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Cells contain enormous amounts of DNA. The single, circular chromosome of this Bdellovibrio bacteriovorus cell contains 3,782,950 individual nucleotide pairs, which means that if the circle were cut and laid out as a long piece, it would be about one thousand times longer than the cell itself. To fit and function inside the cell, the chromosome has to be extraordinarily organized and packed, a feat we still do not understand. Some of this packing is evident in nearly every cell: the center of the cell tends to have very few large macromolecular complexes like ribosomes, because they are excluded by the densely-packed chromosome(s). Look for these ribosome-excluding zones in the rest of the book; they indicate the location of the bulk of the cell’s DNA. Since this region is not enclosed by an internal membrane, it is not called a nucleus (the “karyon” that defines eukaryotes). Instead, we use the term nucleoid to describe the cytoplasmic region where most of the DNA is concentrated.
At times, the nucleoid becomes easier to see. Imagine that your cell wanted to decrease its gene expression (we will discuss why in Chapters 8 and 9). One approach is simply to pack the chromosome so tightly that the transcriptional machinery cannot access the genes. This cell has done just that, condensing its nucleoid into a dense twisted braid we can easily visualize.