Although the nucleic acids have been known to chemistry since the nineteenth century, their importance to living systems was not appreciated until recently.

Of the four basic molecules of life that we have mentioned, there is more research at the present time in the nucleic acids than in any of the others. In fact, one of the nucleic acids, deoxyribonucleic acid is sometimes called “the basic molecule flied’.

The role of the nucleic acids as the genetic material which directs and controls the metabolism of living systems will be discussed in a later chapter. Our aim at present is that of gaining some insight into the structure of these large organic molecules. Such insight is essential to an understanding of their functional roles on living systems.

The nucleic acids are threadlike macromolecules composed of thousands of atoms. There are two different types of nucleic acids, deoxyribonucleic acid, and ribonucleic acid. The difference between the two will become apparent as we consider the micro molecular units which make up these large macromolecules.

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By treating DNA or RNA with mild acids, it is possible to hydrolyze them into smaller units called nucleotides. The nucleotides may be considered the micro molecular building blocks of the large macromolecular nucleic acids.

However, the nucleotides themselves are relatively large molecules, and when they are hydrolyzed further with stronger acids, they break down into nucleosides and phosphoric acid. The nucleosides, under treatment with still stronger acids, break down into pentose sugars and nitrogen- rich bases. Thus, a nucleotide is composed of a molecule of phosphoric acid, one of a pentose sugar, and one of a nitrogenous base. There are five major nitrogenous bases involved, and these fall into two categories, the pyrimidines and pureness. Three of the five are pryrimidines and the other two are pureness.

Although it is not essential at this point that we distinguish between pyrimidines and pureness from a technical standpoint, it will be helpful to look at the formula of each base. As you can see, the three pyrimidines are single rings, and the two purenesses are double-ringed molecules. The two pentose sugars found in nucleic acids are ribose and deoxyribose.

You will recall that we gave the formula for ribose when we discussed carbohydrates and we mentioned its importance in the nucleic acid molecule at that point. The difference between the structure of ribose and that of deoxyribose is the absence of oxygen at position 2 in the deoxyribose molecule. The absence of oxygen is implied in the prefix dioxin-, which means “take away oxygen”. The pentose sugar found in RNA is ribose, while that found in DNA is deoxyribose.

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This difference and the substitution of Uralic for thymine in RNA constitute the primary structural differences between DNA and RNA. A nucleotide is formed by adding phosphoric acid to a nucleoside. By this means, a nucleotide may be helix, that is, that certain bases must pair off with certain other bases.

The diameter of 20 A across the helix suggested that a pyramiding must bond with a urine. You will recall that a Purina molecule involves a double ring, and as consequence, it is of greater diameter than a pyramiding molecule.

A distance of 20 A would not accommodate two pureness, according to size estimates, and it appeared to be too great a distance for a pair of pyrimidines. However, it was estimated that urine bonded to a pyramiding ought to fit the space very nicely. Upon hydrolyzing any given type of DNA completely, the relative proportions of the four nitrogenous bases may be determined. When this is done, the following relationships are evident.

It is observed from these relationships that adenine must be bonded to thymine and cytosine must be bonded to guanine across the double helix. In other words, for every adenine in a DNA molecule there must be a thymine, and for every cystosine there must be a guanine.

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It is extremely important to note that this point, however, that this characteristics of the DNA molecule does not affect the variability which can exist among DNA molecule does not affect the variability which can exist among DNA molecules with regard to the vast array of sequence of nucleotides which may exist in the chain in an “up and down” direction.

It should also be pointed out that there are a few cases in which DNA apparently exists as a single strand rather than as a double helix.

The Watson – Crick Model has withstood the rigors of numerous experimental tests since 1953. It has emerged as the best working model available for the structure of DNA. In fact, Watson, Crick, and Wilkins were awarded the Nobel Prize in physiology and medicine in 1962 for their formulation of this model. The precise structure of RNA is much less clear than that of DNA. In some cases it appears to be single-stranded, in others double-stranded, and in still others a combination of both.