The passage of a wave of tritiated. thymidine‐labelled cells through each of the phases of mitosis is analysed in erythroblasts of four vertebrates species, selected because of their widely differing diploid DNA content. Mitotic time (tm) and relative durations of separate mitotic phases as well as that of the G2 period are thus accurately determined. Mean duration of tm appears to vary according to the species specific DNA amount. This relation is approximately the same as for S, for G1 and for the generation time, which have been previously determined.

Erythroblasts of four vertebrate species (Triturus cristatus, Rana esculenta, Lacerta viridis and Gallus domesticus) differing markedly in their nuclear diploid DNA content, are used to study a possible relationship between cell cycle duration and DNA content. DNA is determined cytophotometrically and fluorometrically. The cell cycle is analysed by evaluating labelled mitoses after an injection of tritiated thymidine and also by double labelling with 14C‐ and 3H‐thymidine. A direct but non‐linear relationship is demonstrated between DNA content of erythroblast nuclei and the duration of DNA‐synthesis.

In the circulating blood of anemic ducks, 5 % of all erythroid cells synthesize DNA. Immature erythroblasts, at all stages of differentiation, synthesize DNA although to a varying degree, while reticulocytes and erythrocytes do not. In the erythroid cell population labeled in vitro 2 h with 32Pi, half of the labeled DNA sediments as small-molecular-weight molecules, suggesting that these molecules fail to integrate into the high-molecular-weight components.Labeled DNA is found in the cytoplasmic postmitochondrial fractions and it is in a form of deoxyribonucleoproteins which cosediment with ribosomes as well as subribosomal particles in sucrose gradients. However, fixation with HCHO and centrifugation to equilibrium in CsCl gradient of these particles shows that the deoxyribonucleoprotein bands at the density different than the ribosomes and, thus, not physically linked to them. In EDTA-dissociated ribosomes, the deoxyribonucleoprotein particles cosediment with ribosomal subunits in such a way that the larger the particle, the larger the molecular weight of the DNA cosedimenting with it.The specific radioactivity of the cytoplasmic ribosome-derived and postribosomal-particlederived DNAs and the small molecular-weight nuclear DNA is similar and 10-20-fold higher than that of the bulk nuclear DNA. The former three DNA species sediment between 4-14 S. It is concluded that the cytoplasmic nonmitochondrial DNA species are of the nuclear origin. Less than 0.5 % of the total cellular nonmitochondrial DNA can be purified from the nucleus and the cytoplasm as fast-labeled small-molecular-weight components. All of the cellular nonmitochondrial DNA species band at the same mean buoyant density in Cs2S0,/urea gradients. All behave as native structures in hydroxyapatite and contain less than 5 "/, of their length as single-stranded regions. down product of the nuclear DNA. Furthermore, Fromson and Nemer [7] have attributed the presence of DNA in cytoplasmic nonmitochondrial fractions to contamination by a nuclear leakage. The problem, however, remains unsolved due to recent findings by Koch [8,9] that when compared to the nuclear DNA, the cytoplasmic DNA seems to be relatively enriched in unique sequences.The transfer of the products of transcription from the nucleus to the cytoplasm in the form of mRNA and its translation on ribosomes has been extensively investigated, for instance in avian erythroid cells [lo, 111. The primary transcription product of the globin genes is of the size 5 x lo6 daltons [12,13] and is suggested to be the precursor of the cytoplasmic mRNA (pre-mRNA). Although there is little doubt about the role of this pre-mRNA as intermediary in cellular information transfer, a straightforward scheme of direct