Now that we have examined the various types of DNA sequences found in eukaryotic genomes and how they are organized within genomes, we turn to the question of how DNA molecules as a whole are organized within eukaryotic cells. Because the total length of cellular DNA is up to a hundred thousand times a cell’s diameter, the packing of DNA is crucial to cell architecture. It is also essential to prevent the long DNA molecules from getting knotted or tangled with each other during cell division, when they must be precisely segregated to daughter cells. The task of compacting and organizing chromosomal DNA is performed by abundant nuclear proteins called histones. The complex of histones and DNA is called chromatin.

Chromatin, which is about half DNA and half protein by mass, is dispersed throughout much of the nucleus in interphase cells (those that are not undergoing mitosis). Further folding and compaction of chromatin during mitosis (see Figure 6-3) produces the visible metaphase chromosomes whose morphology and staining characteristics were detailed by early cytogeneticists. Although every eukaryotic chromosome includes millions of individual protein molecules, each chromosome contains just one, extremely long, linear DNA molecule. The longest DNA molecules in human chromosomes, for instance, are 2.8 × 10⁸ bp, or almost 10 cm, in length! The structural organization of chromatin allows this vast length of DNA to be compacted into the microscopic constraints of a cell nucleus (see Figure 8-1). Yet chromatin is organized in such a way that specific DNA sequences within the chromatin are readily available for cellular processes such as the transcription, replication, repair, and recombination of DNA molecules. In this section, we consider the properties of chromatin and its organization into chromosomes. Important features of chromosomes in their entirety are covered in the next section.

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