Molecular Weights of Nucleic Acids

Often only two or three very large DNA molecules are present in the nucleus, and in some kinds of cells all the DNA may be present as a single large molecule of molecular weight 10 -10 . 

While natural DNA samples generally have high molecular weights, the lower end of the polynucleotide molecular weight range is only filled by some varieties of RNA. 

Samples of natural RNA have been divided into three major varieties, each of which has a different metabolic role and has a characteristic molecular weight range. These are known as 

Transfer RNA (t-RNA)—sometimes known as soluble RNA, with a molecular weight range 20,000-40,000 

All these varieties are needed for protein biosynthesis. The relative proportions present in cells usually lie within the approximate ranges  

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Determination of the size or molecular weight of nucleic acids is crucial in molecular biology. This importance is increased by the introduction of the PCR, which allows an individual to be identified (e.g. a pathogen bacteria) or the genetic links between individuals and allows the prediction or diagnosis of diseases. 

The molecular weight of nucleic acids, in various types of virus, varies from 2 to 160 million. RNA and the much larger DNA types are about equally common in viruses infecting Man. Typical DNA viruses are the adenoviruses (causing infections of the respiratory tract), the poxviruses, and the herpesviruses. Typical RNA viruses are the myxoviruses (including influenza), paramyxoviruses (mumps and measles), and those causing yellow fever and encephalitis. The nucleic acids are double-stranded in some types, but single-stranded in others. 

The molecular weights of nucleic acids are subject to numerous errors,... 

Values found for the molecular weight of deoxyribonucleic acids also vary considerably, but probably lie between 1.0 X 106 and 4.4 X 106. Various difficulties encountered in making such measurements have been discussed by Jordan,244 and it is probable that more reliable information will be obtained only when the behavior of polyelectrolytes in general is better understood. Certain of the techniques used are useful in detecting differences between different nucleic-acid preparations, but the discrepancies between the values given by different methods of measurement appear to vary with the degree of polymerization.246... 

The molecular weight or molar mass of proteins and nucleic acids (DNA, RNA) is identical for each specific species, e.g., the molecular weights of all casein molecules from a specific source are identical. These polymers are members of a homologous series and are said to be monodisperse or molecularly homogeneous. 

The technique of gel filtration is widely used in biochemical research. Chapter 3 described the theory and applications of gel filtration to the experimental procedures of desalting, separation and purification of biomolecules, and estimation of molecular weight of biomolecules. In this experiment, gel filtration procedures will be used to study the dynamic binding of small molecules by proteins. Many of the dynamic processes occurring in biological cells and organisms are the result of interactions between molecules. Often these interactions involve one or more smaller molecules binding to a macromolecule (usually a protein or nucleic acid). 

Because of their relatively low molecular weight (70 to 90 nucleotide residues), transfer ribonucleic acids are of special interest for 13C NMR investigations [769, 778, 782-784] of nucleic acids. Using a tube of 20 mm o.d., a sample of thermally denatured yeast... 

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