DNA Damage and Repair Mechanisms

DNA Damage by Radiant Energy

• Radiant energy, such as ultraviolet (UV) irradiation, can cause damage to DNA.
• UV irradiation can lead to the formation of covalent bonds between adjacent thymines on the same strand of DNA, creating thymine dimers.
• When DNA is damaged in this manner, it interferes with replication and transcription processes.

DNA Repair Mechanisms

• Most cells possess mechanisms to repair damage to DNA.

Light Repair (Photolyase)

• One repair mechanism involves an enzyme called photolyase.
• Photolyase can break the covalent bonds between thymine dimers, reversing the damage.
• This enzyme requires visible light to function; the light provides the energy for bond cleavage.
• Because it requires light, this mechanism is called light repair.

Excision Repair (Dark Repair)

• A second repair mechanism involves an enzyme that excises the damaged segment from a single strand of DNA.
• This process is called excision repair, or dark repair, as it does not require visible light.
• Following the excision of the damaged nucleotides, DNA polymerase replaces them with new, undamaged nucleotides.
• DNA ligase then forms the final phosphodiester bond to complete the repair.