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In this article we will discuss about the biochemical evidence that genes are made of DNA.
Quantitative estimations of DNA carried out on various organisms revealed the following constant features, providing additional support for DNA as the genetic material:
1. The amount of DNA per cell in an organism or a given species is remarkably constant and cannot be altered by changes in external or internal environment of the cell.
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2. The amount of DNA per cell is in proportion to the complexity of the cell and to the amount of genetic information it contains. Thus mammals were found to have approximately 6pg per cell, reptiles 5, birds 2, crustaceans 3, sponges 0.1, higher plants 2.5, fungi 0.02-0.17, bacteria 0.002-0.06 and bacteriophage lambda 0.00008. This shows that higher the organism in the evolutionary scale, greater the content of DNA per cell.
3. Germ cells of higher plants and animals are haploid with only one set of chromosomes, and half the amount of DNA present in somatic cells of the some species.
4. The amount of DNA per diploid cell in a species is constant from one cell type to another.
In 1940s definite clues relating genes to DNA came after the discovery that ultraviolet radiation induces gene mutations. Working with maize plants Stadler found that the wavelength of UV light absorbed maximally by proteins was 280 nm, whereas DNA would absorb maximally at 260 nm.
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Since the wavelength most suitable for inducing mutations is also 260 nm, it was inferred that DNA was the target for gene mutations and not proteins. Another confirmation came from studies on the chemical composition of chromosomes. DNA was found to be present exclusively in the nucleus, and absent from locations where chromosomes were not detectable.
Biochemical analysis of the base composition of DNA and its relation with the species members provided unequivocal evidence in favour of DNA as the hereditary material. In the years 1949 to 1953 E. Chargaff and his associates applied quantitative chromatographic methods for the separation and analysis of the four bases of DNA isolated from different species.
They could draw the following conclusions:
(a) The base composition of DNA varies from one species to another;
(b) Different tissues of the same organism have the same base composition in their DNA;
(c) Base composition of DNA in a given species does not alter with age or environment;
(d) The number of adenine residues is equal to the number of thymine residues (A = 1) and the number of guanine residues is equal to the number of cytosine residues (G = C). It follows therefore, that the sum of purine residues equals the sum of pyrimidine residues (A + G = C + T);
(e) The DNA specimens from closely related species have similar base composition, and of unrelated species have widely different base composition.
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Base composition of DNA can be used to classify organisms. An infinite number of different sequences of base pairs are possible in a DNA molecule, thus DNA is capable of encoding an enormous amount of information, and is more likely to be the genetic material of most organisms.
