E. coli cells were grown for many generations in a medium containing only the heavy nitrogen isotope ¹⁵N, so that all their DNA became fully heavy (¹⁵N/¹⁵N). The cells were then shifted to a medium containing only the light isotope ¹⁴N and allowed to divide exactly three times. Assuming semiconservative replication, what fraction of the DNA molecules present after the third division is completely light (¹⁴N/¹⁴N)?