But in a process called asymmetric division, a cell divides into two daughters that are not identicala skin stem cell divides into another skin stem cell and a regular skin cell, for example. In that case, the genetic information within the chromosome copies remains the same, but the type of cell, or "cell fate," is different.
The Yamashita lab used stem cells from the testes of the fruit fly Drosophila to study the process of cell division.
"The Drosophila germ line stem cell can be identified at a single-cell resolution, so they are an ideal model," Yamashita said.
The stem cells cluster and are easy to identify; they divide to produce another germ line stem cell and a differentiating cell called a gonialblast, which goes on to eventually become a sperm cell.
The researchers marked the copies of each chromosome in the Drosophila stem cells as they divided. Using this method, they tracked the tendency of the X and the Y chromosome copies to move to the daughter germ line stem cell or to the gonialblast. They were able to demonstrate that copies of X and Y chromosomes (but not other chromosomes) are distinguished and delivered to the daughter cells with a striking bias.
This is the first direct evidence that cells indeed have an ability to distinguish identical copies of chromosomes and separate them in a regulated manner. This ability has been suspected and hypothesized, but never proven.
"We do not know yet why copies of X and Y chromosomes segregate nonrandomly," Yamashita said. "We think maybe specific epigenetic information is transmitted to the germ line stem cell and to the gonialblast."
The findings suggest that the information on the X and Y chromosomes that makes this division possible is primed during gametogenesisthe process of creating ovum or sperm cellsin the parents.
Many other cells throughout the body are able to divide into two different typ
|Contact: Laura J. Williams|
University of Michigan