Double-strand Breaks in Heterochromatin (IMAGE) DOE/Lawrence Berkeley National Laboratory Caption Heterochromatin (purple) accounts for a third of the chromatin in both humans and fruit flies. Some heterochromatin forms the telomeres that cap the ends of the chromatids, and much is concentrated near the centromere, where sister chromatids are joined. Accurate repair of double-strand breaks in heterochromatin is challenging, because most of its DNA consists of short, repeated sequences. Credit Lawrence Berkeley National Laboratory Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.