A densitometric nondestructive microassay for DNA quantitation.

A nondestructive assay for small quantities of nonradioactive DNA has been developed. Submicroliter volumes of DNA samples of concentrations as low as 10 micrograms/ml in 10 mM Tris, pH 8, containing 10 micrograms/ml ethidium bromide are photographed in 1-microliter microcapillary tubes on a near-uv transilluminator. The negatives are scanned with a densitometer, producing a sharp peak for each capillary; samples whose peak heights show apparent linearity with DNA content are compared to the linear portion of a standard curve constructed from DNA samples of known concentrations. The samples may be recovered undamaged, as introduction into capillaries and illumination for photography do not introduce nicks into the DNA. The DNA samples may be of heterogeneous molecular weight, but all DNAs should be of similar base composition and in the same buffer. Traces of phenol do not interfere with the determination.