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Read this article to get information about the chemical composition of Chromosomes!
The major chemical components of the chromosome are DNA, RNA (nucleic acids), and proteins (histones and nonhistones).
There is, in addition, calcium, which seems to be associated with the DNA (Barton, 1951, Mazia, 1954a). Calcium is important in binding sections of the chromosome together (Mazia, Steffensen, 1955).
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DNA, RNA and protein all contribute to the chromosomal mass.
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Nucleic acids:
These constitute mostly DNA being about 35% along with histone protein (These are basic proteins having amino acids such as arginine and lysine in their molecules.) which is 55%, thus forming deoxyribonucleoprotein comprising 90% of chromosome. The remaining 10% part is called residual chromosome and contains RNA 12 to 14%, DNA 2 to 3%, and residual protein 83 to 86%.
The residual protein (nonhistone proteins) is acidic in nature characterized by the preponderance of amino acids tryptophane and tyrosine. The important nonhistone proteins of chromosomes are phosphoproteins, DNA polymerase, RNA polymerase, DPN pyrophosphorylase and nucleoside triphosphatases. In addition to histone or protamines there is another type of protein called chromosomin by Stedman and Stedman (1943), and containing high tryptophane contents.
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The linkages present between DNA and protein molecules are essentially of unstable ionic nature and are called salt linkages. Specific metallic ions (Mg++, Ca++, etc.) in cells form additional linkages in the chromosomes. These may occur between DNA and protein or between DNA groups.
The quantity of DNA in a cell is directly proportional to the number of chromosomes in the nucleus. It is the primary genetic material since the DNA content and chromosome number are related and it is only restricted to the nucleus and transmitted to the daughter cells during cell division. DNA carries genetic informations of one generation to another (Ris, 1969).
The RNA is transcribed by the DNA and most of the RNA is transported to the cytoplasm but some remains associated with DNA along with proteins. RNA is found relatively abundantly in the particulate fractions of the cytoplasm.
In the nucleus it is a part of the chromosome fabric where it is associated primarily with histone-like proteins, and in addition is found in the nucleolus where it is associated with tryptophane containing proteins (Kaufmann, Gay and McDonald, 1951).
The DNA-RNA ratios in the nucleus vary rather widely-40: 1 ratio in thymus chromosomes; 10: 1 in liver chromosomes; in Kidney nuclear ratio is 10: 1, etc. Usually the amount of DNA is so much greater in the chromosome that RNA is stained and recognized only with difficulty.
RNA content of the nuclei of tumor and leukemic cell is much higher than that of more normal tissues. RNA is more prevalent in active than in quiescent cells. RNA is necessary for protein synthesis. RNA is derived from, or made by, DNA for transport to the cytoplasm where it functions for protein synthesis. Hammerling (1953) believes the nucleolar RNA to be synthesized in the nucleolus itself.
Proteins:
The basic proteins, histone and protamine, the later one are less complex. Protamine has been found only in spermatozoa of some fishes where it appears to have replaced histone. Histone is found in practically all the types of nuclei that have been studied. Histone has arginine and lysine amino acids and little tryptophane or tyrosine.
Protamine has 90% arginine and no tyrosine or tryptophane. Histones are highly viscous complex of DNA and may be removed from the chromosome by NaCl (concentration of one mol/litre) and DNA-histone ratio of the removed complex is about 1.2: 1 to 1.6: 1.
The nonhistone or residual protein remains in the chromosome after DNA and histones have been removed. It contains more tryptophane than in histones. Residual protein (chromosin of Mirsky and Pollister, 1946) and chromosomin of Stedman and Stedman (1947) have been found in chromosome. The amount of DNA are the same in all the cells, regardless of size, but the total protein content is in direct proportion to cellular volume.
