Effect of concentration on the formation of molecular hybrids from T4 DNA.

When the thymine of T(4) DNA is replaced by 5-BU the melting temperature of T(4) DNA is increased from about 83 degrees to about 93 degrees C. Heating and slow cooling of T(4) DNA at concentrations of about 30 mug/ml leads to aggregates which consist of several polynucleotide chains which appear in the electron microscope as a branched structure. The aggregates have regions which are true hybrids. When the concentration of T(4) DNA is lowered to less than 1 mug/ml the products of hybridization are not aggregates but have the morphology of native DNA molecules and the density labels are distributed as expected from the fusing of two chains of approximately equal length.