Epigenetics_Scitable - Epigenetics In 2010 a group of...

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10/30/13 1/3 Epigenetics In 2010, a group of researchers at the Stanford University School of Medicine in California uncovered a set of three mutations in the microscopic roundworm Caenorhabditis elegans , each capable of extending the life span of the worms by up to 30%. This exciting discovery prompted the research team to ask whether descendants of these worms could live longer, even if they didn't inherit the original mutation from their parents. Much to the researchers' surprise, the descendants lacking the original mutation continued to exhibit longer life spans for up to three generations. How could this happen? The longevity characteristic was no longer carried in the gene sequence, yet it was somehow lingering in descendants of mutated ancestors. Where does this type of genetic memory come from? The answer lies in a phenomenon called epigenetics, which describes molecular events that occur on DNA but not within the DNA sequence. Remarkably, epigenetic changes to DNA are branded into the genome in a manner that can sometimes be inherited by future generations. What, then, are these traces — these brands that somehow stick to the lineages of organisms? Transgenerational longevity in the roundworm is just one example among many epigenetic phenomena — differences in the traits of organisms (their phenotypes) that occur without any accompanying changes or mutations in genomic DNA sequences (their genotypes). Epigenetic events have both intrigued and puzzled researchers over the years . How is it that two organisms with identical genotypes can sometimes exhibit strikingly different phenotypes? In the 1980s, researchers discovered that the answer to this question is centered on specific chemical modifications that occur on the genomic DNA and its associated histone proteins, without changing the identity of the base pairs that make up the DNA. What are these modifications, and how do they affect phenotypes? Introductory biology teaches us that DNA is built from four different nucleotides: adenine, cytosine, guanine, and thymine. In epigenetic modification, a methyl group (–CH 3 ) is added to specific cytosine bases of the DNA. This enzymatic process, called DNA methylation, is known to play a key role in both development and disease. Methylation is a physical modification to the DNA that affects the way the molecule is shaped and, consequently, regulates which genes are transcriptionally active.
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