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What is chromatin, euchromatin and heterochromatin?

Chromatin is the molecular substance of a chromosome. It consists of a complex of DNA, RNA and protein in eukaryotic cells. Frequently, people encounter pictures of chromosomes which have a striped pattern of stronger and lighter staining. What are these chromosome bands? Do they represent genes? No, they do not necessarily represent genes nor do they automatically correspond to stretches lacking genes. Chromosomes often presented to the public are most highly compacted metaphase chromosomes or the giant chromosomes (i.e. polytene chromosome) of Drosophila larvae from salivary gland cells. The latter are made up of about 2000 DNA double strands arranged parallel to each other. A single band of a Drosophila giant chromosome can contain about 50.000 nucleotides in a row. But what makes its staining different to an adjacent band? What we see as dark bands is a consequence of an increased concentration of DNA within variably, compacted chromatin that differs in staining on average every 50 kb. But there is also a gradient of diverse higher order degrees of compaction. Further, in the life of a chromosome it alters its appearance. During the so-called interphase it is relaxed with a lot of open chromatin (see FAQ 7). During this phase, some entire chromosomes but mostly only parts thereof retain a strong staining. These subnuclear features are known since the early days of cytology. In 1928, Heitz introducd for them the term 'heterochromatin'. Euchromatin, on the other hand, is highly decondensed chromatin. Chromosomal regions in the genome which lack high numbers of genes are normally compacted in heterochromatin while chromosomal regions with high concentrations of transcribed genes are part of relaxed euchromatin. But these patterns change during development depending on the pattern of particular epigenetic tags present in the chromatin (see FAQ 8).

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