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?
In: 3306
**ELI5:**
Cancer is like a box of pencils. Imagine you have a big box of pencils, that needs to last you until high school.
Losing pencils is bad, because it means you won’t be able to do your homework. It’s especially bad if you lose a lot of them. Imagine if you tripped and broke half of your pencils. That sucks, but it’s okay since you have a pretty big box of pencils, and there’s still a lot in there. You finish 3rd grade with plenty of pencils left.
Over the years, you lose one here or there. You successfully finish some of them, and proudly show off the tiny eraser, telling your mom you worked hard.
By the end of 6th grade, you don’t have that many pencils left. Eventually, you use up the last pencil, and the underpaid teacher can’t afford to buy you a new box of pencils. You don’t get to go to high school because you can’t do your homework. The end. Take care of those pencils, you’ve got a lot of years of school, and they need to last a long time.
**ELI20:**
The body has many mechanisms to prevent cancer, sort of like fail-safes in case any number of them are damaged.
To simplify, let’s just say there’s only two mechanisms: die if DNA is damaged, and grow only when told to.
It’s very unlikely for any one cell, by random chance, to have damage to both of these protective mechanisms.
If you get a bad sunburn, you might damage the first mechanism. So the damaged cell no longer kills itself when it notices excessive DNA damage. Still, it doesn’t grow abnormally, so it’s not cancer, and overall you’re fine.
Most of these cells are pushed out and die off, but the radiation penetrated deep enough to affect the stem cells that are responsible for replenishing the skin layers. The stem cells are never replaced, so the damage sticks around for the rest of your life. All of the offspring of that stem cell will have damage to the first mechanism.
Decades later, it just so happens that UV radiation hits one of the offspring of that damaged stem cell in just the right spot, causing it to grow even when it hasn’t been told to. Now you’ve lost both protection mechanisms, and you have cancer.
If it was any other cell, it would have been fine, because the first protective mechanism would have killed off the damaged cell, preventing the cancer. But because you had a bad sunburn in the past, you’re at a much high risk of cancer in the future. Wear your sunscreen. You only have so many pencils in your box.
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