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The DNA genomes of organisms whose cells possess nuclei are packaged in a highly characteristic fashion. Most of the DNA is tightly wrapped around protein particles called nucleosomes, which are connected to each other by flexible DNA segments, like pearls on a necklace. This arrangement plays a major role in deciding which genes are actively expressed, and thus which proteins can be synthesized in a given cell. The biophysicists Professor Ulrich Gerland and Wolfram Moebius from Ludwig-Maximilians-Universitaet (LMU) in Munich have recently developed a model which explains the distribution of nucleosomes around the functionally crucial transcription start sites. Transcription is the first step in the process that converts genetic information into proteins. At the transcription start sites the DNA must be free of nucleosomes. The two researchers discovered that distinct stop signals positioned on either side of these zones must actively prevent the formation and sliding of nucleosomes. "Our model provides a useful tool for dissecting the so-called chromatin code, which determines how the DNA is packed and selectively made accessible for transcription", says Gerland.(PLoS Computational Biology, 19 August 2010)
In higher organisms, the genetic material in each cell is packed in the form of compact chromosomes in the nucleus. The basic structural unit of a chromosome is the nucleosome. The nucleosomes, each made up of two copies of four different histone proteins, provide a kind of spool on which the DNA strands are wound, and are linked together by more flexible sections of DNA, like beads on a string. But nucleosomes are not just passive packages that keep the DNA in a compact form. "They have a decisive influence on gene regulation, insofar as they help to control which segments of the DNA can be translated
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