Growth only gets you so far. For a coccoid cell, growth increases volume more rapidly than surface area, which is a problem if the cell relies on nutrients imported from the environment. Even if a cell is rod-shaped (so its surface area to volume ratio remains relatively constant with growth), increased volume increases diffusion times, making metabolism less efficient. So what can you do to keep your cell thriving?
It may be time to divide. In essence, division simply splits a cell, the “mother,” into two “daughters.” Each daughter will be roughly half the size of the mother. A fair split, though, particularly of critical components like the genome, requires careful coordination. Think about the things in your cell. Some of them are present in many copies, like the lipids in the membrane(s) and most of the proteins. Others are present in very few copies, like the chromosome(s). Conceptually, we can sort components into two broad categories: high copy-number and low copy-number items. How would you split the high copy-number items? Easy, right? Just split the cell in the middle and each half will have plenty. This is true, for instance, for the ribosomes you see in this Thiomonas intermedia cell.
What about low copy-number components? Look at the poly-phosphate storage granules in the same cell. To ensure each daughter cell gets the same number, they are evenly spaced along the length of the cell, ready for division in the middle.
