Deoxyribonucleic acid (DNA) carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms. Maintaining DNA integrity is crucial, especially when exposed to external stressors like ultraviolet (UV) radiation from sunlight. Exposure to UV-B light causes the formation of lesions known as thymine dimers, a common type of DNA damage. A thymine dimer forms a covalent bond between two adjacent thymine bases on the same DNA strand, creating an abnormal structure. This defect physically distorts the DNA helix, which prevents cellular machinery from accurately reading the genetic code. This damage effectively blocks both DNA replication and gene transcription unless it is quickly addressed and repaired.
Direct Reversal of Thymine Dimers
Some organisms possess a simple, highly efficient mechanism to fix thymine dimers called photoreactivation. This process is a form of direct reversal, meaning the damaged structure is restored to its original state without cutting out or replacing any DNA segments. The repair is catalyzed by a single enzyme known as photolyase, which binds specifically to the thymine dimer lesion.
Photolyase activity is dependent on light, absorbing energy from the blue or near-UV range of the visible spectrum. Once activated, the enzyme uses a radical mechanism to split the covalent bonds linking the two thymine bases. This action immediately restores the two separate thymine bases, making the DNA strand functional once again.
This direct reversal pathway is the primary method of thymine dimer repair in a wide variety of life forms, including bacteria, fungi, plants, and many non-placental animals. However, humans and most other placental mammals lack a functional version of the photolyase enzyme. Consequently, human cells must rely on a more complex, multi-step system to remove this common type of DNA damage.
The Steps of Nucleotide Excision Repair
In humans, the main defense against thymine dimers is Nucleotide Excision Repair (NER). This pathway fixes bulky lesions that significantly distort the DNA helix, such as those caused by UV radiation. The process begins with damage recognition, handled by a complex of proteins that constantly scan the entire genome.
The recognition phase often involves the protein complex XPC-RAD23B, which detects the structural abnormality caused by the dimer. Once the distortion is found, the damaged site is stabilized, and a large multi-protein complex, including transcription factor II H (TFIIH), is recruited. This recruitment marks the beginning of the DNA unwinding necessary to access the lesion.
TFIIH contains helicase subunits (XPD and XPB) that utilize energy to unwind the DNA double helix around the dimer, creating a localized bubble about 20 nucleotides long. Following the unwinding, the act of incision, or cutting, is performed by two specialized endonucleases.
The enzyme XPG makes a cut on the 3′ side of the dimer, while the XPF-ERCC1 complex cuts on the 5′ side. These two incisions occur only on the damaged strand, bracketing the lesion and releasing a short oligonucleotide segment. This excised segment, which contains the thymine dimer, is typically 24 to 32 nucleotides in length.
The removal of this damaged segment leaves a single-stranded gap in the DNA helix. The final phase, synthesis and ligation, begins when DNA polymerase fills this gap. Using the undamaged complementary strand as a template, the polymerase accurately synthesizes the missing nucleotides. Finally, DNA ligase seals the last break in the phosphate backbone, completing the repair and restoring the DNA to its undamaged form.
When DNA Repair Systems Fail
Defects in the genes encoding NER proteins can have severe consequences for human health. Unrepaired thymine dimers rapidly accumulate in cells with a compromised NER system, increasing the risk of mutation and disease. The most striking example is the rare genetic disorder known as Xeroderma Pigmentosum (XP).
XP is caused by inherited mutations in any of the genes responsible for the NER pathway, such as XPA through XPG. Individuals with this condition cannot effectively repair UV-induced DNA damage, resulting in extreme sensitivity to sunlight. Even minimal sun exposure leads to severe sunburn, blistering, and an accumulation of unrepaired thymine dimers in skin cells.
The most serious outcome of XP is the high incidence of skin cancer, including basal cell carcinoma, squamous cell carcinoma, and melanoma, often appearing in childhood. These cancers arise because unrepaired thymine dimers are highly mutagenic, leading to permanent changes in the genetic code that drive uncontrolled cell growth. For the general population, chronic, excessive UV exposure can overwhelm a functional NER system, contributing significantly to skin cancer development.