nucleic acid

Any of the organic compounds making up the genetic material of living cells. Nucleic acids direct the course of protein synthesis, thereby regulating all cell activities. Their transmission from one generation to the next is the basis of heredity. The two main types, DNA and RNA, are composed of similar materials but differ in structure and function. Both are long chains of repeating nucleotides. The sequence of purines and pyrimidines (bases)—adenine (A), guanine (G), cytosine (C), and either thymine (T; in DNA) or uracil (U; in RNA)—in the nucleotides, in groups of three (triplets, or codons), constitutes the genetic code.

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A nucleic acid is a macromolecule composed of chains of monomeric nucleotides. In biochemistry these molecules carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are universal in living things, as they are found in all cells and viruses. Nucleic acids were first discovered by Friedrich Miescher.

Artificial nucleic acids include peptide nucleic acid (PNA), Morpholino and locked nucleic acid (LNA), as well as glycol nucleic acid (GNA) and threose nucleic acid (TNA). Each of these is distinguished from naturally-occurring DNA or RNA by changes to the backbone of the molecule.

Chemical structure

The term "nucleic acid" is the generic name for a family of biopolymers, named for their role in the cell nucleus. The monomers from which nucleic acids are constructed are called nucleotides.

Each nucleotide consists of three components: a nitrogenous heterocyclic base, which is either a purine or a pyrimidine; a pentose sugar; and a phosphate group. Nucleic acid types differ in the structure of the sugar in their nucleotides - DNA contains 2-deoxyriboses while RNA contains ribose (where the only difference is the presence of a hydroxyl group). Also, the nitrogenous bases found in the two nucleic acid types are different: adenine, cytosine, and guanine are found in both RNA and DNA, while thymine only occurs in DNA and uracil only occurs in RNA. Other rare nucleic acid bases can occur, for example inosine in strands of mature transfer RNA.

Nucleic acids are usually either single-stranded or double-stranded, though structures with three or more strands can form. A double-stranded nucleic acid consists of two single-stranded nucleic acids held together by hydrogen bonds, such as in the DNA double helix. In contrast, RNA is usually single-stranded, but any given strand may fold back upon itself to form secondary structure as in tRNA and rRNA. Within cells, DNA is usually double-stranded, though some viruses have single-stranded DNA as their genome. Retroviruses have single-stranded RNA as their genome.

The sugars and phosphates in nucleic acids are connected to each other in an alternating chain, linked by shared oxygens, forming a phosphodiester bond. In conventional nomenclature, the carbons to which the phosphate groups attach are the 3' end and the 5' end carbons of the sugar. This gives nucleic acids polarity. The bases extend from a glycosidic linkage to the 1' carbon of the pentose sugar ring. Bases are joined through N-1 of pyrimidines and N-9 of purines to 1' carbon of ribose through N-β glycosyl bond.

Types of nucleic acids

Ribonucleic acid

Ribonucleic acid, or RNA, is a nucleic acid polymer consisting of nucleotide monomers, which plays several important roles in the processes of transcribing genetic information from deoxyribonucleic acid (DNA) into proteins. RNA acts as a messenger between DNA and the protein synthesis complexes known as ribosomes, forms vital portions of ribosomes, and serves as an essential carrier molecule for amino acids to be used in protein synthesis.

Deoxyribonucleic acid

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. The main role of DNA molecules is the long-term storage of information and DNA is often compared to a set of blueprints, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.

Nucleic acid components

Nucleobases

Nucleobases are heterocyclic aromatic organic compounds containing nitrogen atoms. Nucleobases are the parts of RNA and DNA involved in base pairing. Cytosine, guanine, adenine, thymine are found predominantly in DNA, while in RNA uracil replaces thymine. These are abbreviated as C, G, A, T, U, respectively.

Nucleobases are complementary, and when forming base pairs, must always join accordingly: cytosine-guanine, adenine-thymine (adenine-uracil when RNA). The strength of the interaction between cytosine and guanine is stronger than between adenine and thymine because the former pair has three hydrogen bonds joining them while the latter pair have only two. Thus, the higher the GC content of double-stranded DNA, the more stable the molecule and the higher the melting temperature.

Two main nucleobase classes exist, named for the molecule which forms their skeleton. These are the double-ringed purines and single-ringed pyrimidines. Adenine and guanine are purines (abbreviated as R), while cytosine, thymine, and uracil are all pyrimidines (abbreviated as Y).

Hypoxanthine and xanthine are mutant forms of adenine and guanine, respectively, created through mutagen presence, through deamination (replacement of the amine-group with a hydroxyl-group). These are abbreviated HX and X.

Nucleosides

Nucleosides are glycosylamines made by attaching a nucleobase (often referred to simply as bases) to a ribose or deoxyribose (sugar) ring. In short, a nucleoside is a base linked to sugar. The names derive from the nucleobase names. The nucleosides commonly occurring in DNA and RNA include cytidine, uridine, adenosine, guanosine and thymidine. When a phosphate is added to a nucleoside (by phosphorylated by a specific kinase enzyme), a nucleotide is produced. Nucleoside analogues, such as acyclovir, are used as antiviral agents.

Nucleotides and deoxynucleotides

A nucleotide consists of a nucleoside and one phosphate group. Nucleotides are the monomers of RNA and DNA, as well as forming the structural units of several important cofactors - CoA, flavin adenine dinucleotide, flavin mononucleotide, adenosine triphosphate and nicotinamide adenine dinucleotide phosphate. In the cell nucleotides play important roles in metabolism, and signaling.

Nucleotides are named after the nucleoside on which they are based, in conjunction with the number of phosphates they contain, for example:

• Adenine bonded to ribose forms the nucleoside adenosine.
• Adenosine bonded to a phosphate forms adenosine monophosphate.
• As phosphates are added, adenosine diphosphate and adenosine triphosphate are formed, in sequence.

See also

• Nucleic acid methods

External links

• Interview with Aaron Klug, Nobel Laureate for structural elucidation of biologically important nucleic-acid protein complexes provided by the Vega Science Trust.
• Nucleic Acid Research Journal

References

• Keith Roberts, Martin Raff, Bruce Alberts, Peter Walter, Julian Lewis and Alexander Johnson, Molecular Biology of the Cell 4th Edition, Routledge, March, 2002, hardcover, 1616 pages, 7.6 pounds, ISBN 0-8153-3218-1