The Mitotic and Endomitotic Nuclear Cycle in Allium Carinatum II. Relations Between DNA Replication and Chromatin Structure

SUMMARY

The relations between DNA replication and nuclear chromatin structure were studied in root tips of Allium carinatum by autoradiography, using ³H-thymidine as precursor. The findings give evidence for the existence of significant structural differences between nuclei in G₁ and in G₂, and for DNA replication of heterochromatin in the condensed (= chromocenter) state.

A « dispersion stage », showing decondensation of the chromocenters but no DNA synthesis, occurs in early mitotic prophase and in endomitosis. It is interpreted as morphological expression of the chromatid separation.

The similarity of the Angiosperm endomitosis to the early stage of mitotic prophase indicates its nature as short-cut of the cell cycle in early prophase, but not as endoreduplication, which would mean an interphasic event.