Exposure to UV light from the sun or from tanning beds can inflict certain type of DNA damage that causes skin cancer even in the dark, the findings showed.

The study could lead to new preventive tools, such as an 'evening-after' sunscreen, the researchers from Yale Yale School of Medicine added.

In the current study associate research scientist Sanjay Premi, professor Douglas Brash and co-authors first exposed mouse and human melanocyte cells to radiation from a UV lamp.

Melanocytes cells make the melanin that gives skin its colour even in the dark, The radiation caused a type of DNA damage known as a cyclobutane dimer (CPD), in which two DNA 'letters' attach and bend the DNA, preventing the information it contains from being read correctly.

To the researchers' surprise, the melanocytes not only generated CPDs immediately but continued to do so hours after UV exposure ended. Cells without melanin generated CPDs only during the UV exposure.

This finding showed that melanin had both carcinogenic and protective effects.

"If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," said Brash. "But it is doing both good and bad things," he added.

The researchers next tested the extent of damage that occurred after sun exposure by preventing normal DNA repair in mouse samples.

They found that half of the CPDs in melanocytes were 'dark CPDs' CPDs created in the dark.

The study was published online in the journal Science.

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