Question

This is about experimental foundation for thinking about models for chromosome folding and nuclear organization. SAR/MAR sequences were postulated to form the bases of these radial loops.  SAR/MAR sequences were defined experimentally by one approach for SARs and by a different experimental approach for MARs. SAR/MAR sequences later turned out to be enriched in DNA sequences that show high binding affinity for topoisomerase 2.  Attention then turned to identifying proteins that bind to SAR/MAR sequences in mitotic chromosomes.  One protein in fact was found to be topoisomerase 2. The radial loop model was criticized because harsh conditions were used in the experimental approaches taken to identify both SAR sequences and proteins binding to SARs.

Just in the last couple of years a new experimental approach using single molecule fluorescence methods has described a new enzymatic activity for this protein complex.

What is that enzymatic activity and how might it relate to the formation of radial loops in mitotic chromosomes?

Answer 1

According to the radial loop model, the mechanism for chromosome condensation was based on self-assembly of chromosome scaffold and anchoring chromosomal loops. Based on a new experimental approach using single-molecule fluorescence methods, SCII (scaffold associated protein II) has been identified apart from topoisomerase 2.

Topoisomerase II and SCII, two major protein components of mitotic chromosome scaffold, have DNA topological roles. During the formation of radial loops in mitotic chromosomes, SCII has been identified as a component of a mitotic condensing complex that introduces positive supercoils in DNA in presence of topoisomerase II.