UV RADIATION TYPES AND THEIR CARCINOGENECITY

Answer for Old question Paper

Three types of UV radiation:

• UV-A
• UV-B
• UV-C

UV-A and its carcinogenecity

• Wavelenght range: 320 - 400 nm
• As the wavelength is long they reach the deeper layers of the skin unlike UV-B which reaches only the stratum corneum (visible layer of the skin containing dead cells)
• UV-A radiation like the other two forms destroy collagen fibres.
  
• UV-A radiation is not at all absorbed by the ozone layer and it reaches the earth directly.
• Causes sun burns quickly. It triggers the melanocytes to produce melanin responsible for the color of the skin burn lesions.
• The lesions later transform into melanoma, a malignant form of skin cancer.
• This is due to uncontrolled division of the melanocytes.
• The effects of UV-A radiation is due to indirect effects unlike the UV-B radiation.
• UV-A radiation leads to the generation of highly reacting species like hydroxyl and free oxygen radicals, which damages the DNA leading to cancerous growth.
• Sunscreens donot usuallly contain protectants for UV-A. Recently materials like titanium oxide that confer protection against UV-A radiation are being incorporated into the sunscreens.
  

UV-B and its carcinogenecity

• Wavelength range: 280-320 nm
• It reaches the stratum corneum of the skin (visble layer of the skin with dead cells)
• Even UV-B radiation causes skin burns. but it tkaes 2 days of latent period for the lesions to appear. The melanocytes produces melanin thats responsible for the color.
• Melanin is a photoprotectant. It absorbs the radiation and releases the energy in the form of heat.
• The carcinogenecity of UV-B radiation is due to the direct DNA damage. UV-B radiation excite the DNA molecules leading to the formation of covalent bonds between the adjacent CC residues in the same DNA strand. When the Replication process commences, this CC dimer is read as AA. This results in a TT residue in the daughter strand. This mutation is referred to as 'classical C-T mutation'. The mutations leads to cancerous growth.
• Sunscreens contain protectants that confer protection from UV-B radiation.

UV-C and its carcinogenecity:
Wavelength Range : 100-280 nm
This is high energy radiation like the UV-B radiation.
It is completely absorbed by the earthds atmosphere.
But artificial sources are the causes for UV-C exposure.
Like the UV-B radiations it may directly damage the DNA molecules leading to cancerous growth.

Activated RAS oncogenes were identified in human skin cancer cells. Most of the mutations in the RAS gene were found in pyramidine rich residues. this indicates that pyramidines are the targets of UV induced mutations.
P53 helps in removing the damages caused by radiation