Such highly condensed chromatin cannot be transcribed, so transcription ceases as chromatin condensation takes place. As discussed in Chapter 4, the chromatin in interphase nuclei condenses nearly a thousand fold during the formation of metaphase chromosomes. The condensation of interphase chromatin to form the compact chromosomes of mitotic cells is a key event in mitosis, critical in enabling the chromosomes to move along the mitotic spindle without becoming broken or tangled with one another. MPF induces multiple nuclear and cytoplasmic changes at the onset of M phase, both by activating other protein kinases and by phosphorylating proteins such as condensins and the nuclear lamins. Cytokinesis usually begins during late anaphase and is almost complete by the end of telophase, resulting in the formation of two interphase daughter cells. Mitosis ends with telophase, during which nuclei re-form and the chromosomes decondense. The transition from metaphase to anaphase is triggered by breakage of the link between sister chromatids, which then separate and move to opposite poles of the spindle. Most cells remain only briefly at metaphase before proceeding to anaphase. At this stage, the cell has reached metaphase. The chromosomes shuffle back and forth until they eventually align on the metaphase plate in the center of the spindle. The kinetochores of sister chromatids are oriented on opposite sides of the chromosome, so they attach to microtubules emanating from opposite poles of the spindle. During prometaphase the microtubules of the mitotic spindle attach to the kinetochores of condensed chromosomes. In these cells the spindle pole bodies are embedded within the nuclear envelope, and the nucleus divides in two following migration of daughter chromosomes to opposite poles of the spindle.įollowing completion of prophase, the cell enters prometaphase-a transition period between prophase and metaphase. In particular, yeasts and many other unicellular eukaryotes undergo “closed mitosis,” in which the nuclear envelope remains intact (see Figure 8.30). As discussed in Chapter 8, however, nuclear envelope breakdown is not a universal feature of mitosis. In higher eukaryotes the end of prophase corresponds to the breakdown of the nuclear envelope. Chromatin is stained blue, keratin is stained red, and microtubules are stained green. There they serve as the two poles of the mitotic spindle, which begins to form during late prophase.įluorescence micrographs of chromatin, keratin, and microtubules during mitosis of newt lung cells. The centrosomes (which had duplicated during interphase) separate and move to opposite sides of the nucleus. In addition to chromosome condensation, cytoplasmic changes leading to the development of the mitotic spindle initiate during prophase. The condensed sister chromatids are then held together at the centromere, which (as discussed in Chapter 4) is a DNA sequence to which proteins bind to form the kinetochore-the site of eventual attachment of the spindle microtubules. These newly replicated DNA molecules remain intertwined throughout S and G 2, becoming untangled during the process of chromatin condensation. The beginning of prophase is marked by the appearance of condensed chromosomes, each of which consists of two sister chromatids (the daughter DNA molecules produced in S phase). Mitosis is conventionally divided into four stages- prophase, metaphase, anaphase, and telophase-which are illustrated for an animal cell in Figures 14.23 and 14.24. Sister chromatids then separate from each other and move to opposite poles of the spindle, followed by the formation of daughter nuclei. These basic events of mitosis include chromosome condensation, formation of the mitotic spindle, and attachment of chromosomes to the spindle microtubules. Although many of the details of mitosis vary among different organisms, the fundamental processes that ensure the faithful segregation of sister chromatids are conserved in all eukaryotes.
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