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
