How Do We Tan?

How does our skin "see" UV light?  How does your skin know to get darker?   To flush with blood during a sun-burn?

If I understand correctly...

• UV radiation around the 300 nm wavelength hits DNA (chromosome telomeres are particularly vulnerable because of their repeat series of thymine)
• The photon elevates an electron to an outer shell on part of the molecule
• This change in atom shape changes the molecules shape
• The change in molecule shape makes a hickey on the DNA spiral called a pyrimidine dimer
• The dimer stops proper DNA replication creating incomplete single stranded DNA sequences
• These sequences combine with an enzyme (Lex A ) which then causes the expression of genes that produce enzymes that repair UV damaged DNA (nucleotide excision repair)
• The presence of the repair enzymes (somehow) causes...
• Flushing of blood, aka a sun burn
• This results in
• More repair
• Removal of dead cells (apoptosis)
• Increased melanin production (by melanocytes) thereby "shading" the cells from further damage
• We know the DNA repair enzymes communicate to our skin to make more melanin because we can artificially add them to skin cell sample or cause their numbers to rise by chemically damaging DNA and it produce more melanin without UV exposure.  More here.

Real guy, real sunburn

Melanin

• Melanin dissipates 99.9% of UV radiation as heat
• Melanin is also in parts of the fluid in your inner ear, medula, zona reticularis, adrenal gland and other neurons in the form of neuromelanin

Who knew there was more than one kind of melanin?

• Phaeomelanin that's yellow or red
• More common in whites/asians
• Eumelanin that's brown

Vitamin D3

• So why aren't all humans black?  If melanin is so great for protecting us from DNA mistakes and cancer why don't we all have it in abundance?
• We need vitamin D3 to make healthy bones
• UV light turns 7-dehydrocholesterol deep within the skin into vitamin D3
• I speculate that the heavy use of clothing in colder climates, thus covering more skin, would cause a greater need for lighter skin that is better at producing vitamin D3.  Meaning, that perhaps it wasn't less UV light towards the poles that made some skin colors so light as much as it was heavy use of clothing over evolutionary time.  

What about freckles?

• Why are freckles patchy?  Why not uniform hue across the skin?  Why do they tend to be on extremely fair skinned people?  Do they use more phaeomelanin over eumelanin?  Are they an adaptation to deal with a loss of function mutation?  Please let me know if you find any answers...
• Freckles apparently aren't a concentration of melanin producing cells (that is what a mole is), but only a patch of normal density melanocytes that has produced melanin.

Pictures from here, here, here, here, here.

Works sighted [sic]:
https://mail-attachment.googleusercontent.com/attachment?ui=2&ik=456a53bd10&view=att&th=1320e998bf429a6b&attid=0.1&disp=inline&realattid=f_grvgy34w0&safe=1&zw&saduie=AG9B_P8GDCc0dwBqoQc8G36dp0NA&sadet=1314556509504&sads=XcarW9THBtclWSIb4mapkRCLbw4

http://en.wikipedia.org/wiki/Melanin
http://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.1982.tb04362.x/abstract;jsessionid=042D2310D61189980612F01997231EDD.d01t04
http://www.sciencedirect.com/science/article/pii/S0027510704004907
http://en.wikipedia.org/wiki/Vitamin_D
http://en.wikipedia.org/wiki/Xeroderma_pigmentosum