How many DNA-containing cells are needed for STR analysis?

• A. Only the equivalent of 18 DNA-containing cells are needed.
• B. 5 cells
• C. 100 cells
• D. 1 cell

Answer: (A)

Stripping down to the practical idea: STR analysis is extremely sensitive and can produce usable profiles from surprisingly small amounts of DNA. A human diploid cell contains about 6.6 picograms of DNA, so the equivalent of 18 DNA-containing cells is roughly 120 picograms. Modern PCR-based STR kits can generate readable profiles from DNA in the low hundreds of picograms, though at that level you may see more stochastic effects like allele drop-out. That makes the equivalent of around 18 cells enough in many cases to obtain a profile, which is why this option is the best choice. Using more cells would simply provide more DNA and reduce dropout risk, but it isn’t necessary to achieve a readable STR profile. Fewer cells, such as a single cell, are often at too low a level and prone to incomplete results.