image: A double-helix DNA is wrapped around histone octamers to form nucleosomes, structural units of chromatin in eukaryotic cells, which are then connected by naked linker DNA. DNA breathing, a process by which the DNA double helix denatures spontaneously to produce single-stranded DNA bubbles, tends to happen to the naked linker DNA compared to the nucleosomal DNA. Cytosine deamination (i.e., loss of -NH2 from cytosine), a major source of cytosine to thymine mutations that are frequently involved in diseases, including cancers, results from attack of water (H2O) that is accessible to the cytosine only when the DNA is single-stranded. Therefore, nucleosomes suppress cytosine to thymine mutations by protecting the DNA from water, an endogenous DNA mutagen. A, T, G, and C stand for adenine, thymine, guanine, and cytosine, the four bases that constitute DNA. This image relates to a paper that appeared in the March 9, 2012, issue of Science, published by AAAS. The paper, by X. Chen at Sun Yat-sen University in Guangzhou, China, and colleagues was titled, "Nucleosomes Suppress Spontaneous Mutations Base-Specifically in Eukaryotes." view more
Credit: Image courtesy of Xiaoshu Chen, 2012