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Cohesin protein complexes are a molecular glue that holds two sister chromatids together. They play an important role both in mitosis and meiosis. In mitosis, all cohesin complexes present on the chromosomes are removed before the start of the anaphase stage.
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Topoisomerases are enzymes that relax overwound DNA molecules during various cell processes, including DNA replication and transcription. These enzymes regulate positive and negative DNA supercoiling without changing the nucleotide sequence. DNA overwinding in a clockwise direction results in positively supercoiled DNA, whereas underwinding in a counterclockwise direction produces negatively supercoiled DNA.
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For successful DNA replication, the unwinding of double-stranded DNA must be accompanied by stabilization and protection of the separated single strands of the DNA. This crucial task is performed by single-strand DNA-binding (SSB) proteins. They bind to the DNA in a sequence-independent manner, which means that the nitrogenous bases of the DNA need not be present in a specific order for binding of SSB proteins to it. The binding of SSB proteins straightens single-stranded DNA (ssDNA) and makes...
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DNA unwinding helicase enzymes are a type of motor protein. Motor proteins can translocate along filaments or polymers using energy generated from ATP hydrolysis. Helicases are involved in all the important cellular processes where DNA unwinding is required, such as DNA replication, repair, recombination, and transcription. They are present in all living organisms, but vary in their structure, function, and mechanism of action. For example, in prokaryotes, DnaB helicase binds and translocates...
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Studying DNA Looping by Single-Molecule FRET
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Cohesin supercoils DNA during loop extrusion.

Iain F Davidson1, Roman Barth2, Kota Nagasaka1

  • 1Research Institute of Molecular Pathology, Vienna BioCenter, Vienna, Austria.

Cell Reports
|June 14, 2025
PubMed
Summary
This summary is machine-generated.

Cohesin (a protein complex) negatively supercoils DNA during loop extrusion, twisting it between its hinge and clamp. This supercoiling is essential for forming and maintaining genome architecture.

Keywords:
CP: Molecular biologySMC complexescohesinloop extrusionsupercoilingtopoisomerases

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Area of Science:

  • Molecular Biology
  • Genetics
  • Biochemistry

Background:

  • Cohesin is crucial for organizing genomic DNA into loops.
  • These loops influence chromatin assembly, gene regulation, and recombination.
  • The precise mechanism of cohesin-mediated DNA translocation during loop extrusion remains unclear.

Purpose of the Study:

  • To investigate the biophysical mechanism by which cohesin extrudes DNA loops.
  • To determine if DNA supercoiling is involved in cohesin's loop extrusion process.
  • To elucidate the role of DNA supercoiling in genome architecture.

Main Methods:

  • In vitro biochemical assays to study cohesin-DNA interactions.
  • Analysis of cohesin mutants defective in ATPase activity or DNA binding.
  • Cellular assays to assess loop formation and genome structure.
  • Depletion of topoisomerase I to study its effect on loop extrusion.

Main Results:

  • Cohesin actively generates negative supercoiling in DNA during loop extrusion.
  • This supercoiling depends on the engagement of cohesin's ATPase heads and a DNA-binding site on its hinge.
  • A cohesin mutant impaired in supercoiling produces shorter DNA loops in cells.
  • Depletion of topoisomerase I also leads to shorter loops, suggesting a role for DNA relaxation.

Conclusions:

  • Negative DNA supercoiling is an integral component of the cohesin loop extrusion mechanism.
  • The twisting of DNA by cohesin is critical for its function in genome organization.
  • Relaxation of supercoiled DNA by factors like topoisomerase I is necessary for efficient cohesin-mediated loop extrusion and maintaining genome architecture.