Nucleic Acids and Nucleotides: Structure, Function, and Biological Significance

Nucleic Acids: Fundamental Molecules of Life

Nucleic acids contain the instructions necessary for life processes and are responsible for all basic functions of living beings. They are heritable, possess the characteristic of self-duplication, are stable, contain the hereditary material of living things, allow for changes (mutation and recombination), and produce various molecules.

Nucleotide Structure

Nucleotides are the fundamental units that form nucleic acids. They are composed of a pentose sugar, a nitrogenous base, and one or more phosphate groups. Both the nitrogenous base and the phosphate groups are attached to the pentose, which is always an aldopentose.

Nitrogenous Bases

Nitrogenous bases can be of two types: purines, derived from the purine ring, or pyrimidines, derived from the pyrimidine ring. The existence of several radical groups allows for several nitrogenous bases:

• Purines: Adenine (A), Guanine (G)
• Pyrimidines: Cytosine (C), Thymine (T), Uracil (U)

Nucleosides: Pentose & Base

The composite formed by a pentose and a nitrogenous base is called a nucleoside. There are several types, specific to RNA and DNA:

• RNA Nucleosides: Adenosine, Guanosine, Cytidine, Uridine
• DNA Nucleosides: Deoxyadenosine, Deoxyguanosine, Deoxycytidine, Deoxythymidine

Importance of Nucleotides

Some nucleotides with more than one phosphate group function in energy production. For example:

• Adenosine Triphosphate (ATP): When energy is available, a molecule of adenosine diphosphate (ADP) is used to attach a third phosphate group to the other two, forming ATP. This process is called phosphorylation. The bond formed is highly efficient, meaning its formation requires a considerable amount of energy.
• Cyclic AMP (cAMP): Cyclic AMP plays a crucial role in triggering cell responses. Before receiving external environmental signals, it acts as a mediator between external information and the final cellular response.

Nucleotides as Coenzymes

Several nucleotides function as coenzymes, vital for metabolic processes:

• Nicotinamide Adenine Dinucleotide (NAD+): A derivative of vitamin B3.
• Nicotinamide Adenine Dinucleotide Phosphate (NADP+): Similar to NAD+ but with an additional phosphate group at the 2'-carbon.
• Flavin Adenine Dinucleotide (FAD): A derivative of vitamin B2.

These three coenzymes participate in dehydrogenation reactions, which are fundamental to cellular catabolism. In their reduced state, they readily yield H+ and electrons to other molecules, playing a basic role in multiple metabolic processes.

Nucleotide Linkages: Phosphodiester Bonds

Nucleotides are linked together by phosphodiester bonds, typically in a 5'-3' orientation. The existence of hydroxyl groups in both the pentose and the phosphate allows the union of nucleotides through the formation of bonds between them. This is a condensation reaction that yields a compound called a dinucleotide and releases a molecule of water. The new linkage is called a phosphodiester bond.

The hydrolysis of a dinucleotide releases two mononucleotides. Nucleotides can bind to form trinucleotides, tetranucleotides, and so on. The union of many nucleotides forms polynucleotides, which are the nucleic acids.

A polynucleotide chain has a 3' end with a free hydroxyl (-OH) group on the 3' carbon of a pentose, and a 5' end with a free phosphate group attached to the 5' carbon of another pentose.