The molecular weight cut-off (MWCO) is a term used in membrane filtration to denote the pore size distribution and retention capability of a membrane. In general, for optimal recovery of most macromolecules, the recommendation is to use an ultrafiltration membrane with an MWCO that’s 1/3 to 1/2 of the target molecule’s molecular weight. 

MWCO Selection for DNA

• Calculate the molecular weight of the target DNA based on its nucleotide count.
• Choose an ultrafiltration membrane with an MWCO around one-third of the DNA's weight.

For instance, the average molecular weight of deoxyribnucleotide in DNA is 309 Da. 

Hence, a DNA strand with 1000 nucleotides would have a molecular weight of 309 x 1000 = 309,000 Da = 309 kDa. 

In this case, you would need a membrane with an MWCO of 103 kDa (1/3 of 309) for effective retention.