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The fourth type of organic compound important to human structure and function is nucleic acid. Nucleic acids are polymers, and nucleotides are their monomers. A nucleotide is one of a class of organic compounds composed of three subunits:
The two forms of nucleic acids differ in their type of pentose sugar.
Deoxyribonucleic acid (DNA) is a nucleotide that stores genetic information. DNA contains deoxyribose sugar (so-called because it has one less atom of oxygen than ribose) plus one phosphate group and one nitrogen-containing base. Ribonucleic acid (RNA) is a ribose-containing nucleotide that helps transfer the genetic code in DNA to protein. RNA contains ribose, one phosphate group, and one nitrogen-containing base.
There are five nitrogenous bases that exist and are grouped into two categories based on their chemical structure—purines and pyrimidines. A purine is a nitrogen-containing molecule with a double ring structure, which accommodates several nitrogen atoms. There are two purines: adenine and guanine. A pyrimidine is a nitrogen-containing base with a single ring structure. There are three pyrimidines: cytosine, thymine, and uracil. Each form of nucleotide is only created with four of the bases. DNA nucleotides are made with adenine, cytosine, guanine, and thymine. RNA is made with adenine, cytosine, guanine, and uracil.
Bonds formed by dehydration synthesis between the pentose sugar of one nucleic acid monomer and the phosphate group of another form a “backbone.” Each nucleotide’s base sticks out.
RNA often contains just one such backbone (single-stranded), whereas DNA contains two (double-stranded). The bases project away from the backbone.
In double-stranded nucleic acids, bases of opposite strands project toward each other and associate using hydrogen bonds. Adenine bonds with thymine in DNA (or uracil in RNA), and cytosine bonds with guanine. DNA then twists to form a shape known as a double helix, which resembles a spiral staircase.
The sequence of nitrogen-containing bases within a strand of DNA forms the genes that act as molecularly encoded instructions for the cell. Copies of this code can be made using RNA nucleotides that are used to form proteins.
The nucleotide adenine is also used as a form of stored energy in the body. A nucleotide contains a single phosphate group. However, when additional phosphate groups are added, they repel one another, thereby storing energy that can be released at a later time.
Adenosine triphosphate (ATP) is an adenine base connected to a ribose sugar and three phosphate groups, as shown in the image below, and is a means of short-term energy storage in the human body. ATP is classified as a high-energy compound because the two covalent bonds linking its three phosphates store a significant amount of energy. In the body, the energy released from these high-energy bonds helps fuel the body’s activities, from muscle contraction to the transport of substances in and out of cells to anabolic chemical reactions.
When the bond connecting ATP’s third phosphate is broken, the products are adenosine diphosphate (ADP) (di, two) and inorganic phosphate (Pᵢ). This hydrolysis reaction can be written:
Removal of a second phosphate leaves adenosine monophosphate (AMP) and two phosphate groups. These reactions also free the energy that had been stored in the phosphate-phosphate bonds.
These reactions are reversible, too. ADP undergoes phosphorylation, which is the addition of a phosphate group to an organic compound resulting in the formation of ATP. In such cases, the same level of energy that had been released during hydrolysis must be reinvested to power dehydration synthesis.
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