GCSE Biology

Could someone please explain the following formula: Length of time in stage = (observed number of cells in stage / total number of cells observed) * total length of cell cycle. I don't understand how you're supposed the calculate the length of time in a stage just by counting the number of cells.

Assuming that the cells aren't synchronised (i.e. they don't all do the same thing at the same time), we can expect that, at a point in time, the cells will be in different stages of the cell cycle. We can assume that the proportion of cells in a particular stage at any particular time will be proportional to the duration of that stage of the cell cycle.


Imagine you're walking to the shop, and come back a different route – on a map this would look like a "loop". When you're out on this loop, walking at a constant speed, you might spend some time on a pavement, some time on a road, some time by a canal, and some time in a field. So it makes sense that if you're spending a lot of time on a pavement, then a lot of the "loop" will be on the pavement.

Now imagine your whole school (a lot of people!) is walking this loop. If you count the number of people in each type of environment at any given time, you'll probably find that there are a lot of people on a pavement, and different numbers of people in fields and on roads and by the canal depending on their length in proportion to the length of the whole loop. So, if you know how many people there are in total, how many are on a pavement, and how long the total loop takes, you can estimate the length of time a person spends on a pavement.

Same goes for the cell cycle. The cells aren't walking round in a physical loop, but their "state" is changing. The calculation is just an estimate, but with enough cells your estimate will be pretty close to the actual length of time spent in a stage