Genetic information is written in the DNA molecule in the cells (see DNA). Human DNA in the cell nucleus has an overall length of about two metres. In order for it to fit into the cell, it is wound around specialized proteins (histones) and, together with them, folded many times and wound in chromosomes, rod-like shapes usually with a length of several thousandths of a millimetre. For example, human beings have 46 of these species, which differ in size and shape, in the cell nucleus. Each chromosome is formed of two identical chromatids, whose DNA was formed by copying the chain of a DNA molecule originally contained in one chromatid (Fig. 3.1 in the book). During nuclear division, the two chromatids separate to the opposite ends of the cell, ensuring fair (even) distribution of the genetic material between the two daughter cells. When cells are not dividing, the chromosomes are loosened and are not visible without using special microscope techniques. They change into their characteristic form observable under a normal (optical) microscope during cell division.
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