  Cell growth and gene expression, essential elements of all living systems,
have long been the focus of biophysical interrogation. Advances in single-cell
methods have invigorated theoretical studies into these processes. However,
until recently, there was little dialog between the two areas of study. Most
theoretical models for gene regulation assumed gene activity to be oblivious to
the progression of the cell cycle between birth and division. But there are
numerous ways in which the periodic character of all cellular observables can
modulate gene expression. The molecular factors required for transcription and
translation increase in number during the cell cycle, but are also diluted due
to the continuous increase in cell volume. The replication of the genome
changes the dosage of those same cellular players but also provides competing
targets for regulatory binding. Finally, cell division reduces their number
again, and so forth. Stochasticity is inherent to all these biological
processes, manifested in fluctuations in the synthesis and degradation of new
cellular components as well as the random partitioning of molecules at each
cell division. The notion of gene expression as stationary is thus hard to
justify. In this review, we survey the emerging paradigm of cell-cycle
regulated gene expression, with an emphasis on the global expression patterns
rather than gene-specific regulation. We discuss recent experimental reports
where cell growth and gene expression were simultaneously measured in
individual cells, providing first glimpses into the coupling between the two.
While the experimental findings, not surprisingly, differ among genes and
organisms, several theoretical models have emerged that attempt to reconcile
these differences and form a unifying framework for understanding gene
expression in growing cells.
