The time of day, determined by a cell's internal clock, has a stronger influence on cell division than previously thought, according to a study published last week in Proceedings of the National Academy of Sciences.
Several factors are thought to play a role in a cell's decision to divide, including the size of the cell, the time of day, and cues from the environment, such as the amount of light. Now, scientists from the University of Cambridge and Imperial College London have shown that the circadian clock continuously influences cell division throughout the day and night.
The team designed a set of experiments with colonies of cyanobacteria to pick apart the influence of time of day, size of cell, and the presence of light on cell division. First, they observed division rates for cyanobacteria altered to lack circadian clocks, as well as rates of unaltered cells under constant light conditions. Using the division patterns from these experiments, and what was thought to influence them, Imperial mathematicians and collaborators then designed models to predict what would happen if the light changed over the course of future experiments.
In the subsequent experiments, the team found that rather than the circadian clock acting like an on/off switch or 'gate', forbidding cell division at certain times, it acts to fine tune the process by decreasing division at certain times and accelerating it at others.
"Cells born in the early part of the day grow to a smaller size before dividing again, because they seem to be in a 'rush' to divide before the end of the day. In contrast, cells born later in the day are in less of a 'rush', and therefore they grow to a bigger size, and avoid dividing in the period that normally corresponds to darkness at night," explained lead author of the study Dr Bruno Martins from the University of Cambridge.
Image: A graph of cell size, an image of a clock with day/night portion, and a graph of daylight and nighttime across 24 hours, all feeding into a graph showing different lengths of cells dividing at different times of the day. Image courtesy of Bruno Martins.