Question

Describe the excision repair process in DNA, using the excision of thymine dimers as an example.

Short answer

The excision of thymine dimers involves recognition, incision, removal, synthesis, and ligation steps.

Step-by-step solution

Identify Damage in DNA

The first step in the excision repair process is the recognition of damage in the DNA. In this case, thymine dimers, which are abnormal covalent bonds between two adjacent thymine bases caused by UV radiation, are identified as the damage.

Action of UV-Specific Endonuclease

An enzyme called UV-specific endonuclease, also known as UvrABC endonuclease in prokaryotes, recognizes the thymine dimer and makes incisions on either side of the dimer in the DNA strand. This effectively cuts out a short segment of the DNA strand containing the dimer.

Removal of Damaged DNA

The damaged segment of DNA, containing the thymine dimer, is removed by a DNA helicase enzyme that unwinds and separates the damaged segment from the rest of the DNA molecule.

DNA Synthesis

DNA polymerase fills in the gap by synthesizing a new DNA strand complementary to the undamaged template strand. This 'repair synthesis' ensures that the segment is replaced accurately.

Ligation

Finally, DNA ligase seals the newly synthesized segment into the existing DNA strand by forming a covalent bond between the adjacent nucleotides, thus completing the repair process.

Key concepts

Thymine Dimers

Thymine dimers are a type of DNA damage caused primarily by ultraviolet (UV) radiation. When two adjacent thymine bases on a DNA strand are exposed to UV light, they can form abnormal covalent bonds with each other. This creates a form of mutation that distorts the DNA structure, making it difficult for the DNA to replicate and be read by cellular machinery.
Thymine dimers interfere with normal DNA replication because the cellular enzymes can't properly read the distorted sections of DNA. As a result, this can lead to errors in the genetic code, which may potentially cause mutations, cell malfunction, or diseases like skin cancer if not repaired.
To counteract this, cells implement a biological mechanism known as the excision repair process, specifically targeting these types of distortions.

UvrABC Endonuclease

UvrABC endonuclease is a critical enzyme involved in the DNA excision repair process, particularly in prokaryotic cells. This enzyme complex is responsible for recognizing and cutting out damaged DNA segments, such as thymine dimers.
The process begins when the UvrA protein detects distortions in the DNA caused by a thymine dimer. Once identified, UvrA binds to the site, followed by the recruitment of the UvrB and UvrC proteins to form the complete UvrABC complex. This complex makes precise cuts on either side of the dimer, creating small incisions.

• UvrA detects the damage.
• UvrAB binds to the DNA.
• UvrC makes incisions.

These incisions allow the removal of the damaged DNA segment, paving the way for further repair.

DNA Synthesis

Once the damaged DNA segment, such as a thymine dimer, is removed, the DNA synthesis phase begins. This phase is crucial for filling the gap left behind in the DNA strand. DNA polymerase, an enzyme that synthesizes DNA molecules, steps in to fill this gap. Using the undamaged complementary strand as a template, DNA polymerase matches the correct nucleotides to form a new DNA segment.
This repair synthesis is meticulously precise to ensure that the new segment is an accurate replication of the original genetic sequence. Following this synthesis, another enzyme, DNA ligase, comes into play. DNA ligase seals the newly synthesized segment into the existing strand by forming covalent bonds between the adjacent nucleotides, effectively completing the repair process.

• DNA polymerase synthesizes the new DNA.
• DNA ligase seals the new segment.
• Ensures the integrity of the DNA.

By the end of this process, the DNA molecule is restored to its original, undamaged state.