Martin Krupa
medical student
University of California, San Diego

Sarah, NY, US

During telophase, when new daughter cells form how do they "know" what type of cell they will become?

Martin Krupa
HHMI medical fellow,
medical student,
School of Medicine,
University of California, San Diego
When a cell divides, its first generation offspring is largely restricted to becoming much like its parent because physically it is a clone of its parent. It inherits the parent’s membrane, cytoplasm, and nucleus, along with DNA and all the proteins. What is more, because nuclear transcription factors are also inherited, the daughter cell’s expression profile is the same, further ensuring it will remain like its parent for at least the near future. Left to its own devices, the cell might continue to proliferate into an army of clones.

But in an organism, the cell is not alone. Surrounding cells and tissues secrete protein growth factors that bind to receptors on the daughter cell’s membrane. Those receptors then act as a switch activating or inhibiting a cascade of cellular proteins, which in turn regulate the expression of genes and control the growth and polarization of new daughter cells. Nonprotein growth factors, on the other hand, such as hydrophobic steroid molecules, may permeate the cell membrane and translocate into the nucleus, where they directly control gene expression.

In addition to the outside environment, cell growth is controlled from the inside. While the DNA sequence is the same in each cell of an organism, not all of it is always available for transcription. One mechanism by which the cell controls which parts of its DNA are “turned on” and which are “off” is methylation—that is, adding a carbon group to the gene. The second mechanism is chromatin remodeling, which physically rearranges the DNA packaging proteins, chromatin, in such a way that certain parts of the DNA sequence become hidden inside and thus are unavailable. In the eye, for example, a retinal stem cell adds methyl groups around genes involved in light transduction in order to increase expression of these genes. Similarly, it rearranges the chromatin around genes involved in muscle formation so as to hide them from transcription since they are not necessary for retinal function.

Such epigenetic changes, as well as those caused by the environment, are passed on at division, but because the changes are minute, it takes several generations for the cell to mature to its terminal destination.

03/08/12 05:43