At the cellular level things tend to hapepen rather quickly and on small scale. For example you can have a small amount of bacteria, which are single cells, reproduce rather quickly into massive cell counts, and similarly you have human cells which can reproduce just as quickly in numbers sufficient to mantain tissues and organs. In reality, Human cells are constantly dying with new cells taking their place. This is how, for example, youre able to loose a small amount of blood containing millions of RBCs, without becoming anemic.

Hence, it therefore follows, given the general rule that cellular reproduction = some probability of DNA mutation, that the DNA is constantly changing on the extreme small scale, such that we might not even be able to detect the change due to its small scale, so long as the multicellular organisim is alive. This is why we have RNA and ribosomes - to avoid large mutatuons in DNA.