Nucleosomes can change position to allow transcription of genes
Nucleosomes can slide along DNA. When nucleosomes are spaced closely together (top), transcription factors cannot bind and gene expression is turned off. When the nucleosomes are spaced far apart (bottom), the DNA is exposed. Transcription factors can bind, allowing gene expression to occur. Modifications to the histones and DNA affect nucleosome spacing.
Source
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"OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013."
http://cnx.org/content/m44536/latest/Figure_16_03_02.png
OpenStax CNX
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