In their study, a team at Northwestern University has discovered a new mechanism for cell fate determination - how one cell, the daughter, becomes dramatically different from the mother.

In fact, the study of yeast has shown why mothers and daughters differ in how they express their genes, the PLoS Biology journal reported.

Many of the fundamental mechanisms for cell division in yeast are conserved, or very similar, in mammals; many of the proteins involved in human disease are related to proteins that are involved in yeast cell division.

According to researchers, when a yeast cell divides it produces a mother cell and a smaller, different daughter cell.

The daughter cell is the one that actually performs the final act of separation, cutting its connection to the mother cell. And the daughter takes longer than the mother to begin the next cycle of division, since it needs time to grow.

The key to the researchers' discovery of how this differentiation works is the gene regulator Ace2, a protein that directly turns genes on. The team found that the protein gets trapped in the nucleus of the daughter cell, turning on genes that make daughter different from mother.

The researchers are the first to show the regulator is trapped because a signalling pathway (a protein kinase called Cbk1) turns on and blocks Ace2 from actually interacting with the cell's nuclear export machinery.

Without this specific block, the machinery would move the regulator out of the nucleus, and the daughter cell would be more motherlike -- not as different.

"Daughter-cell gene expression is special, and now we know why," lead researcher Eric L. Weiss said.