page_title: Kinetochore
section_title: Capturing MTs
hierarchical_section_title: Kinetochore / Function / Chromosome anchoring to MTs in the mitotic spindle / Capturing MTs
context_page_description: A kinetochore is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis. Its proteins also help to hold the sister chromatids together and play a role in chromosome editing. Details of the specific areas of origin are unknown.
  Monocentric organisms, including vertebrates, fungi, and most plants, have a single centromeric region on each chromosome which assembles a single, localized kinetochore. Holocentric organisms, such as nematodes and some plants, assemble a kinetochore along the entire length of a chromosome.
  Kinetochores start, control, and supervise the striking movements of chromosomes during cell division. During mitosis, which occurs after chromosomes are duplicated in S phase, two sister chromatids are held together by a centromere. Each chromatid has its own kinetochore, which face in opposite directions and attach to opposite poles of the mitotic spindle apparatus.
context_section_description: During the synthesis phase (S phase) in the cell cycle, the centrosome starts to duplicate. Just at the beginning of mitosis, both centrioles in each centrosome reach their maximal length, centrosomes recruit additional material and their nucleation capacity for microtubules increases. As mitosis progresses, both centrosomes separate to establish the mitotic spindle. In this way, the spindle in a mitotic cell has two poles emanating microtubules. Microtubules are long proteic filaments with asymmetric extremes, a "minus"(-) end  relatively stable next to the centrosome, and a "plus"(+) end  enduring alternate phases of growing-shrinking, exploring the center of the cell. During this searching process, a microtubule may encounter and capture a chromosome through the kinetochore. Microtubules that find and attach a kinetochore become stabilized, whereas those microtubules remaining free are rapidly depolymerized. As chromosomes have two kinetochores associated back-to-back (one on each sister chromatid), when one of them becomes attached to the microtubules generated by one of the cellular poles, the kinetochore on the sister chromatid becomes exposed to the opposed pole; for this reason, most of the times the second kinetochore becomes attached to the microtubules emanating from the opposing pole, in such a way that chromosomes are now bi-oriented, one fundamental configuration (also termed amphitelic) to ensure the correct segregation of both chromatids when the cell will divide.
  When just one microtubule is anchored to one kinetochore, it starts a rapid movement of the associated chromosome towards the pole generating that microtubule. This movement is probably mediated by the motor activity towards the "minus" (-) of the motor protein cytoplasmic dynein, which is very concentrated in the kinetochores not anchored to MTs. The movement towards the pole is slowed down as far as kinetochores acquire kMTs (MTs anchored to kinetochores) and the movement becomes directed by changes in kMTs length. Dynein is released from kinetochores as they acquire kMTs and, in cultured mammalian cells, it is required for the spindle checkpoint inactivation, but not for chromosome congression in the spindle equator, kMTs acquisition or anaphase A during chromosome segregation. In higher plants or in yeast there is no evidence of dynein, but other kinesins towards the (-) end might compensate for the lack of dynein.
  Another motor protein implicated in the initial capture of MTs is CENP-E; this is a high molecular weight kinesin associated with the fibrous corona at mammalian kinetochores from prometaphase until anaphase. In cells with low levels of CENP-E, chromosomes lack this protein at their kinetochores, which quite often are defective in their ability to congress at the metaphase plate. In this case, some chromosomes may remain chronically mono-oriented (anchored to only one pole), although most chromosomes may congress correctly at the metaphase plate.
  It is widely accepted that the kMTs fiber (the bundle of microtubules bound to the kinetochore) is originated by the capture of MTs polymerized at the centrosomes and spindle poles in mammalian cultured cells. However, MTs directly polymerized at kinetochores might also contribute significantly. The manner in which the centromeric region or kinetochore initiates the formation of kMTs and the frequency at which this happens are important questions, because this mechanism may contribute not only to the initial formation of kMTs, but also to the way in which kinetochores correct defective anchoring of MTs and regulate the movement along kMTs.
caption: Chromosomes attach to the mitotic spindle through sister kinetochores, in a bipolar orientation
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