The key is to use fresh samples and not to overload the column. Low yield or purity of genomic DNA is usually due to incomplete digestion or incomplete Lysis of the sample. Starting with a maximum amount or volume of samples does NOT usually give the best yield of DNA. On the contrary, it usually results in incomplete sample Lysis and degradation of proteins, thus making extraction of all DNA from the sample unfeasible. Further, it always requires subsequent removal of undigested residues and yields viscous sample lysate. When the lysate is too viscous, it not only has difficulty in passing the column, but also indicates the presence of an abundant amount of contaminants such as proteins and salts. High amounts of contaminants not only affect DNA binding, but also may not be washed off completely, leading to carry over to the eluted genomic DNA. Therefore, a good quality and yield of DNA is only expected when a sample is completely digested. We advise starting with half of the maximum amount of sample suggested. When there aren’t any problems with digestion or passing the lysate through the column, the sample amount can be increased gradually in the subsequent preparations.
What is the key to successfully isolating genomic DNA of good yield and quality? "