What effect does ultraviolet light have on DNA?

Ultraviolet (UV) light has a well-documented effect on DNA, primarily by causing the formation of thymine dimers. When DNA is exposed to UV light, especially at specific wavelengths, the energy from the UV rays can lead to the bonding of adjacent thymine bases in the DNA strand. This abnormal bond forms what is known as a thymine dimer.

The presence of thymine dimers can disrupt the normal structure of the DNA helix and interfere with DNA replication and transcription, leading to errors and potentially causing mutations if not properly repaired. Cells contain mechanisms to recognize and repair these dimers; however, if the damage is extensive or the repair mechanisms fail, this can result in permanent changes to the DNA sequence.

The other effects, such as causing mutations or enhancing replication, do not accurately capture the immediate interaction between UV light and DNA. Additionally, UV light does not protect against damage; rather, its exposure is harmful to the integrity of DNA.