What is a chromosome and how does it differ from chromatin? What substances is the chromosome made of? Explain the differences between euchromatin and heterochromatin. What are the reasons for these differences?
DNA molecules are not visible in the nucleus because they are in the form of thin strands of chromatin, which stains more intensively than the rest of the nucleoplasm (Fig. 64). Chromatin is a complex of DNA strands associated with histone proteins. In the period between cell divisions, DNA strands are mainly despiralized. They are concentrated in the center of the nucleus, weakly stained with dyes, and are called euchromatin. They contain genes that are actively involved in RNA synthesis. However, part of the DNA molecules remains in a spiral, tightly laid state, so it stains intensely – this is heterochromatin. These parts of DNA molecules are inactive or inactive, so they are tightly packed. During cell division, DNA molecules strongly spiral, thicken, form complexes with a protein and turn into well-marked structures – chromosomes. Each DNA molecule has one chromosome.
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