There are two kinds of NA. One ribonucleic acid (RNA) is so called because its sugar moiety is the pentose ribose. In the other, the sugar moiety is very similar but has one less oxygen atom than ribose and is called deoxyribose. This NA is called DNA. In RNA the purine bases may be adenine or guanine and the pyrimidine bases, cytosine or uracil. The bases in DNA are the same except that thymine occurs in place of uracil.
The nucleotides (base + pentose + phosphate) in RNA are adenylic acid (or adenosine-5′-monophosphate, familiar as AMP and in NAD), guanylic acid, cytidylic acid and uridylic acid; in DNA: deoxyadenylic, deoxycy tidy lie, deoxyguanylic and deoxythymidylic acids.
All are nucleoside phosphates. While discussing nucleotides it is of interest to note that nucleotides other than adenylic acid also serve as coenzymes. Among these are uridylic acid or uridine-5′-monophosphate or UDP; guanylic acid or guanosine-5′-monophosphate or GDP. Adenylic acid is also found in FAD and Co A.
The full story of synthesis of the NA macromolecules logically begins with synthesis of the nucleotides and of the ribose, purines and pyrimidines of which they are composed. These details are not essential to the present discussion; we stipulate the ready formed nucleotides.
The formation of the diester bonds linking the pentose-phosphate groups of the various nucleotides into a long chain, the backbone of NA, occurs by the familiar anhydrosynthesis.
The free nucleotides occur as triphosphates. In the presence of a NA polymerase enzyme system they join, each giving off pyrophosphate, leaving the energy of the phosphate bond in the polymer chain.
DNA, as is generally known, is the substance of genes, chromosomes and inheritance. In all “higher” cells DNA is confined to the chromosomal structures inside the nuclear membrane.
In all bacterial cells’ DNA is confined to the chromosomal equivalent, the fibrillar nucleoid material which is not enclosed in a nuclear membrane. In both “higher” as well as in bacterial cells RNA appears in both nuclear and cytoplasmic regions.
As will be explained, RNA does not determine heredity, but it acts as an intermediary between the DNA of the nucleus and all synthetic functions in the cell, which are carried on mainly in the cytoplasm. With regard to viruses, in any true virus (i.e., excluding the bacterium-like “large viruses”) there is only one kind of NA, either RNA or DNA, never both.
The importance of DNA or RNA in all forms of life can hardly be overstated. In all forms of life it is the DNA (or RNA) of the genes and chromosomes that undergoes the alterations called mutations.
Mutations, with natural selection, are the basis of the great pageant of evolution of all forms of life on this earth, from primeval specks of sub vital marine slime to modern astronauts and, who knows, to “moon men.” An understanding of the structure, replication and alterations in NA’s is the key to the real “facts of life.”