Nonetheless, the ebb and flow of molecules in a cell must reliably convey instructions for such essential processes as DNA replication and cell division.
How big are the molecular fluctuations expected in a single yeast cell? Physicists estimate molecular fluctuation using a rule-of-thumb that the size of typical fluctuations is the square root of the average number of molecules. "If there are on average 900 molecules of a particular protein in a cell, then we can expect fluctuations of plus or minus 30 molecules, or 3.3 percent," said Tyson. "That is not too bad."
For DNA there might be a severe problem, Tyson noted, "because there is only one copy of every gene in a yeast cell. But cells are equipped with an elaborate and expensive mechanism to replicate DNA molecules and not allow the random fluctuations predicted by statistical physics."
The weak link in the is mRNA: the molecule that carries information from the gene to the cell's ribosomes, where proteins are made.
The literature reports that there is on average only 1 mRNA molecule per gene per cell, in yeast, and that each mRNA molecule lives, on average, for 15 to 20 minutes before it is degraded. "This is intriguing," said Tyson, "because the physicist's rule-of-thumb would predict very large fluctuations in mRNA abundance sometimes 1, sometimes 0, sometimes 2 or 3 or 4 -- which means the noise among mRNA molecules is huge, and it propagates to the level of the encoded protein."
The noisy fluctuations in protein level may be 50 percent instead of 3 percent. "There is no way the control system can work in the face of such large fluctuations," Tyson said. "It would be completely unreliable."
Progression through the cell cycle is indeed a noisy process, with typical flucutations of 15 to 20 percent for the time taken to complete the process. To achieve this level
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