FAQ: How does excess residual protein in blood and tissue preps affect the purification process?

If there is excess protein in the lysate prior to the addition of isopropanol for the precipitation step, the contaminating protein inhibits proper binding of the DNA to the beads. Excess protein in the lysate causes the precipitated DNA to float within the lysate and does not allow it to bind to the beads efficiently. Additionally, the glass beads will tend to get stuck in the bottom of the 2 ml tubes. It is therefore very important to remove as much of the hemoglobin-containing supernatant as possible from the erythrocyte pellet during a blood prep; do not leave more than 20 μl behind. The same is true during tissue preps; exceeding the maximum input amounts will also lead to excess protein in the lysate and DNA will not bind efficiently to the beads.