At the restriction point (R), mammalian cells irreversibly commit to divide. R has been viewed as a point in G1 that is passed when growth factor signaling initiates a positive feedback loop of Cdk activity. However, recent studies have cast doubt on this model by claiming R occurs prior to positive feedback activation in G1 or even before completion of the previous cell cycle. Here we reconcile these results and show that whereas many commonly used cell lines do not exhibit a G1 R, primary fibroblasts have a G1 R that is defined by a precise Cdk activity threshold and the activation of cell-cycle-dependent transcription. A simple threshold model, based solely on Cdk activity, predicted with more than 95% accuracy whether individual cells had passed R. That a single measurement accurately predicted cell fate shows that the state of complex regulatory networks can be assessed using a few critical protein activities.

In a process similar to exon splicing, ciliates use DNA splicing to produce a new somatic macronuclear genome from their germline micronuclear genome after sexual reproduction. This extra layer of DNA rearrangement permits novel mechanisms to create genetic complexity during both evolution and development. Here we describe a chimeric macronuclear chromosome in Oxytricha trifallax constructed from two smaller macronuclear chromosomes. To determine how the chimera was generated, we cloned and sequenced the corresponding germline loci. The chimera derives from a novel locus in the micronucleus that arose by partial duplication of the loci for the two smaller chromosomes. This suggests that an exon shuffling-like process, which we call MDS shuffling, enables ciliates to generate novel genetic material and gene products using different combinations of genomic DNA segments.

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