NDA (II) 2017 General Ability Test · Question 65 of 150

Correct — A, Ultraviolet radiation. Melanin is made by melanocytes in the base of the epidermis and passed to the surrounding skin cells, where the granules gather on the sunward side of the nucleus like a parasol over the DNA. The pigment absorbs ultraviolet light strongly and also mops up the reactive oxygen species that ultraviolet exposure generates, so it reduces the DNA damage that leads to sunburn, premature ageing of the skin and skin cancer. Tanning is this defence in action — ultraviolet exposure stimulates melanocytes to make more pigment. The same reasoning explains the broad global pattern by which populations long settled in strongly sunlit latitudes carry more melanin.

• (b)Infrared radiation — Infrared is the long-wavelength radiation we feel as radiant heat. Its photons carry too little energy to break chemical bonds in DNA, so there is nothing here for a pigment to shield; the body handles heat by sweating and by dilating skin blood vessels instead.
• (c)X-ray radiation — X-rays are far more energetic than ultraviolet and pass straight through skin, which is precisely why they are used to image bone. A surface pigment a fraction of a millimetre thick cannot stop them; lead shielding is what is used.
• (d)Short wave radio radiation — Radio waves sit at the low-energy end of the spectrum, with wavelengths measured in metres. They are non-ionising, they pass through the body without damaging it, and no biological pigment evolved to block them.

Where a radiation sits on the electromagnetic spectrum decides what it does to living tissue. Radio waves and infrared are non-ionising and mainly deposit heat. Ultraviolet is energetic enough to be absorbed by DNA and to produce lesions in it, but weak enough to be stopped in the outer skin — the one band where a pigment barrier is both necessary and sufficient. X-rays and gamma rays are strongly ionising and penetrate deeply, so they require dense external shielding rather than anything the body can make.

Order the four options by energy and the answer falls out. Only ultraviolet is both damaging and superficial, which is exactly the niche a skin pigment can fill. Two useful balancing facts: ultraviolet is not wholly harmful, since the skin uses it to make vitamin D, so heavy pigmentation in weakly sunlit latitudes carries its own cost; and the atmosphere does much of the work already, with stratospheric ozone absorbing the most damaging ultraviolet before it reaches the ground. Melanin also appears in hair and in the iris, where it determines eye colour, and its absence in albinism brings both light sensitivity and a high risk of sun damage.

• Melanin is produced by melanocytes in the basal layer of the epidermis and transferred to neighbouring skin cells.
• It absorbs ultraviolet radiation and neutralises the reactive oxygen species that ultraviolet exposure creates.
• Ultraviolet damage to skin DNA underlies sunburn, photo-ageing and skin cancer.
• Melanin also colours hair and the iris of the eye; its absence causes albinism.
• Stratospheric ozone absorbs most of the harmful ultraviolet before it reaches the earth's surface.

• Picking infrared because sunlight feels hot; heat is not what melanin defends against.
• Assuming a body pigment could stop X-rays, which pass through soft tissue entirely.
• Forgetting that ultraviolet is also useful, since the skin needs it to make vitamin D.

NDA sets one biological agent against four bands of the spectrum, so the way in is to rank the options by energy and by how deeply each penetrates.

• practice — not a real PYQ

  Which layer of the atmosphere absorbs most of the harmful ultraviolet radiation from the Sun?

  • (a)The stratosphere, through its ozone
  • (b)The troposphere, through its water vapour
  • (c)The mesosphere, through its carbon dioxide
  • (d)The exosphere, through its hydrogen

  Answer(a) The stratosphere, through its ozone — the ozone layer is the earth's first line of defence against ultraviolet, and melanin the body's second.

• practice — not a real PYQ

  Sunlight falling on human skin is needed for the synthesis of

  • (a)Vitamin D
  • (b)Vitamin C
  • (c)Vitamin K
  • (d)Vitamin B12

  Answer(a) Vitamin D — ultraviolet light converts a cholesterol derivative in the skin into a vitamin D precursor, which is why ultraviolet is not purely harmful.