Question

Three common ways to repair changes in DNA structure are nucleotide excision repair, mismatch repair, and homologous recombination repair. Which of these three mechanisms would be used to fix the following types of DNA changes? A. A change in the slructure of a base caused by a mutagen in a nondividing eukaryotic cell B. A change in DNA sequence caused by a mistake made by DNA polymerase \(\mathrm{C}\) A thymine dimer in the DNA af an actively dividing bacterial cell

Short answer

A. Nucleotide Excision Repair. B. Mismatch Repair. C. Nucleotide Excision Repair.

Step-by-step solution

Analyzing Scenario A

In scenario A, a change in the structure of a base caused by a mutagen in a nondividing eukaryotic cell is described. This pertains to nucleotide excision repair which removes damaged portions of DNA and it operates evenly throughout the cell cycle, making it suitable for non-dividing cells.

Analyzing Scenario B

In scenario B, a change in DNA sequence caused by a mistake made by DNA polymerase is described. This pertains to mismatch repair, which identifies and repairs erroneous insertion, deletion, and mis-incorporation of bases that were not corrected during DNA Replication.

Analyzing Scenario C

In scenario C, a thymine dimer in the DNA of an actively dividing bacterial cell is described. The repair mechanism for this scenario is the nucleotide excision repair, which is suitable for rapidly dividing cells given that it has the capability to repair thymine dimers.

Key concepts

Nucleotide Excision Repair

Nucleotide excision repair (NER) is a crucial mechanism that cells use to fix DNA damage, particularly those that cause bulky distortions in the DNA helix. This can happen due to external factors like UV radiation or certain chemicals that lead to issues such as thymine dimers.

NER operates by recognizing the distortion in the DNA double helix. Proteins involved in this process locate the damaged spot and excise a short single-stranded DNA segment containing the lesion. This leaves a gap, which DNA polymerase fills in using the undamaged strand as a template, followed by DNA ligase sealing the nick in the sugar-phosphate backbone.

• Works during all phases of the cell cycle
• Effective in both dividing and non-dividing cells, making it versatile against various types of DNA damage
• Useful for thymine dimers and chemical-induced base changes

Mismatch Repair

Mismatch repair (MMR) is an essential process to maintain DNA fidelity, particularly following DNA replication. Despite DNA polymerase having proofreading capabilities, errors can still slip through during rapid cell division.

The MMR system corrects mismatches that occur when non-complementary nucleotides are incorporated. It starts with proteins that recognize the mismatch and distinguish the newly synthesized strand from the old template, usually by identifying nicks in the new DNA. Once identified, the incorrect section is excised, and the gap is filled correctly by DNA polymerase.

• Critical for correction post-DNA replication
• Targets errors such as insertions, deletions, and incorrect base pairing
• Helps prevent mutation accumulation, maintaining genetic stability

Homologous Recombination Repair

Homologous recombination repair is a precise method of repair used to fix double-strand breaks (DSBs) in DNA and ensure genome integrity, particularly important during and after S-phase in the cell cycle.

In this mechanism, a similar or identical DNA sequence from a sister chromatid serves as a template to mend the break. This approach utilizes a complex of proteins to pair the broken DNA with its homologous sequence and precisely repair the break without losing genetic information.

• Effective only when a sister chromatid is available, mainly during and post-replication
• Crucial for repair of DSBs, minimizing risks of chromosomal rearrangements
• Considered high-fidelity repair due to accurate sequence reconstruction