Read this article to get information about the chemical composition of chromosomes.
DNA, RNA (nucleic acids), and proteins (histones and non-histones) make up the majority of the chromosome’s chemical elements. There is, in addition, calcium, which seems to be associated with the DNA. Calcium is important in binding sections of the chromosome together. DNA, RNA and protein all contribute to the chromosomal mass.
Together with the histone protein, they make up around 35% of the DNA (These are basic proteins having amino acids such as arginine and lysine in their molecules.) It accounts for 55 percent, creating deoxyribonucleoprotein, which makes up 90 percent of the chromosome. The remaining 10% is known as the residual chromosome, and it contains RNA (12–14%), DNA (2%–3%), and residual protein (83–86%).
The residual protein (nonhistone proteins) is acidic in nature, with a high concentration of the amino acids tryptophane and tyrosine. Phosphoproteins, DNA polymerase, RNA polymerase, DPN pyrophosphorylase, and nucleoside triphosphatases are important nonhistone proteins of chromosomes. In addition to histones and protamines, Stedman and Stedman (1943) discovered chromosomin, a protein with a high tryptophane content.
The salt linkages that exist between DNA and protein molecules are essentially of an unstable ionic nature. Specific metallic ions (Mg++, Ca++, etc.) in cells form additional chromosomal linkages. These can happen between DNA and protein or between different DNA groups.
The number of chromosomes in the nucleus is directly proportional to the amount of DNA in the cell. It is the primary genetic material because DNA content and chromosome number are related, and it is only found in the nucleus and is passed down to daughter cells during cell division. DNA transmits genetic information from one generation to the next.
Figure 1: Chemical composition of chromosomes
The DNA transcribes RNA, and most of it is transported to the cytoplasm, but some remains associated with DNA and proteins. RNA is found relatively abundantly in the particulate fractions of the cytoplasm.
It is a component of the chromosome fabric in the nucleus, where it is primarily associated with histone-like proteins, and it is also found in the nucleolus, where it is associated with tryptophane-containing proteins.
The DNA-RNA ratios in the nucleus vary greatly: 40: 1 in thymus chromosomes, 10: 1 in liver chromosomes, 10: 1 in kidney nuclear ratio, and so on. Typically, the amount of DNA in the chromosome is so great that RNA is stained and recognized only with difficulty.
The RNA content of tumor and leukemic cell nuclei is much higher than that of more normal tissues. RNA is more abundant in active cells than in dormant cells. RNA is required 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.
The basic proteins are histone and protamine, the latter being less complex. Protamine has only been discovered in the spermatozoa of some fishes, where it appears to have replaced histone. Histone has been found in almost every type of nucleus studied. Histone contains arginine and lysine amino acids but 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.