BIOLOGY Volume 1 - A Guide to General Biology - 2004

3. CHEMICAL COMPONENTS OF LIVING ORGANISMS

3.6. DNA and RNA - Nucleic Acids

Nucleic Acids, like Proteins, are essential for life. They constitute the genetic material of All living organisms, from complex life forms down to the simplest Viruses. The name "nucleic acids" reflects the fact that they are localized primarily in the Cell Nucleus (from the Latin nucleus). When stained specifically for nucleic acids, nuclei become clearly visible under a Light Microscope.

Elucidating The Structure of DNA (deoxyribonucleic acid)—one of the two existing Types of Nucleic acids—ushered in a new era in biology. It finally made it possible to understand how living organisms store the information necessary to regulate their vital processes and how they pass this information on to their offspring. As noted above (see Fig. 3.4), nucleic acids are composed of monomer units called NUCLEOTIDES. Nucleotides join together to form extremely long molecules known as polynucleotides. Therefore, to understand the structure of polynucleotides, we must first examine how nucleotides are constructed.

3.6.1. Structure of Nucleotides

A nucleotide molecule consists of three parts: a five-carbon sugar, a nitrogenous base, and phosphoric acid.

SUGAR. The sugar component of a nucleotide contains five carbon atoms, making it a pentose. Depending on the type of pentose present, There are two types of nucleic acids: Ribonucleic Acids (RNA), which contain ribose, and Deoxyribonucleic Acids (DNA), which contain deoxyribose. In deoxyribose, the —OH group at the 2nd carbon atom is replaced by an —H atom, meaning it contains one less oxygen atom than ribose (Fig. 3.37).

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Fig. 3.37. Components of nucleotides.

BASES. Both types of nucleic acids contain four different kinds of bases: two belong to the purine class and two to the pyrimidine class. The Basic Properties of these compounds are due to the nitrogen incorporated into the ring structure. Purines include adenine (A) and guanine (G), while Pyrimidines include cytosine (C) and thymine (T) or uracil (U) (found in DNA and RNA, respectively). Thymine is chemically very similar to uracil (it is 5-methyluracil, meaning uracil with a methyl group attached to the 5th carbon atom). Purine molecules contain two rings, whereas pyrimidine molecules contain only one.

Bases are typically designated by the first letter of their name: A, G, T, U, and C.

PHOSPHORIC ACID (Fig. 3.37). Nucleic acids are classified as acids because their molecules contain phosphoric acid.

Figure 3.38 illustrates how the sugar, base, and phosphoric acid combine to form a nucleotide molecule. The joining of the sugar and the base occurs with the release of a Water molecule, which is a Condensation reaction. Formation of a complete nucleotide requires a second condensation reaction—between the sugar and the phosphoric acid.

Different nucleotides vary depending on the specific sugars and bases that make up their structure.

The Role of nucleotides in the body is not limited to serving as the Building Blocks of nucleic acids; several important Coenzymes are also nucleotides. Examples include adenosine triphosphate (ATP), cyclic adenosine monophosphate (cAMP), coenzyme A, nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and flavin adenine dinucleotide (FAD) (Sec. 4.5).

Fig. 3.38. Formation of a nucleotide.