if our skin cells are constantly dying and being replaced by new ones, how can a bad sunburn turn into cancer YEARS down the line?
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Not all cells die, the new ones don’t appear out of thin air.
The skin cells of the superficial layer (Epidermis) of the skin – which is divided into four or five sublayers – is formed at the bottom layer, as cells there divide. Some of these cells migrate to become more superficial (and then later on die), while some, “immortal cells” (also called stem cells), in the deepest layer stay there to divide and produce more cells.
If the cells which divide to form new cells are damaged, then this damage will be in every cell born when they divide.
Once you have one cancer cell, it will take a very long time (up to years) until you actually can detect the cancer. The growth is exponential, which means it will be fast later on, but it also means the growth can be really slow in the beginning.
Also, damages accumulate and add up over time. Some damage can be a prerequisite for cancer. Simplified, once you get type A damage, this will not cause cancer. Neither will damage B, which is different from A. But once you get damage types A and B in the same cell, you will get a cancer cell.
Cells can also damage in a way that the cells don’t die, despite they would have been meant to be programmed to die. So, despite a cell which is not supposed to be “immortal” (or a stem cell) can start acting like one. This is bad and will result in cancer.
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