Method for the selective size-fractionated separation and isolation of nucleic acid mixtures

A method for fractionated size-dependent isolation of a nucleic acid from a mixture of nucleic acids with different numbers of base pairs,

• the method comprising:
• a) adding, in the absence of an aliphatic alcohol, a first binding buffer, which comprises at least one chaotropic salt and at least one substance that raises the pH of the binding buffer to a volume of the mixture of nucleic acids;
• b) binding on a first solid phase and separation of the nucleic acids bound to said first solid phase, from a filtrate;
• c) mixing a second binding buffer, which has a lower pH than the first binding buffer, with the filtrate from said b) binding;
• wherein said second binding buffer comprises at least one selected from the group consisting of a nonionic surfactant and an alcohol;
• d) binding on a second solid phase and separation of the nucleic acids bound to said second solid phase;
• e) washing and elution of the nucleic acid isolated after said b) binding and said d) binding,
• with the result that the nucleic acid isolated after said b) binding not only has a larger number of base pairs than a nucleic acid isolated under said d) binding, but also that, both after both said b) binding and after said d) binding, individual, particular nucleic acid fractions with a particular number of base pairs are isolated that were not isolated in the respective other step wherein,
• prior to fractionation, first of all the total nucleic acid is concentrated in a sample volume of any desired size by reducing said sample volume.