sugar but lacking a phosphate. Without phosphates, the four deoxynucleosides are called deoxyadenosine, deoxycytidine, deoxyguanosine, and deoxythymidine. As shown in Figure 1.2, the deoxynucleotides have one, two, or three phosphates attached to the sugar and are known as deoxynucleoside monophosphates, diphosphates, or triphosphates, respectively. The individual deoxynucleoside monophosphates, called deoxyguanosine monophosphate etc., are often abbreviated dGMP, dAMP, dCMP, and dTMP, where the d stands for deoxy; the G, A, C, or T stands for the base; and the MP stands for monophosphate. In turn, the diphosphates are abbreviated dGDP, dADP, dCDP, and dTDP, and the triphosphates are abbreviated dGTP, dATP, dCTP, and dTTP. The phosphate attached to the sugar is called the α phosphate, while the next two are called the β and γ phosphates, respectively, as shown in the figure. Collectively, the four deoxynucleoside triphosphates are often referred to as dNTPs.