Scientists clarify a mechanism of epigenetic i...( Although letters representing the th...)

Although letters representing the three billion pairs of molecules that form the rungs of the helical DNA ladder are routinely called the human genetic code, the DNA they comprise transmits traits across generations in a variety of ways, not all of which depend on the sequence of letters in the code.

In some cases, rather than the sequence of letters, it is the physical manner in which DNA is spun around protein spools called histones and tightly packed into chromosomes that determines whether or with what intensity specific genes are expressed. A team of scientists at Cold Spring Harbor Laboratory (CSHL) has solved another in a series of mysteries about this critical mechanism of gene expression, described in a paper in the April 8 issue of Current Biology.

According to CSHL professor Rob Martienssen, Ph.D., who led the research team, about a tenth of our DNA stands aloof, spending its time in tightly packed clumps called heterochromatin, and unwinding only to replicate when a cell divides. After copying, both of the resulting DNA molecules to the surprise of many have been observed to form reclusive clumps in the same places as the original one did.

This inherited clumping of DNA, which causes genes to be expressed in distinctive ways, is one of a series of phenomena that scientists call epigenetic. The same sequence of nucleotides in two people can produce different patterns of gene expression if the way the DNA is clumped happens to be different.

We have not understood epigenetic inheritance very well, says Dr. Martienssen, a plant geneticist and one of the pioneers in the study of epigenetics. To explore this process, he and his team are studying the way DNA is packed in yeast, and how this packing can be transmitted across generations. The single-cell yeast organism is easy to study, in part because it lacks other epigenetic inheri
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