Genome complexity: DNA packing helps cell division (Introduction)

by David Turell , Thursday, April 06, 2023, 17:54 (387 days ago) @ David Turell

The structuring is called nucleosomes:

https://phys.org/news/2023-04-dna-packaging-cell-division.html

"The DNA molecule is located in the cell nucleus as a densely packed complex of DNA and protein, known as chromatin. Wrapped in sections around a core of special proteins known as histones, the DNA forms so-called nucleosomes, which are organized along the DNA like pearls on a string.

"For one of the fundamental processes of life, replication, the doubling of DNA, the standard textbook view tends to see this literally tangled structure as a hindrance that has to be loosened up and overcome with the expenditure of energy.

"A team led by molecular biologist Dr. Christoph F. Kurat from LMU's Biomedical Center (BMC) has now shown that this is not the whole truth: At certain places in the genome, the starting points of replication, a characteristic nucleosome structure is crucial for replication to get going in the first place, as the researchers report in the journal Nature.

"Before a cell can divide, its DNA has to double. This process does not initiate at one place only, rather the molecular machines of replication get to work simultaneously at many starting points along the chromosomes. Human cells possess around 30,000 of these so-called replication origins, while the single-celled baker's yeast with a small genome, which Kurat's team investigated as a model organism, possesses around 400 of them.

"Some time ago, researchers had discovered characteristic chromatin structures at these origins: The nucleosomes at these sites are arranged in very regular fashion, "in a much more orderly way than in the rest of the genome," says Kurat.

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"...the researchers were able to demonstrate which factors generate the regular chromatin structure at the origins and how important this is for the replication machinery to get going at their starting points.

"'Mutated cells without this chromatin structure are not viable," says Erika Chacin, lead author of the study. A key factor for the start of replication is the protein complex ORC (Origin Recognition Complex), of which it has long been known that it recruits the necessary parts of the replication machines.

"To their surprise, the researchers discovered that this complex has a second function: It plays a crucial role in the genesis of the highly ordered chromatin structure at the origins, by arranging the nucleosomes accordingly in conjunction with so-called chromatin remodeler complexes."

Comment: such a complex set of coordinated reactions which must be created all at once to work. Another example of irreducible complexity requiring that it must be designed all at once.