What is the small band below my fragment of interest on an agarose gel after DNA cleanup using QIAquick?
FAQ ID -148

Occasionally, DNA fragments eluted from the silica matrix of QIAquick, MinElute or QIAEX II Kits will contain denatured single-stranded DNA (ssDNA), appearing as a smaller band on an analytical gel. Under certain conditions, chaotropic agents (present in all silica-based DNA purification methods) can denature DNA fragments. This is a rare event that may be influenced by sequence characteristics such as the presence of inverted repeats or A–T-rich stretches.

Because salt and buffering agents promote renaturation of DNA strands, the following tips are recommended:

  • use the eluted DNA to prepare your downstream enzymatic reaction, but omit the enzyme. Incubate the reaction mix at 95°C for 2 minutes to reanneal the ssDNA, and allow the tube to cool slowly to room temperature before adding the enzyme and proceeding
  • alternatively, the DNA can be eluted from the silica-gel membrane or resin in 10 mM Tris buffer containing 10 mM NaCl. However, the salt concentration of the eluate must then be taken into consideration in downstream applications.