When cells divide and transfer copies of genes to daughter cells, the process includes a phase where the replicated chromosomes are tightly condensed into durable packages called mitotic chromosomes. These chromosomes must survive the trying ordeal of holding firm during cell division. Some scientists think protein provides the scaffolding-like support in this process.
But researchers at the University of Illinois at Chicago have discovered it is not protein, but chromatin, the portion of the cell nucleus that contains DNA, which provides this support.
"What we've shown is that the mitotic form of chromosomes, when they're folded up for transmission during cell division -- the long, chromatin fibers made of DNA complexed with a lot of protein -- are linked at cross-link points, kind of like playground monkey bars," said John Marko, associate professor of physics at UIC. "And the bars are just chromatin."
The findings will be reported in the online edition of the Proceedings of the National Academy of Sciences the week of Oct. 28.
Marko, who also holds a courtesy appointment in bioengineering, was assisted by Michael Poirier, a former Ph.D. student in physics at UIC, who is now doing post-doctoral research at the Université Louis Pasteur in Strasbourg, France.
Marko and Poirier discovered the protein-support idea literally did not hold up following a series of experiments involving newt chromosomes, chosen because they are large and easy to grasp and manipulate under the microscope. After treating the chromosomes with low concentrations of enzymes that cut DNA at specific points, they discovered there was no support left, and the chromosomes dissolved.
The chromosomes were removed from cells and held in place by hair-like glass tube pipettes. The DNA digestion was done in free solution. This methodology was never previously used. The technique may prove useful in unlocking the puzzle of how cells divide.
Marko began his work about seven years ago while a research fellow at Rockefeller University in New York. He and Poirier began work on this reported research at UIC about four years ago.
"We realized that although a lot was understood biochemically about how chromosomes were organized, not so much was understood about what the physical properties of chromosomes were and how that might be a useful probe to look at their structure," Marko said.
"We're trying to understand the mechanics of cell division, which is still largely considered as a 'black box,' where chromosomes go in, and duplicate chromosomes come out. There are still a lot of questions about how this machinery works, particularly about maintenance, duplication and segregation of the DNA."
"Our study can provide very severe constraints on how other researchers should think about the state of the chromosome during cell division. We say there is no internal protein structure perhaps made by all these proteins sticking together. Instead, these proteins are distributed through a network of chromatin."
The research was funded by grants from the University of Illinois, the National Science Foundation, the Whitaker Foundation, a Johnson & Johnson corporation award, a Research Corporation award and a grant from the Petroleum Research Foundation of the American Chemical Society.