Cell Division: Stages of Mitosis | Learn Science at Scitable

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.

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 nuclearand.) 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 theportion of thefrom chromosomes undergoing mitosis, or the portion of the cell cycle during which thedivides (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.