The structure of nucleic acids

Nucleic acids are key components of the mechanism of storage and transfer of genetic information in biological cells. Deoxyribonucleic acid (DNA) contains the instructions for protein synthesis, which is carried out by different forms of ribonucleic acid (RNA). In this section, we discuss the main structural features of DNA and RNA. Both DNA and RNA are polynucleotides (2), in which base–sugar–phosphate units are linked by phosphodiester bonds. In RNA the sugar is β-d-ribose and in DNA it is β-d-2-deoxyribose (as shown in 2). The most common bases are adenine (A, 3), cyto sine (C, 4), guanine (G, 5), thymine (T, found in DNA only, 6), and uracil (U, found in RNA only, 7). At physiological pH, each phosphate group of the chain carries a negative charge and the bases are deprotonated and neutral. This charge distribution leads to two important properties. One is that the polynucleotide chain is a polyelectrolyte, a macromolecule with many different charged sites, with a large and negative overall surface charge. The second is that the bases can interact by hydrogen bonding, as shown for A-T (8) and C-G base pairs (9). The secondary and tertiary structures of DNA and RNA arise primarily from the pattern of this hydrogen bonding between bases of one or more chains. In DNA, two polynucleotide chains wind around each other to form a double helix (Fig. 19.34). The chains are held together by links involving A-T and C-G base pairs that lie parallel to each other and perpendicular to the major axis of the helix. The structure is stabilized further by interactions between the planar π systems of the bases. In B-DNA, the most common form of DNA found in biological cells, the helix is right-handed with a diameter of 2.0 nm and a pitch of 3.4 nm. Long stretches of DNA can fold further into a variety of tertiary structures. Two examples are shown in Fig. 19.35. Supercoiled DNA is found in the chromosome and can be visualized as the twisting of closed circular DNA (ccDNA), much like the twisting of a rubber band.

The extra -OH group in β-D-ribose imparts enough steric strain to a polynucleotide chain so that stable double helices cannot form in RNA. Therefore, RNA exists primarily as single chains that can fold into complex structures by formation of A-U and G-C base pairs. One example of this structural complexity is the structure of transfer RNA (tRNA), shown schematically in Fig. 19.36 in which base-paired regions are connected by loops and coils. Transfer RNAs help assemble polypeptide chains during protein synthesis in the cell.

Fig. 19.34 DNA double helix, in which two polynucleotide chains are linked together by hydrogen bonds between adenine (A) and thymine (T) and between cytosine (C) and guanine (G).

Fig. 19.35 A long section of DNA may form closed circular DNA (ccDNA) by covalent linkage of the two ends of the chain. Twisting of ccDNA leads to the formation of supercoiled DNA.

Fig. 19.36 Structure of a transfer RNA (tRNA).

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مواضيع ذات صلة

Units

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption