As the onion root tip grows deeper into the ground, the cells at the tip continue to divide. One of the resulting cells will remain in place and differentiate into a mature cell, such as a vessel element or a sieve cell. The other cell will remain meristematic, continuing to divide as the root grows deeper into the soil. The word meristem, unfortunately, doesn’t have a useful root to learn. It is perhaps ironic that the word meristem cannot be divided up into interpretable segments, as the term is derived from a Greek word that means “to divide”.
Cells that remain in the meristematic region will remain in the cell cycle, entering a period called interphase, which is composed of three distinct phases. G1 (gap 1), is when cells duplicate the contents of the cytoplasm, including the organelles and cytosol. At the end of G1, they will reach a checkpoint. If duplication of cell contents was successful, they will pass into S-phase (synthesis). During S-phase, DNA is replicated and each chromosome becomes a set of sister chromatids. After S-phase, there is another checkpoint to ensure that replication of cell contents, specifically the DNA, was successful. Finally, the cell enters G2 (gap 2), where it passes a final checkpoint before division. The cell enters cell division again, completing mitosis and cytokinesis to produce two identical daughter cells.
Cells that differentiate exit the cell cycle into G0 (resting). In this stage, cells can begin to express different genes and specialize for specific tasks. You will see these cells in the upper portion (away from the tip) of the onion root. Note that these cells are much larger than the actively dividing cells in the meristematic region of the root.
Why are so many checkpoints necessary during this process? What do you think would happen if there weren’t checkpoints?