Biorientation is the phenomenon whereby
microtubules emanating from different
microtubule organizing centres (MTOCs) attach to
kinetochore
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 ...
s of
sister chromatid
A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the dup ...
s. This results in the sister chromatids moving to opposite poles of the cell during
cell division
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
, and thus results in both daughter cells having the same genetic information.
Kinetochores link the chromosomes to the mitotic spindle - doing so relies on intricate interactions between microtubules and kinetochores. It has been shown that, in fission yeast, microtubule attachment can make frequent erroneous attachments early in mitosis, which are then often corrected prior to anaphase onset by a system which uses protein kinase to affect kinetochore microtubules in the absence of astriction between sister chromatids.
Proper biorientation allows correct chromosomal segregation in cell division.
Although this process is not well understood, high-resolution imaging of live mouse
oocytes
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female g ...
has revealed that chromosomes form an intermediate chromosomal configuration, called the ''prometaphase belt'', which occurs prior to biorientation. Kitajima, et al. estimate that about 90% of chromosomes require correction of the kinetochore-microtubule attachments (using Aurora kinase )prior to obtaining correct biorientation.
This suggests a possible cause for the elevated frequency of abnormal chromosome counts (
aneuploidy
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with a ...
) in mammals.
Several methods are postulated by which chromosomes biorient when they are located far from the pole with which they need to connect. One mechanism involves the kinetochore meeting microtubules from the distal pole. Another method described is based on observations that the kinetochore of one pole-oriented chromosome attaches to kinetochore fibers of an already bioriented chromosome. These two mechanisms possibly work in concert - certain chromosomes may biorient via encounters with microtubules from distal poles, which is then followed by kinetochore fibers that speed up biorientation with already-oriented chromosomes.
Researchers have detached grasshopper spermatocytes from spindle fibers and moved them away from the metaphase plate via micromanipulation. Several chromosomes instantly bioriented, as deduced from the observation that, upon reattachment, the chromosomes moved to the metaphase plate without moving to the poles.
References
Organelles
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