Genome complexity: horizontal gene transfer (Introduction)

by David Turell , Monday, June 13, 2016, 15:30 (3086 days ago) @ David Turell

New computer simulation research on cell division mechanics may have found the way that human cells organize 46 chromosomes into 23 pairs from six feet of DNA when packed into a nucleus. The nucleus opens and the pairs are split, all in a 20 minute space of time as the cell divides in two:-http://phys.org/news/2016-06-chromosomes-tiny-loops-molecular-motors.html-
"Human cells contain 23 pairs of chromosomes that form a loosely organized cluster in the cell nucleus. When cells divide, they must first condense these chromosomes—each of which when fully extended is a thousand times longer than the cell's nucleus and physically indistinguishable from the others—into compact structures that can be easily separated and packaged into their offspring. -"An MIT-led team has now developed a model that explains how cells handle this difficult task. In computer simulations, the researchers demonstrate that certain molecular "machines" can transform chromosomes from a loosely tangled rope into a series of tiny loops that condense each chromosome and allow it to extricate itself from the others.-"Moreover, the researchers demonstrate that a similar model explains how chromosomes are organized when cells are not dividing, and they hypothesize that loop extrusion by molecular motors splits chromosomes into separate domains, helping to control which genes are expressed in a given cell.-"This mechanism, outlined in three recent papers published in Cell Reports, eLife, and Biophysical Journal, suggests that chromosome organization relies on proteins that act as molecular motors that pull strands of DNA into progressively larger loops. The MIT team suggests that two proteins thought to function primarily as "staples" that hold DNA together, cohesin and condensin, can also actively manipulate DNA.-"'Nobody has ever directly observed this mechanism of loop extrusion. If it exists, it will solve lots of problems," says Leonid Mirny, a professor of physics in MIT's Institute for Medical Engineering and Sciences, who led the research. "We will know how chromosomes condense, how they segregate, how genes talk to enhancers. Lots of things can be solved by this mechanism."-"'What we believe is happening is each condensin complex consists of two motors, each moving along the chromosome in the opposite direction," Mirny says. "Each loop extrusion event is tiny compared to the size of a chromosome. Nevertheless, collectively they can self-organize this chromosome into an early-metaphase, condensed chromosome."-"In human cells, this process usually takes about 20 minutes.-"The loops seen during interphase are constantly being formed and then dismantled in a dynamic process. When a cell begins preparing to divide, the researchers believe the boundaries are removed and condensin comes in to form the many more loops needed to condense the chromosomes for cell division.-"Although biologists have not demonstrated that condensin and cohesin can act as molecular motors, there is speculation that they can because they are structurally similar to other cellular proteins that function as molecular motors.-"'Combined, these papers constitute the most important papers in our field in the last five years," says Job Dekker, a professor of biochemistry and molecular pharmacology at the University of Massachusetts Medical School, who was not involved in the research. "These papers explain how a very simple method of loop extrusion can explain, to an extraordinary level, the structures of chromosomes in both mitotic and nondividing cells.'"-Comment: Cell division is a constant activity in life. The DNA in its chromosome form must be split accurately every time. Obviously there are mistakes, but controls reduce them to a very bare minimum. Look at this website to see how complex the many phases are:- https://askabiologist.asu.edu/content/cell-division-Not by chance!