The number-average molecular weight, Mn, of a random distribution of single-stranded DNA molecules can be calculated directly from the profile obtained after alkaline sucrose gradient sedimentation. For such a distribution Mn can also be obtained from the weight-average molecular weight, <tex-math>$M_{w}$</tex-math>, and the median molecular weight, <tex-math>$M_{{\textstyle\frac{1}{2}}}$</tex-math>. When the size distributions are not random the latter procedures are erroneous. Nonrandom distributions of the type to be expected after X-irradiation of cellular DNA and its repair were generated by computer using Monte Carlo procedures. These distributions were then utilized as examples to illustrate the types of error which can be encountered when <tex-math>$M_{w}$</tex-math> and <tex-math>$M_{{\textstyle\frac{1}{2}}}$</tex-math> are used to calculate Mn from nonrandom distributions.

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