Chemical Structure of Nucleic Acids

DNA and RNA are linear polymers made of subunits known as nucleotides. The information in each gene is determined by the order of the different nucleotides, just as the information in this sentence is due to the order of the 26 possible letters of the alphabet. There are four different nucleotides in each type of nucleic acid and their order determines the genetic information (Fig. 3.01).

Each nucleotide has three components: a phosphate group, a five-carbon sugar, and a nitrogen-containing base (Fig. 3.02). The phosphate groups and the sugars form the backbone of each strand of DNA or RNA. The bases are joined to the sugars and stick out sideways.

In DNA, the sugar is always deoxyribose; whereas, in RNA, it is ribose. Both sugars are pentoses, or five-carbon sugars. Deoxyribose has one less oxygen than ribose (Fig. 3.03). It is this chemical difference that gave rise to the names deoxyribonucleic acid and ribonucleic acid. Both sugars have five-membered rings consisting of four carbon atoms and an oxygen. The fifth carbon forms a side chain to the ring. The five carbon atoms of the sugar are numbered 1', 2', 3', 4' and 5' as shown in Fig. 3.02. By convention, in nucleic acids, numbers with prime marks refer to the sugars and numbers without prime marks refer to the positions around the rings of the bases.

Nucleotides are joined by linking the phosphate on the 5'-carbon of the (deoxy) ribose of one nucleotide to the 3'-position of the next as shown in Fig. 3.04. The phosphate group is joined to the sugar on either side by ester linkages, and the overall structure is therefore a phosphodiester linkage. The phosphate group linking the sugars has a negative charge.

base Alkaline chemical substance, in molecular biology especially refers to the cyclic nitrogen compounds found in DNA and RNA

deoxyribonucleic acid (DNA) Nucleic acid polymer of which the genes are made deoxyribose The sugar with five carbon atoms that is found in DNA

DNA Deoxyribonucleic acid, nucleic acid polymer of which the genes are made nucleic acid Class of polymer molecule consisting of nucleotides that carries genetic information nucleotide Monomer or subunit of a nucleic acid, consisting of a pentose sugar plus a base plus a phosphate group pentose A five carbon sugar, such as ribose or deoxyribose phosphate group Group of four oxygen atoms surrounding a central phosphorus atom found in the backbone of DNA and RNA phosphodiester The linkage between nucleotides in a nucleic acid that consists of a central phosphate group esterified to sugar hydroxyl groups on either side ribonucleic acid (RNA) Nucleic acid that differs from DNA in having ribose in place of deoxyribose ribose The 5-carbon sugar found in RNA

Chemical Structure of Nucleic Acids 53

; 5-carbon sugar

(4 alternatives)

One nucleotide

FIGURE 3.01 The Order of the Nucleotides Encodes the Genetic Information

Nucleotides are ordered along a string of DNA or RNA. It is the ordering of the different nucleotides that dictates the nature of the information within the nucleic acid.

The three components of a nucleotide are shown to the left. The structures on the right show the pentose sugar (deoxyribose) connected to the phosphate and the base.

FIGURE 3.03 The Sugars Composing RNA and DNA

Ribose is the five-carbon sugar (pentose) found in RNA. Deoxyribose is the pentose of DNA. It has one less oxygen than ribose as it has a hydrogen in place of the hydroxyl group on position 2' of the ribose ring.

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