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. Author manuscript; available in PMC: 2014 Jan 1.
Published in final edited form as: Biochim Biophys Acta. 2012 Sep 6;1829(1):76–83. doi: 10.1016/j.bbagrm.2012.08.015

Fig. 5.

Fig. 5

Mechanism of transcription through chromatin by Pol II. After initiation of transcription by Pol II, the enzyme can be paused within the first nucleosome positioned on the gene (1). After overcoming the initial nucleosomal barrier, at a low Pol II density transcription is accompanied by transient displacement/exchange of H2A/H2B dimer(s); the nucleosome structure is recovered before arrival of the next Pol II complex (2). This Pol II-specific mechanism allows survival of H3/H4 histones and their modifications on DNA during transcription. At a higher density Pol II complexes encounter hexasomes that are missing H2A/H2B dimer(s) (3). In this case an unstable intermediate with a smaller number of DNA-histone contacts is formed, resulting in eviction of the histone hexamer from DNA [62]; therefore all core histones are evicted and exchanged (3). Thus proximity of Pol II complexes to each other dictates the fate of nucleosomes on transcription.