Genome complexity: controls for splitting DNA (Introduction)

by David Turell , Tuesday, March 30, 2021, 21:00 (1333 days ago) @ David Turell

More is teased apart as the controls are further studied:

https://phys.org/news/2021-03-dna-replication.html

"DNA replication is fundamental for life—indeed, we copy a light-year's length of DNA over the course of our lifetimes. This copying must be carried out accurately by a molecular machine consisting of protein components, and as such, it is carefully controlled. However, how this control of the protein components is achieved in the watery environment of the cell is by no means clear. (my bold)

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"...the researchers established for the first time that the protein that sets into motion the building of the replication machinery (the origin recognition complex (ORC) – originally discovered by the winner of the 2020 Dr. H.P. Heineken Prize for Biophysics and Biochemistry, Dr. Bruce Stillman) is a mobile protein that diffuses along the DNA. Importantly, however, its motion is brought to a halt at particular DNA sequences that ORC was previously established to have high affinity for. This reduction of ORC mobility at these sequences then facilitates the assembly of additional components of the replication machinery at these locations on the DNA.

"Using this approach, the researchers also found that the key motor component of the replication machinery (known as MCM) can stably associate with the DNA not only in its canonical double hexameric form, but also in a hitherto unknown single hexameric form uncoupled from ORC that displays significant mobility. This expansion of knowledge about the forms that MCM adopts on DNA may imply that its role inside cells is larger than what textbook knowledge currently indicates.

"With this work, the TU Delft and Francis Crick Institute researchers shed new light on a fundamental biological process that affects our daily lives through its connections to embryo development, tissue regeneration, and cancer progression. Quoting the first author of the work, Dr. Humberto Sánchez, "We are just at the beginning of a molecular journey that will be full of surprises."

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"Abstract (original article)
DNA replication in eukaryotes initiates at many origins distributed across each chromosome. Origins are bound by the origin recognition complex (ORC), which, with Cdc6 and Cdt1, recruits and loads the Mcm2-7 (MCM) helicase as an inactive double hexamer during G1 phase. The replisome assembles at the activated helicase in S phase. Although the outline of replisome assembly is understood, little is known about the dynamics of individual proteins on DNA and how these contribute to proper complex formation. Here we show, using single-molecule optical trapping and confocal microscopy, that yeast ORC is a mobile protein that diffuses rapidly along DNA. Origin recognition halts this search process. Recruitment of MCM molecules in an ORC- and Cdc6-dependent fashion results in slow-moving ORC-MCM intermediates and MCMs that rapidly scan the DNA. Following ATP hydrolysis, salt-stable loading of MCM single and double hexamers was seen, both of which exhibit salt-dependent mobility. Our results demonstrate that effective helicase loading relies on an interplay between protein diffusion and origin recognition, and suggest that MCM is stably loaded onto DNA in multiple forms."

Comment: Obviously there is much more to be learned, and my bold above again points to the problems that can happen as molecules are free to make mistakes. It is obvious there are various forms of control, but never 100% perfect. Certainly a complexly designed system.