Nuclear pores during the cell cycle in a slime mold, physarum polycephalum.

Freeze-fracture and thin sectioning techniques were used to follow in large synchronous plasmodia of Physarum polycephalum the changes in number and distribution of nuclear pores during the cell cycle. Using freeze-fracture, we determined that average pore frequency rises gradually from 14/micrometers(2) of nuclear envelope surface at early S to a value of about 22 just before prophase. Nuclear diameter averaged 3.3 micrometers at early S and increased to 4.3 micrometers at late G2. Calculating nuclear volume and average chromatin volume per nucleus with respect to time in the cell cycle leads to the conclusion that number of nuclear pores appears to be most directly related to amount of chromatin present per nucleus and to be independent of nuclear surface area.