Theoretically, the minimal sample requirement for polymerase chain reaction analysis is a single cell. A single cell has approximately 6 pg of DNA. This number is derived from the molecular weight of A/T and G/C base pairs (617 and 618 g/mol, respectively). There are about three billion base pairs in one copy of the human genome; therefore for one genome copy:
1.85 X 1012 g/mol X 1 mol/6.023 X 1023 molecules = 3.07 X 10-12 g = 3pg
A diploid cell has two genome copies, or 6 pg of DNA. One ng (1000 pg) of DNA should, therefore, contain 333 copies (1000 pg/3pg/genome copy) of each locus.
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