Structure

The nucleic acids are fundamental to genetics and to metabolism. Deoxyribonucleic acid (DNA) is found in the chromosomes within the nucleus and the mitochondria, and it contains the genetic information. Ribonucleic acid (RNA) is found both in the nucleus and in the surrounding cytoplasm, and its various forms fulfill several tasks associated with the transfer of the genetic message and its eventual translation into proteins.

Each nucleic acid is a linear polymer of nucleotides (Figure 1A). Nucleosides, the related small molecules, consist of a pentose sugar bound to the N-9 atom of a purine or to the N-1 of the nh2

II 2

C43^CH N

O II

II I

Guanine

~h2N

II 2

x-cV. HCs 4%N II 31 Cytosine

Adenosine 5'-triphosphate (ATP)

H3C.

O II

Thymine

Figure 1 (A) Schematic representation of part of a DNA strand showing the structural formulas of the four constituent bases, adenine, guanine, cytosine, and thymine, linked via the 3'-OH group of the deoxyribose moiety to the 5'-phosphate group of the next nucleotide. Also shown is the numbering of the atoms in the deoxyribose, as well as the pyrimidine and purine rings. The latter consist of a six-membered pyrimidine ring fused to a five-membered imidazole ring. (B) Structural formula of ATP indicating that the ribose, as distinct from deoxyribose, has an OH group at the 2' position on the pentose ring.

pyrimidine ring: With one or more phosphate groups at the 5' position of the sugar, the molecule is a nucleotide (Figure 1B). When nucleoside triphosphates (NTP) are linked through the 5' phosphate groups to the 3' position of the previous residue on the growing chain, the chemical energy for the polymerization is provided by the removal of the second and third phosphate groups.

In DNA the pentose is 2'-deoxyribose (Figure 1A) and the bases are adenine (A), guanine (G), cytosine (C), and thymine (T). Two strands are wound in opposing chemical directions in the well-defined double-helix structure of DNA, with each nucleotide of one strand linked by hydrogen bonding to a complementary nucleotide on the other (A-T and

G-C). The deoxyribose and phosphate groups form the outer sides of the 'ladder.'

The RNA molecule is chemically single stranded, but double-helical regions arise when stretches of complementary sequences allow hairpin loops to form. In addition, the base uracil (U) is found instead of thymine, and the pentose is ribose.

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