![]() The sister chromatids, in turn, become "glued" together by a protein complex named cohesin. 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). In addition, chromosomal DNA is duplicated during a subportion of interphase known as the S, or synthesis, phase. 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. 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. The difference in DNA compaction between interphase and mitosis is dramatic. With very few exceptions, mitosis occupies a much smaller fraction of the cell cycle than interphase. 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). (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. In mammalian cells, late anaphase follows shortly after early anaphase and extends the spindle to around twice its metaphase length in contrast yeast and certain protozoa use late metaphase as the main means of chromosome separation and can extend the spindle to up to 15 times its metaphase length in the process.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. The contributions of early anaphase and late anaphase to anaphase as a whole vary with cell type. Late anaphase involves both the elongation of overlap microtubules and the use of two distinct sets of motor proteins: one of these pulls overlap microtubules past each other, the other pulls on astral microtubules that have attached to the cell cortex.During this, motor proteins at the kinetochores pull on the kinetochore microtubules. Early anaphase involves shortening kinetochore mictrotubules by depolymerization at both ends.These two processes were originally distinguished by their different sensitivities to drugs, and mechanically they are distinct processes. The third force for the separation of chromosomes involves lengthening the polar microtubules at the plus end. A second force involves pulling of the microtubules by cortex-associated cytosolic dynein. Kinesin proteins attached to polar microtubules push the microtubules past one another. Anaphase B drives separation of the sister centrosomes to their opposite poles through three forces. This involves the polar microtubules elongating and sliding relative to each other to drive the spindle poles further apart. When the chromatids are fully separated late anaphase (or Anaphase B) begins.This is achieved by shortening of the spindle microtubules, and forces are mainly exerted at the kinetochores. ![]() During early anaphase (or Anaphase A) the chromatids abruptly separate and move towards the spindle poles.Within anaphase two distinct processes occur.
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