Biological clocks and timing of development and division

4 problems

• How noisy coupled clock cells produce sharp segment boundaries

  Partially resolved

  Each cell of the tissue that forms backbone segments carries its own noisy genetic clock, and neighbors keep each other in step by signaling through contact. How precisely segment boundaries can be placed, given this noise, cell mixing and cell division, is not quantified.

• What sets the species-specific period of the segmentation clock

  Partially resolved

  The genetic oscillator that times backbone segments has a period of roughly 2 to 3 hours in mouse and about 5 hours in human cells grown in a dish. Which single cellular property slows every step of the clock by the same factor is not established.

• Molecular mechanism of the adder in bacterial size control

  Contested

  Bacteria such as E. coli add a fixed amount of volume between birth and division, whatever size they were born at, which keeps the population at a stable size. Which molecular process enforces this rule is disputed.

• Mechanism of temperature compensation in circadian clocks

  Open

  Most chemical reactions speed up strongly when warmed, yet circadian clocks, the cell's 24-hour timers, keep nearly the same period over a wide range of temperatures. How the clock cancels the temperature dependence of its reactions is not understood.