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How long do melanocytes and melanosomes continue to protect DNA after UV exposure? Basically, if skin is tanned, then over the next month it is shed, will melanocytes continue to produce high levels of melanin to keep the new skin tanned without requiring more UV exposure? The cells react by producing more melanin after exposure but will they continue to do this in new skin cells?

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The tanning response to UV occurs in two phases:

  1. The immediate response begins at exposure to UV-A, reaches a max at 1-2 hours, and fades between 3 and 24 hours. This article speculates that this is due to the redistribution of melanosomes rather than additional melanin production.
  2. The delayed response is more durable, occurring by repeated exposure to UV-B, as well as as UV-A and visible light. "It is a gradual process in which the skin starts darkening 48–72 h after irradiation, reaches a maximum ∼3 wk after exposure, and the skin does not return to its original melanin content until ∼8–10 months later." This source says this is due to additional melanocytes, melanosomes, and more melanin, in general.

Both processes are influenced by genetic factors.

Source for the initial points, which also contains interesting details on the enzymes involved in these processes.

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Thanks for the resources, I'll need some time to read them :-D – Nick Apr 24 '13 at 23:54
Not a problem - it was interesting. I might start with the one source I linked to twice in the body of the answer. Although it's older, it is also not written by Avon scientists, and is less likely to be biased. – dd3 Apr 25 '13 at 3:40

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