Figure 1.

Asymmetrical division in S. cerevisiae provokes a conceptual problem in transcription rate control along successive cell generations. (A) In symmetrical cell division increased cell size is paralleled by nTR in such a way that SR is kept constant, and both identical daughter cells have approximately the same volume, nTR and SR, as their previous generation. This has been observed in S. pombe and human fibroblasts (4,5), where no change in [mRNA] and, therefore in the mRNA half-life (HL), has been detected. (B) With asymmetrical division that produces a large mother (M) and a small daughter (D) cells, a similar model for nTR control would produce daughter cells with higher SRs than the previous generation, which would render this model unsatisfactory to explain actual behavior in S. cerevisiae. (C) We propose a model in which nTR remains constant with volume, which would provoke a lower SR inversely to increased volume. If [mRNA] ribostasis is strictly conserved, an increase in HL will appear. Numbers inside cells represent SR values. Note that genome replication occurs in between second and third growth stages in the three models shown. In the case of model C nTR should be duplicated at the end of the replication (2) to be then divided (1+1) between daughter and mother cells.