Unlike carbohydrates, lipids and proteins, nucleic acid (NA’s) contribute little as energy sources or structural members of cell. Nucleic acids are like the pilothouse of a ship, contributing nothing as fuel or cargo, hull or super-structure, yet they guide the entire vessel, its contents and crew.
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As the name implies, NA’s were formerly thought to occur only in the nucleus of cells. As will be explained, some NA’s are strictly intranuclear, some are not. In the nucleus (or the bacterial nucleoid) NA’s are loosely combined (conjugated) with proteins as nucleoproteins.
The NA’s are easily separated from their protein conjugates by mild hydrolytic processes. Structurally, NA’s are long-chain heteropolymers. Unlike polymers made up of simple molecules like glucose or amino acids, NA’s are made up of complex units called nucleotides: i.e., they are polynucleotides.
Each nucleotide in NA is made up of three components: one molecule of phosphoric acid, one of the pentose ribose (or deoxyribose) and either a purine base (adenine or guanine) or a pyrimidine base (cytosine, thymine or uracil).
Alternate forms of cytosine (methyl cytosine or hydroxymethyl cytosine) may occur in some forms of nucleotides.
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The nucleotide units are joined by phosphate diester bonds between alternating phosphate and sugar groups: sugar-phosphate-sugar-phosphate sugars and so on.
To each sugar group a purine or pyrimidine base is attached by covalent C to N bonds. The chains of nucleotides in NA’s are immensely long. In mammalian cells the number of nucleotides per chain is thought to be of the order of 10® = 1 billion.