The process of removing and replacing damaged stretches of DNA involves the coordinated action of two primary enzymes: a nuclease enzyme, which cuts out the damaged section, and a DNA polymerase, which then synthesizes and fills in the new, correct nucleotides.
This vital repair mechanism ensures the integrity of the genetic code, preventing mutations that could lead to various diseases.
How Damaged DNA is Repaired
DNA is constantly susceptible to damage from various sources, including UV radiation, chemical exposure, and errors during replication. Cells have evolved sophisticated repair systems to counteract this. A common repair pathway, often called nucleotide excision repair (NER), illustrates the roles of nuclease and DNA polymerase.
1. Cutting Out the Damaged Section: Nuclease
When DNA damage is detected, a nuclease enzyme (specifically, an exonuclease or endonuclease depending on the type of repair) plays the crucial role of removing the incorrect or damaged segment.
- Mechanism: The nuclease enzyme recognizes the damaged DNA strand and cuts it at two distinct points – one on each side of the damaged section. This precisely excises the faulty nucleotides, leaving a gap in the DNA strand.
2. Replacing the Damaged Section: DNA Polymerase
Once the damaged stretch has been removed, the gap is accurately filled in by a DNA polymerase enzyme.
- Mechanism: The DNA polymerase binds to the single-stranded template on the undamaged strand and begins synthesizing new DNA. It adds complementary nucleotides, effectively filling in the missing section with the correct sequence.
- Final Step: After the DNA polymerase has filled the gap, another enzyme, DNA ligase, forms the final phosphodiester bond, sealing the newly synthesized DNA into the existing strand.
Summary of Enzyme Roles
| Enzyme | Primary Function | Role in DNA Repair Process |
|---|---|---|
| Nuclease | Cuts DNA strands | Identifies and excises the damaged segment of DNA by making precise cuts on either side. |
| DNA Polymerase | Synthesizes new DNA; fills in gaps | Adds correct, complementary nucleotides to fill the gap left by the nuclease, using the undamaged strand as a template. |
| DNA Ligase | Joins DNA fragments; forms phosphodiester bonds | Seals the newly synthesized DNA segment into the existing strand, completing the repair. |
This coordinated action of enzymes is fundamental to maintaining genomic stability and preventing the accumulation of harmful mutations, underpinning processes like DNA replication and gene expression. The efficiency of these repair systems is critical for preventing diseases such as cancer and for the overall health of an organism.
