Photoprotection
A 2005 article in Mutation Research ("Melanogenesis: a photoprotective response to DNA damage?") explains that the body darkens the skin in response to UV light in order to protect the skin cells from becoming damaged. Ultraviolet light (UV light) is a wavelength of light rays that are not visible to the human eye but are present in sunlight and some forms of artificial light. UV light is dangerous for the skin because the light is able to penetrate cells and cause DNA damage. UV light rays can cause parts of DNA (called nucleotides) to fuse together or get fragmented. These genetic changes can result in genetic mutations, which can either kill the cell or cause it to become cancerous. UV light can also cause damage to proteins and other molecules within the cell. Special skin cells called melanocytes secrete pigments (which darken the skin) in response to UV light to protect skin cells.
Melanocytes Stimulation
A 2007 article in The FASEB Journal ("Human skin pigmentation: melanocytes modulate skin color in response to stress") details how UV light is able to cause the body to darken the skin. DNA and protein damage from UV light activates certain stress proteins (proteins that get activated when cells are exposed to physiological stress) in human skin cells. These stress proteins can stimulate melanocytes, in essence telling these pigment-producing cells that UV light is causing DNA damage. This causes the activation of certain proteins melanocytes use to make protective skin pigments. In addition, UV light can change the chemical environment of the melanocytes themselves, which leads to the activation of these same pigment-producing proteins. Regardless of the mechanism, UV light signals melanocytes to make pigments that will darken the skin.
Melanin Synthesis
The main pigment melanocytes make to protect the skin is called melanin. Melanin absorbs UV light, thus preventing further damage. Melanin is made from the amino acid tyrosine. When melanocytes need to make melanin, they use an enzyme called tyrosinase to start a series of chemical reactions that converts tyrosine into melanin. Stress proteins from nearby skin cells cause melanocytes to make more tyrosinase, which speeds the production of melanin. Because the stress proteins are a response to UV light, UV light is essentially able to increase the amount of tyrosinase in melanocytes. This cause more melanin to be produced, which leads to darkening of the skin.
