Nucleic acids are huge macromolecules composed of nucleotides. Every nucleotide is a molecular complex of three types of subunit molecules-a phosphate (phosphoric acid), a pentose sugar, and a nitrogen-containing base:
Nucleic acids store hereditary information that determines which proteins a cell will have. Two classes of nucleic acids are in cells: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA makes up the hereditary units called genes. Genes pass on from generation to generation the instructions for replicating DNA, making RNA, and joining amino acids to form the proteins of a cell. RNA is an intermediary in the process of protein synthesis, conveying information from DNA regarding the amino acid sequence in proteins. The nucleotides in DNA contain the 5-carbon sugar deoxyribose; the nucleotides in RNA contain the sugar ribose. This difference accounts for their respective names. As indicated in Figure 2.17, there are four different types of bases in DNA: A = adenine, T = thymine, G = guanine, and C = cytosine.
The base can have two rings (adenine or guanine) or one ring (thymine or cytosine). In RNA, the base uracil replaces the base thymine. These structures are nitrogen-containing bases-that is, a nitrogen atom is a part of the ring. Like other bases, the presence of the nitrogen-containing base in DNA and RNA raises the pH of a solution.
Figure 2.17 Overview of DNA structure. a. Double helix. b. Complementary base pairing between strands. c. Ladder configuration. Notice that the uprights are composed of phosphate and sugar molecules and that the rungs are complementary paired bases.
DNA Structure Compared to RNA Structure
The nucleotides in DNA and RNA form a linear molecule called a strand. A strand has a backbone made up of phosphatesugar - phosphate-sugar, with the bases projecting to one side of the backbone. Because the nucleotides occur in a definite order, so do the bases. Any particular DNA or RNA has a definite sequence of bases, although the sequence can vary between molecules. RNA is usually single-stranded, while DNA is usually double-stranded, with the two strands twisted about each other in the form of a double helix. The molecular differences between DNA and RNA are listed in Table 2.1. In DNA, the two strands are held together by hydrogen bonds between the bases (see Fig. 2.17). When unwound, DNA resembles a stepladder. The sides of the ladder are made entirely of phosphate and sugar molecules, and the rungs of the ladder are made only of complementary paired bases. Thymine (T) always pairs with adenine (A), and guanine (G) always pairs with cytosine (C) (see Fig. 2.17). This is called complementary base pairing. Complementary bases pair because they have shapes that fit together. We shall see that complementary base pairing allows DNA to replicate in a way that ensures the sequence of bases will remain the same. When RNA is produced, complementary base pairing occurs between DNA and RNA in which uracil takes the place of thymine. Then, the sequence of the bases in RNA determines the sequence of amino acids in a protein because every three bases code for a particular amino acid. The code is nearly universal and is the same in other organisms as it is in humans.
ATP (Adenosine Triphosphate)
Individual nucleotides can have metabolic functions in cells. Some nucleotides are important in energy transfer. When adenosine (adenine plus ribose) is modified by the addition of three phosphate groups, it becomes ATP (adenosine triphosphate), the primary energy carrier in cells. Cells require a constant supply of ATP. To obtain it, they break down glucose and convert the energy that is released into ATP molecules. The amount of energy in ATP is just right for more chemical reactions in cells. As an analogy, the energy in glucose is like a $100 bill, and the energy in ATP is like a $20 bill. Just as you might go to the bank to change a $100 bill (glucose) into $20 bills (ATP molecules), in order to spend money, cells “spend” ATP when cellular reactions require energy. Therefore, ATP is called the energy currency of cells. Cells use ATP when macromolecules such as carbohydrates and proteins are synthesized. In muscle cells, ATP is used for muscle contraction, and in nerve cells, it is used for the conduction of nerve impulses.
ATP is sometimes called a high-energy molecule because the last two phosphate bonds are unstable and easily broken. Usually in cells, the terminal phosphate bond is hydrolyzed, leaving the molecule ADP (adenosine diphosphate) and a molecule of inorganic phosphate, "P" (Fig. 2.18). The terminal bond is sometimes called a high-energy bond, symbolized by a wavy line. But this terminology is misleading-the breakdown of ATP releases energy because the products of hydrolysis (ADP and "P" ) are more stable than ATP. After ATP breaks down and the energy is used for a cellular purpose, ATP is rebuilt by the addition of "P" to ADP again; this can be seen by reading Figure 2.18 from right to left. There is enough energy in one glucose molecule to build 36 ATP molecules in this way. Homeostasis is only possible because cells continually produce and use ATP molecules. The use of ATP as the energy currency of cells also occurs in other organisms, ranging from bacteria to humans.
Figure 2.18 ATP reaction. ATP, the universal energy currency of cells, is composed of adenosine and three phosphate groups (called a triphosphate). When cells require energy, ATP undergoes hydrolysis, producing ADP "P" , with the release of energy. (The "P" stands for inorganic phosphate.) Later, ATP is rebuilt when energy is supplied and ADP joins with "P".