Question

optical density of a 400 base pair long 1ml DNA solution was found to be 0.052. How many DNA molecules are present in the solution

Answer 1

1 optical density is equal to 50μg/ml of dsDNA.
0.052 corresponds to 2.6μg/ml of dsDNA.
Average molecular weight of 1 nucleotide base pair is 650 dalton. So, mol. wt. of 400bp is 650x400=260000.
Number of moles of DNA is 2.6/260000=1x10^​-11
One mole contain 6x10²³ molecules. So, 1x10^​-11 moles will contain 6x10²³x1x10^​-11=6x10¹² DNA molecules.