Problem 2 - Biochemistry: Concepts and Connections

Question

For each DNA repair process in column I, list all characteristics from column II that correctly describe that process. I (a) Nucleotide excision repair (b) Photoreactivation (c) Base excision repair (d) Recombinational repair (e) SOS-driven error-prone repair (f) Alkyltransferase repair (g) Mismatch repair (h) Double-strand break repair II 1\. RecA protein participates. 2\. Damaged nucleotides are removed by nick translation. 3\. A free radical mechanism is involved. 4\. The repair enzyme functions only once. 5\. The key enzyme contains a bound folate cofactor. 6\. No bases or nucleotides are removed from the DNA. 7\. Deficiency of this enzyme in humans greatly increases the risk of skin cancer. 8\. This system is chiefly responsible for the mutagenic effect of ultraviolet light. 9\. This process begins with cleavage of two phosphodiester bonds. 10\. This process begins up to \(1 \mathrm{kbp}\) away from the site to be repaired. 11\. DNA ligase catalyzes the final reaction. 12\. This process also occurs in meiotic recombination. 13\. Replication fork regression might occur during this process.

Step-by-Step Solution

Answer

Each repair process's characteristics are as follows: (a) 2, 11; (b) 7; (c) 3, 11; (d) 1, 12, 11; (e) 8; (f) 4; (g) 10; (h) 1, 12, 11, 13.

Step 1: Nucleotide Excision Repair

Nucleotide excision repair is responsible for removing bulky DNA lesions. Matching characteristics include: 2. Damaged nucleotides are removed by nick translation, and 11. DNA ligase catalyzes the final reaction.

Step 2: Photoreactivation

Photoreactivation directly reverses UV-induced DNA damage using visible light. The key feature is: 7. Deficiency of this enzyme in humans greatly increases the risk of skin cancer.

Step 3: Base Excision Repair

Base excision repair targets small, non-helix distorting base lesions. Matching characteristics include: 3. A free radical mechanism is involved, and 11. DNA ligase catalyzes the final reaction.

Step 4: Recombinational Repair

Recombinational repair deals with DNA double-strand breaks and utilizes homologous recombination. Characteristics include: 1. RecA protein participates, 12. This process also occurs in meiotic recombination, 11. DNA ligase catalyzes the final reaction.

Step 5: SOS-Driven Error-Prone Repair

The SOS response is an emergency repair process that involves mutagenesis. It is characterized by: 8. This system is chiefly responsible for the mutagenic effect of ultraviolet light.

Step 6: Alkyltransferase Repair

Alkyltransferase repair directly removes alkyl groups from bases. Characteristic: 4. The repair enzyme functions only once.

Step 7: Mismatch Repair

Mismatch repair corrects errors that escape proofreading during DNA replication. Features include: 10. This process begins up to \(1 \mathrm{kbp}\) away from the site to be repaired.

Step 8: Double-Strand Break Repair

Double-strand break repair fixes breaks through non-homologous end joining or homologous recombination. Characteristics: 1. RecA protein participates, 12. This process also occurs in meiotic recombination, 11. DNA ligase catalyzes the final reaction, 13. Replication fork regression might occur during this process.

Key Concepts

Nucleotide Excision Repair

Nucleotide Excision Repair (NER) is an essential DNA repair mechanism responsible for removing bulky lesions that cause distortions in the DNA double helix. These lesions can include damage from ultraviolet (UV) light, such as thymine dimers. NER is a multi-step process beginning with the recognition of the DNA damage. An enzyme complex then cuts the DNA strand on either side of the damage.
The excised segment is removed, and the missing part is resynthesized using the undamaged strand as a template. Notably, DNA ligase catalyzes the final reaction, sealing the repaired segment back into the DNA strand, thus ensuring continuity of the genetic information.

Base Excision Repair

Base Excision Repair (BER) is specifically designed to fix small, non-helix distorting base lesions, such as those caused by oxidation, alkylation, or deamination. It starts with the identification of the damaged base by a DNA glycosylase enzyme, which excises the altered base, creating an abasic site. An AP endonuclease then cuts the DNA backbone at this site. Further processing involves removing the sugar-phosphate residue, followed by synthesis of the correct nucleotide in place.
The final step involves DNA ligase, which seals the repaired strand, ensuring the integrity of the DNA is restored. BER is essential for maintaining cellular genomic stability.

Recombinational Repair

Recombinational Repair is a crucial DNA repair process that fixes double-strand breaks (DSBs) in DNA. This type of repair utilizes homologous recombination, a process that requires a sister chromatid as a template to accurately repair the break. The RecA protein is a critical player in this process, facilitating strand invasion that pairs broken DNA ends with homologous sequences.
Double-strand break repair through recombination is meticulously controlled and also occurs during meiosis to exchange genetic material, enhancing genetic diversity. DNA ligase and other enzymes assist in completing the repair, ensuring the fidelity of the genome is maintained.

SOS Repair Mechanisms

The SOS Repair Mechanisms are an inducible response to extensive DNA damage, particularly that caused by UV irradiation. This system encompasses a network of genes, including those encoding repair proteins and transcription factors that are typically repressed. When DNA damage is detected, the SOS system can be triggered, leading to the expression of these repair genes.
Characterized by its error-prone nature, this mechanism can introduce mutations as it attempts to bypass DNA lesions. This error-prone repair is chiefly responsible for the mutagenic effects of UV light, providing a last-ditch effort to preserve cell survival despite extensive damage.

Mismatch Repair

Mismatch Repair (MMR) serves as a guardian of the genome, correcting errors that escape the proofreading activities of DNA polymerases during replication. These errors can include base pair mismatches and small insertions or deletions. The MMR system recognizes such mismatches and excises the incorrect nucleotides, generally starting up to 1 kilobase pair (kbp) away from the mismatch site.
Once the incorrect segment is removed, a new DNA segment is synthesized using the correct template. Proper function of the MMR system is crucial to reduce mutation rates and prevent certain types of cancer, notably Lynch syndrome.

Photoreactivation

Photoreactivation is a straightforward repair mechanism that directly reverses UV-induced DNA damage using visible light energy. It specifically targets and repairs thymine dimers, which form when two adjacent thymines on a DNA strand become covalently bonded due to UV exposure. The enzyme photolyase binds to the dimer and, upon absorbing light, performs a photochemical reaction that cleaves the dimer.
This repair method is efficient and crucial for preventing skin cancer, as a deficiency in photoreactivation can significantly increase susceptibility to such conditions, especially in sunlight-exposed areas.

Alkyltransferase Repair

Alkyltransferase Repair is a unique and direct DNA repair mechanism that specifically removes alkyl groups from damaged bases. Alkylating agents can add ethyl or methyl groups to bases, and one of the key enzymes in repairing this is O⁶-methylguanine-DNA methyltransferase (MGMT). This enzyme transfers the alkyl group from the damaged base to itself, thereby repairing the DNA.
The fascinating aspect of this repair mechanism is that the enzyme functions only once per alkylation event, sacrificially and irreversibly inactivating itself in the process of the repair, which underscores the specificity and efficiency of alkyltransferase repair.

Double-Strand Break Repair

Double-Strand Break Repair (DSBR) is vital for maintaining genome stability, especially since double-strand breaks are among the most lethal types of DNA damage. DSBR can proceed through two main pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). NHEJ directly ligates the broken DNA ends, which may be error-prone, while HR uses a homologous sequence as a template for accurate repair.
RecA protein is involved in homologous recombination, and the process may also involve replication fork regression. Homologous recombination is particularly prominent during meiotic recombination, enabling genetic diversity. DNA ligase plays a critical role in the sealing of repaired DNA, ensuring successful restoration of the DNA structure.