When and where
DNA replication happens before cells divide. In hours old embryos that develop into multicellular organisms, all the cells replicate their DNA and divide into new cells. Hours or days later, multicellular organisms have developed specific meristem regions where new cell production occurs. A visual example of meristem cells where DNA replication is needed is the tip of a plant root (Figure 10). New cells made at the root tip allow the root to grow. The root must also replace the cells that are damaged as the growing root moves through soil. In Figure 10, the meristem region (1) contains meristem cells that are replicating their chromosomes, then dividing into two identical cells by mitosis. Cells on the tip part of the meristem specialize in root cap cells (2,3) with the job of protecting the meristem. The fate of these cells is to die (4), and they need to be replaced by new cells made in the meristem. Cells made on the other half of the meristem (5) can elongate and eventually specialize. These root cells will then function for the plant, all season for annual plants, years for perennial plants. Replication of DNA will only happen in the meristem cells.
Figure 10. Plant root tip (left) and Skin cell layers (right). Left: Root tip by E. Boutet, 2007. CC BY-SA 2.5. Right: Epidermis-delimited, by K., 2010. Public Domain.