| The relative amounts of reassociation of Agropyron and wheat as
well as Agropyron and rye DNA's compared to reassociation of homologous
DNA's are shown in Table 2. Invariably, the amount
of reassociation between heterologous DNA's was lower than that between
homologous DNA'S. From the DNA-DNA hybridization experiment in which Agropyron
DNA was filter-immobilized and wheat or rye DNA was in solution, it appears
that rye DNA nucleotide sequences differ more from those of Agropyron
DNA than from those of wheat. However, in the reciprocal hybridization in
which Agropyron was in solution and either rye or wheat DNA was filter-immobilized,
rye and wheat nucleotide sequences differ equally from Agropyron
DNA. The comparison of wheat with rye DNA was not included because it has been carried out by BENDICH and MCCARTHY (1970a). They observed that when wheat DNA was filter-immobilized and rye DNA was in solution, the reassociation between the DNA's of wheat and rye was only 60% of that of the homologous control. However, in the reciprocal hybridization the amount of binding of wheat DNA to filter-immobilized rye DNA was equal to that of the homologous control. The lack of reciprocity in the amount of reassociation between DNA of wheat and rye was also noticed in a DNA-DNA hybridization experiment employing a DNA-agar technique (BENDICH and BOLTON, 1967). Thermal Stability of Reassociated DNA/DNA Duplexes Another estimate of nucleotide sequence homology among the DNAs mentioned above was based on differences in the thermal dissociation profiles of homologous and heterologous reassociated DNA/DNA duplexes. The thermal dissociation of double-stranded DNA is proportional to the number of hydrogen bonds per unit of the DNA double helix (MARMUR and DOTY, 1962). Imperfect matching of strands in reassociated DNA due to the presence of noncomplementary bases results in a lower number of hydrogen bonds per unit of DNA/DNA duplexes and, consequently, in a decrease in thermal stability as measured by the Tm (BAUTZ and BAUTZ, 1964). The decrease in the Tm of heterologous DNA duplexes compared to the Tm of homologous DNA duplexes will be referred to as the deltaTm. LAIRD et al. (1968) estimated that a deltaTm of 1C is approximately equal to the mismatching of 1.5% of the nucleotides in the reassociated DNA. The thermal dissociation of homologous or heterologous DNA/DNA duplexes was investigated according to a procedure previously described by BENDICH and MCCARTHY (1970a). The deltaTm's of heterologous DNA/DNA duplexes involving the DNA's of Agropyron and wheat or Agropyron and rye (Table 3) follow a pattern similar to the relative amounts of reassociation between these DNAs (Table 2). The deltaTm of wheat-Agropyron heterologous DNA/DNA duplexes was 2.4C smaller than the deltaTm of rye-Agropyron heterologous DNA/DNA duplexes. Again the DNA nucleotide sequences of Agropyron appear to be more like wheat than those of rye. The reciprocal hybridizations involving rye and Agropyron DNAs did not give identical results. While the deltaTm was 4.6C for heterologous DNA/ DNA duplexes from hybridization in which Agropyron DNA was immobilized and rye DNA was in solution, the deltaTm of DNA/DNA heterologous duplexes from the reciprocal hybridization was only 0.8C. |
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