In all organisms, DNA and RNA are each made up of only four different nucleotides. Recall from Chapter 2 that all nucleotides consist of an organic base linked to a five- carbon sugar that has a phosphate group attached to the 5′ carbon. In RNA, the sugar is ribose; in DNA, deoxyribose. The nucleotides used in synthesis of DNA and RNA contain five different bases. The bases adenine (A) and guanine (G) are purines, which contain a pair of fused rings; the bases cytosine (C), thymine (T), and uracil (U) are pyrimidines, which contain a single ring. Three of these bases—A, G, and C—are found in both DNA and RNA; however, T is found only in DNA and U only in RNA. (Note that the single-letter abbreviations for these bases are also commonly used to denote the entire nucleotides in nucleic acid polymers.)

A single nucleic acid strand has a backbone composed of repeating pentose-phosphate units from which the purine and pyrimidine bases extend as side groups. Like a polypeptide, a nucleic acid strand has an end-to-end chemical orientation: the 5′ end has a hydroxyl or phosphate group on the 5′ car bon of its terminal sugar; the 3′ end usually has a hydroxyl group on the 3′ carbon of its terminal sugar (Figure 1). This directionality, plus the fact that synthesis proceeds 5′ to 3′, has given rise to the convention that polynucleotide sequences are written and read in the 5′→3′ direction (from left to right); for example, the sequence AUG is assumed to be (5′)AUG(3′). As we will see, the 5′→3′ directionality of a nucleic acid strand is an important property of the molecule. The chemical linkage between adjacent nucleotides, commonly called a phosphodiester bond, actually consists of two phosphoester bonds, one on the 5′ side of the phosphate and another on the 3′ side.

Fig1. Chemical directionality of a nucleic acid strand. Shown here are alternative representations of a single strand of DNA containing only three bases: cytosine (C), adenine (A), and guanine (G). (a) The chemical structure shows a hydroxyl group at the 3′ end and a phosphate group at the 5′ end. Note also that two phosphoester bonds link adjacent nucleotides; this two-bond linkage is commonly referred to as a phosphodiester bond. (b) In the “stick” diagram (top), the sugars are indicated as vertical lines and the phosphodiester bonds as slanting lines; the bases are denoted by their single-letter abbreviations. In the simplest representation (bottom), only the bases are indicated. By convention, a polynucleotide sequence is always written in the 5′→3′ direction (left to right) unless otherwise indicated.

The linear sequence of nucleotides linked by phosphodiester bonds constitutes the primary structure of a nucleic acid molecule. Like polypeptides, polynucleotides can twist and fold into three-dimensional conformations stabilized by non-covalent bonds. Although the primary structures of DNA and RNA are generally similar, their three-dimensional conformations are quite different. These structural differences are critical to the different functions of the two types of nucleic acids.

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مواضيع ذات صلة

DNA Can Undergo Reversible Strand Separation

Native DNA Is a Double Helix of Complementary Antiparallel Strands

Abnormalities of Chromosome Structure

Impact of Genomic Variation

Variation in individual Genomes

Variation in gene Expression and its relevance to medicine

Allelic Imbalance in gene expression

Gene expression as the integration of Genomic and Epigenomic signals

Epigenetic and Epigenomic aspects of Gene expression

DNA Microarray Technology

RNA Splicing

Initiation of Transcription