Question

The structure formed by coiling and supercoiling of chromatin fibres.​

Answer 1

Answer:

Explanation:

Supercoiling is a fundamental property of DNA and chromatin. It is modulated by polymerase and topoisomerase activities and, through regulated constraint, by DNA/chromatin binding proteins. As a non-covalent and elusive topological modification, supercoiling has proved intractable to research despite being a crucial regulator of nuclear structure and function. Recent studies have improved our understanding of the formation, regulation and organisation of supercoiling domains in vivo, and reinforce the prospect that the propagation of supercoiling can influence local and global chromatin structure. However, to further our understanding the development of new experimental tools and models are required to better dissect the mechanics of this key topological regulator.

When an activity supercoils DNA the torque generated is transmitted along the molecule. If the ends of the molecule are not fixed (or at least hindered), the supercoiling will dissipate via the unhindered rotation of the helix. Therefore for supercoiling to have a structural or functional influence on DNA or chromatin it must operate within a constrained environment where the energy is at least transiently trapped or restricted. For this reason it is anticipated that genomes need to be organised into supercoiling domains with barriers that prevent the spread of topological stress.

In prokaryotes the Escherichia coli genome has a hierarchical organisation based on large structural macrodomains [3] with the Ter domain being subdivided into smaller, 35 kb domains via MatS/MatP interactions [18]. This organisation establishes a dynamic structural architecture enabling packaging without interfering with transcription or replication. The genome is also separately organised into about 500 independent ∼10 kb supercoiling domains with demarcating barriers stochastically distributed and dynamically maintained [19,20]. However, as there are many more supercoiling domains than structural domains, the factors that define supercoiling boundaries must be distinct from those that characterise structural attachments.