Final answer:
UV-B light can cause a specific type of DNA damage called thymine dimers, leading to mutations and potentially to conditions like skin cancer. The damage is proportional to the amount of UV-B radiation received, as suggested by the linear hypothesis. Cells can repair such damage with enzymes like photolyase, although not all organisms, including humans, possess this repair mechanism.
Step-by-step explanation:
When ultra-violet light, particularly UV-B, strikes DNA, it can cause damage in several ways. One common type of damage is the formation of thymine dimers. This occurs when adjacent thymine bases on a DNA strand form an abnormal covalent bond with each other, creating a bulky dimer that distorts the DNA molecule. As a result, the distorted DNA molecule does not function properly and can lead to mutations. Cells have mechanisms to repair this damage, such as through the enzyme photolyase, which, in some organisms, can directly reverse the damage caused by UV light.
Despite the presence of Earth's ozone layer, which absorbs most UV-B and all UV-C light, sufficient amounts of UV-B can reach the Earth's surface and cause DNA damage. This exposure can lead to skin cancers in humans, such as malignant melanoma, and can also cause cataracts. The linear hypothesis suggests that the extent of this damage is directly proportional to the amount of radiation received.
Furthermore, UV light can also lead to indirect DNA damage through the creation of free radicals, especially if sunscreen has penetrated into the skin. On the other hand, UV-A light mostly creates free radicals, causing indirect DNA damage. Lastly, the primary structure of the DNA helix is most affected by damage, where the bases themselves are chemically modified, which can disrupt the molecule's regular helical structure.