DNA is a very long polymer, up to several µm long. At every stage of the cell cycle, these threads must be packaged into compact structures. A eukaryotic chromosome consists of one or two linear, double-stranded DNA molecules bound with many proteins (the complex of DNA and proteins is referred to as chromatin). Before S phase, each chromosome contains only one double-stranded DNA molecule. After it replicates during S phase, however, there are two double-stranded DNA molecules: the sister chromatids (Figure 11.7). Throughout G2 the sister chromatids are held together along most of their length by a protein complex called cohesin. At mitosis most of the cohesin is removed, except in a region called the centromere, where the chromatids remain held together. At the end of G2 and the beginning of mitosis, a second group of proteins called condensins coats the DNA molecules and makes them more compact.

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If all of the DNA in a typical human cell were put end to end, it would be nearly 2 meters long. Yet the nucleus is only 5 µm (0.000005 meters) in diameter and therefore DNA must be extensively packaged in a highly organized way (Figure 11.8). This packing is achieved largely by proteins called histones (histos, “web” or “loom”), which are positively charged at cellular pH because of their high content of the basic amino acids lysine and arginine. The charged R groups on these amino acids bind to the negatively charged phosphate groups on DNA by ionic attractions. These DNA–histone interactions, as well as histone–histone interactions, result in the formation of beadlike units called nucleosomes (see Figure 11.8).

During interphase, the chromatin that makes up each chromosome consists of single DNA molecules running around vast numbers of nucleosomes that resemble beads on a string. During this phase of the cell cycle, the DNA is accessible to proteins involved in replication and transcription. Once a mitotic chromosome is formed, its compact nature makes it inaccessible to replication and transcription factors, and so these processes cannot occur. Further coiling of the chromatin continues up to the time at which the chromatids begin to move apart.

In mitosis, a single nucleus gives rise to two nuclei that are genetically identical to each other and to the parent nucleus. Mitosis (the M phase of the cell cycle) ensures the accurate segregation of the eukaryotic cell’s multiple chromosomes into the daughter nuclei. While mitosis is a continuous process in which each event flows smoothly into the next, it is convenient to subdivide it into a series of stages: prophase, prometaphase, metaphase, anaphase, and telophase (Figure 11.9, Table 11.2).

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Let’s take a closer look at two cellular structures that contribute to the orderly segregation of the chromosomes during mitosis—the centrosomes and the spindle.