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Updated: Jul 18, 2026
Visualizing Single-Stranded DNA Foci in the G1 Phase of the Cell Cycle
Published on: December 22, 2023
The cell cycle and DNA mismatch repair
Allen G Schroering1, Michael A Edelbrock, Timothy J Richards
1Department of Biochemistry and Cancer Biology, Medical University of Ohio, Toledo, OH 43614-5804, USA.
Abstract:
The DNA mismatch repair (MMR) pathway contributes to the fidelity of DNA synthesis and recombination by correcting mispaired nucleotides and insertion/deletion loops (IDLs). We have investigated whether MMR protein expression, activity, and subcellular location are altered during discrete phases of the cell cycle in mammalian cells. Two distinct methods have been used to demonstrate that although physiological MMR protein expression, mismatch binding, and nick-directed MMR activity within the nucleus are at highest levels during S phase, MMR is active throughout the cell cycle. Despite equal MMR nuclear protein concentrations in S and G(2) phases, mismatch binding and repair activities within G(2) are significantly lower, indicating a post-translational decrease in MMR activity specific to G(2). We further demonstrate that typical co-localization of MutSalpha to late S phase replication foci can be disrupted by 2 microM N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). This concentration of MNNG does not decrease ongoing DNA synthesis nor induce cell cycle arrest until the second cell cycle, with long-term colony survival decreased by only 24%. These results suggest that low level alkylation damage can selectively disrupt MMR proofreading activity during DNA synthesis and potentially increase mutation frequency within surviving cells.
Insights
The DNA mismatch repair (MMR) pathway is active throughout the cell cycle, with highest activity during S phase. Low-level DNA damage can disrupt MMR proofreading, potentially increasing mutation rates.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- The DNA mismatch repair (MMR) pathway is crucial for maintaining genomic stability.
- MMR corrects errors during DNA replication and recombination, including mispaired nucleotides and insertion/deletion loops (IDLs).
Purpose of the Study:
- To investigate alterations in MMR protein expression, activity, and subcellular localization across different cell cycle phases in mammalian cells.
- To determine the impact of DNA damage on MMR function during the cell cycle.
Main Methods:
- Utilized two distinct methods to assess MMR protein expression, mismatch binding, and nick-directed MMR activity.
- Investigated the subcellular localization of MMR proteins, specifically MutSalpha, using replication foci.
- Exposed cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) to evaluate the effects of DNA alkylation damage.
Main Results:
- MMR protein expression, mismatch binding, and nuclear MMR activity are highest during S phase but MMR is active throughout the cell cycle.
- Despite similar nuclear protein levels in S and G(2) phases, MMR activity is significantly reduced in G(2), indicating post-translational regulation.
- Low-level MNNG exposure disrupted MutSalpha localization to replication foci without immediate cell cycle arrest or significant reduction in DNA synthesis.
Conclusions:
- MMR activity is dynamically regulated throughout the cell cycle, with a notable decrease in G(2) phase.
- Low-level alkylation damage can selectively impair MMR proofreading during DNA synthesis, potentially leading to increased mutation frequencies in surviving cells.
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