state fate of chromatid when cell division is completed
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Explanation:
Perhaps the most amazing thing about mitosis is its precision, a feature that has intrigued biologists since Walther Flemming first described chromosomes in the late 1800s (Paweletz, 2001). Although Flemming was able to correctly deduce the sequence of events in mitosis, this sequence could not be experimentally verified for several decades, until advances in light microscopy made it possible to observe chromosome movements in living cells. Researchers now know that mitosis is a highly regulated process involving hundreds of different cellular proteins. The dynamic nature of mitosis is best appreciated when this process is viewed in living cells.
Mitosis Occupies a Portion of the Cell Cycle
An electron micrograph shows a duplicated chromosome. The chromosome has an X-shape, formed by two elongated, side-by-side sister chromatids. The two black chromatids are condensed with fuzzy extensions emanating from their edges, and they are pinched inward and are connected to each other at a point slightly above their center. This constricted region of the chromosome where the pinching occurs is labeled the centromere.
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A circular diagram of arrows pointing in a clockwise direction shows the cell cycle in four phases: G1 on the left, followed by S on the top, followed by G2 on the right, and, finally, M at the bottom. A large, grey arrow is used to shade and group the G1, S, and G2 phases, indicating these are sub-phases of interphase. The M phase is shown with a pink arrow. An illustrated circular cell is shown in every phase. Chromosomal DNA is contained inside a round nucleus or aligned at the cell’s center.
View Full-Size ImageFigure 1
Figure Detail
In his pioneering studies of mitosis, Flemming noted that the nuclear material, which he named "chromatin" for its ability to take up stains, did not have the same appearance in all cells. (We still use the word "chromatin" today, albeit in a more biochemical sense to refer to complexes of nuclear DNA and protein.) Specifically, in some cells, chromatin appeared as an amorphous network, although in other cells, it appeared as threadlike bodies that Flemming named "mitosen." Based on his observations, Flemming had the insight to propose that chromatin could undergo reversible transformations in cells. Today, scientists know that Flemming had successfully distinguished chromosomes in the interphase portion of the cell cycle from chromosomes undergoing mitosis, or the portion of the cell cycle during which the nucleus divides (Figure 1). With very few exceptions, mitosis occupies a much smaller fraction of the cell cycle than interphase.
The difference in DNA compaction between interphase and mitosis is dramatic. A precise estimate of the difference is not possible, but during interphase, chromatin may be hundreds or even thousands of times less condensed than it is during mitosis. For this reason, the enzyme complexes that copy DNA have the greatest access to chromosomal DNA during interphase, at which time the vast majority of gene transcription occurs. In addition, chromosomal DNA is duplicated during a subportion of interphase known as the S, or synthesis, phase. As the two daughter DNA strands are produced from the chromosomal DNA during S phase, these daughter strands recruit additional histones and other proteins to form the structures known as sister chromatids (Figure 2). The sister chromatids, in turn, become "glued" together by a protein complex named cohesin. Cohesin is a member of the SMC, or structural maintenance of chromosomes, family of proteins. SMC proteins are DNA-binding proteins that affect chromosome architectures; indeed, cells that lack SMC proteins show a variety of defects in chromosome stability or chromosome behavior. Current data suggest that cohesin complexes may literally form circles that encompass the two sister chromatids (Hirano, 2002; Hagstrom & Meyer, 2003). At the end of S phase, cells are able to sense whether their DNA has been successfully copied, using a complicated set of checkpoint controls that are still not fully understood. For the most part, only cells that have successfully copied their DNA will proceed into mitosis.
Chromatin Is Extensively Condensed as Cells Enter Mitosis