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The basic reaction of homologous recombination (HR) involves two chromatids that contain DNA sequences sharing a significant stretch of identity. One of these sequences uses a strand from another as a template to synthesize DNA in an enzyme-catalyzed reaction. The final product is a novel amalgamation of the two substrates. To ensure an accurate recombination of sequences, HR is restricted to the S and G2 phases of the cell cycle. At these stages, the DNA has been replicated already and the...
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DNA double-strand break end synapsis by DNA loop extrusion.

Jin H Yang1,2,3, Hugo B Brandão4,5,6,7, Anders S Hansen8,9,10

  • 1Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA.

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|April 6, 2023
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DNA double-strand breaks (DSBs) require efficient repair. This study reveals that DNA loop extrusion, not just passive diffusion, rapidly brings broken DNA ends together for repair via non-homologous end joining (NHEJ).

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Area of Science:

  • Molecular biology
  • Cellular biology
  • Genetics

Background:

  • DNA double-strand breaks (DSBs) are critical DNA lesions that occur during the cell cycle.
  • Non-homologous end joining (NHEJ) is the primary mechanism for repairing DSBs in the G1 phase of the cell cycle.
  • The initial step in NHEJ involves bringing the two broken DNA ends into close proximity, a process known as synapsis.

Purpose of the Study:

  • To investigate the mechanism underlying the rapid synapsis of DNA double-strand breaks.
  • To evaluate the hypothesis that DNA loop extrusion facilitates DSB synapsis.
  • To determine the efficiency and speed of synapsis mediated by different models of DNA loop extrusion.

Main Methods:

  • Experimentally constrained simulations of DNA repair.
  • Theoretical modeling of DNA loop extrusion.
  • Analysis of live-cell imaging data for DNA repair dynamics.

Main Results:

  • Passive diffusion alone is insufficient to explain the observed speed of DSB synapsis.
  • A simple DNA loop extrusion model provides only a modest acceleration of synapsis.
  • An expanded loop extrusion model, incorporating targeted loading, long-lived factors, and stabilization by boundary elements and DSB ends, achieves fast and highly efficient synapsis.

Conclusions:

  • DNA loop extrusion significantly contributes to the rapid and efficient synapsis of DNA double-strand breaks.
  • The findings challenge the traditional view of passive diffusion in DSB repair.
  • This work proposes a refined model for DSB repair involving active DNA organization by loop extruding factors (LEFs).