Repetitive DNA in ThreeGramineaeSpecies with Low DNA Content

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Abstract
The genomes of 3 Gramineae species, i.e., finger millet (Eleusine coracana), pearl millet (Pennisetum americanum) and rice (Oryza sativa) are characterized by studying their DNA denaturation-reassociation properties. The reassociation kinetics measurements of the sonicated DNA (500-700 nucleotide pairs [np]) indicate the presence of a heterogeneous, repetitive DNA fraction accounting for 49-54% of the total DNA in all 3 spp. From the cot1/2 [cot = initial concentration of denatured DNA multiplied by the time of renaturation] value of the slow reassociating DNA, the genome size is estimated as 3.0 .times. 108 np in finger millet, 7.8 .times. 108 np in pearl millet and 9.0 .times. 108 np in rice. The melting patterns of the total DNA reveal a Tm [melting point] value of 88.6.degree. C in the case of pearl millet and 85.0.degree. C in the case of finger millet and rice. Total repetitive and cot 1.0 DNA fractions in all 3 spp. are isolated and their melting properties are compared with those of respective sonicated DNA. In finger millet, the Tm values of cot 25 and cot 1 fractions are lower by 10.8.degree. C and 12.8.degree. C, respectively, than that of sonicated DNA and thus exhibit the presence of a base pair mismatch in the range of 10.8-12.8%. In rice, the Tm values of the fractions cot 50 and cot 1 are slightly lower than that of sonicated DNA and reveal a nucleotide mismatching of only 1.8-3.8%. In the case of pearl millet cot 10 DNA fraction a high-melting DNA component (Tm = 92.degree. C) representing 12% of the total cot 10 DNA and a low-melting component with a Tm of 78.degree. C are present. In cot 1 DNA fraction of pearl millet the proportion of the high-melting component is 35% and it has a Tm of 94.8.degree. C. Optical reassociation studies of cot 1.0 DNA fractions revealed the presence of 2 kinetically distinct components, minor fast-reassociating and major slow-reassociating, having complexities in the range of 330-390 np and 1.28 .times. 105-6.0 .times. 105 np, respectively, in pearl millet and rice and only 1 DNA fraction with an average complexity of 7.7 .times. 104 np in finger millet.

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