Hereditary information flows from parents to offspring not just through DNA but also through the millions of proteins and other molecules that cling to it.
These modifications of DNA, known as "epigenetic marks," act both as a switch and a dial – they can determine which genes should be turned on or off, and how much message an "on" gene should produce.
One way in which epigenetic information is known to be passed from parent to offspring is through the pattern of chemical "caps" added onto certain "letters" of the DNA sequence, ensuring the sequence is "silenced." How these DNA capping patterns, which are inherited, are precisely set is not yet known. But in some cases, enzymes that add these caps are guided to DNA by small RNA molecules. These guides themselves do not carry hereditary information, but they do mark the spots where DNA is to be modified.
A team of scientists at Cold Spring Harbor Laboratory (CSHL) led by Professor Gregory J. Hannon, Ph.D., has now discovered that a class of small RNAs does carry epigenetic information and in fact passes on the trait of fertility from mother to offspring in fruit flies.
A new mechanism of inheritance
In a paper to be published on Nov 27th in Science , the CSHL team reports that maternal small RNAs called Piwi-interacting RNAs (piRNAs) that are deposited into fruit fly embryos "silence" DNA sequences that induce sterility, thus ensuring the fertility of the progeny. "This is a whole new way in which heredity can be transmitted," says Professor Hannon, who is a pioneer in small RNA research. "With this finding we've effectively doubled the number of mechanisms by which epigenetic information is known to be inherited."
The piRNAs are found only in cells of sex organs and partner up with proteins called Piwi to suppress the activity of mobile DNA sequences called transposons. Discovered half a century ago by CSHL scientist and Nobel laureate Barbara McClintock, Ph.D., transposons jump around the genome, inserting themselves into genes and causing mutations. Such genetic havoc is thought to underlie many diseases, including cancer.
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