Cohesin Can Remain Associated with Chromosomes during DNA Replication

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Sister chromatid cohesion is essential for accurate chromosome segregation during anaphase.
• Cohesin, a protein complex, mediates this cohesion, forming a ring structure around sister DNAs.
• The precise mechanism of cohesin loading and ring formation during S phase remains unclear.

Purpose of the Study:

• To investigate whether cohesin complexes bound to chromatin before DNA replication (G1 phase) remain associated after replication (S phase).
• To determine if pre-existing cohesin structures contribute to the establishment of sister chromatid cohesion.

Main Methods:

• Utilized selective photobleaching techniques, specifically pulse-chase Fluorescence Recovery After Photobleaching (pcFRAP).
• Developed a novel non-fluorescent HaloTag ligand to distinguish pre-existing labeled cohesin from newly synthesized protein.
• Examined cohesin dynamics in cells with inactivated cohesin turnover (WAPL deletion).

Main Results:

• Cohesin subunit Scc1 was observed to remain associated with chromatin throughout the S phase in cells with inactivated cohesin turnover.
• The pcFRAP method successfully differentiated between pre-existing and newly synthesized Halo-tagged Scc1.
• These results indicate that cohesin can persist on chromatin across the DNA replication process.

Conclusions:

• Cohesin complexes bound to DNA prior to replication can persist in situ after replication.
• This persistence suggests that sister chromatid cohesion may be established by cohesin already loaded onto unreplicated DNA.
• The findings challenge existing models and provide insight into the temporal regulation of cohesin function during the cell cycle.