Metaphase

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The mitotic spindle checkpoint verifies that all the chromosomes are aligned properly on the metaphase plate and prevents premature entry into anaphase.

Chromosomes lined up on the metaphase plate. Two views with the metaphase plate rotated 60°.

Stages of early mitosis in a vertebrate cell with micrographs of chromatids

Metaphase(fromAncient Greekμετα-(meta-) beyond, above, transcendingand fromAncient Greekφάσις(phásis)'appearance') is a stage ofmitosisin theeukaryoticcell cyclein which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed inanaphase).Thesechromosomes,carryinggenetic information,align in the equator of thecellbetween the spindle poles at themetaphase plate,before being separated into each of the two daughter nuclei. This alignment marks the beginning of metaphase.Metaphase accounts for approximately 4% of thecell cycle's duration.

In metaphase, microtubules from both duplicatedcentrosomeson opposite poles of the cell have completed attachment tokinetochoreson condensed chromosomes. Thecentromeresof the chromosomes convene themselves on the metaphase plate, an imaginary line that is equidistant from the two spindle poles.This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochore microtubules,analogous to a tug-of-war between two people of equal strength, ending with the destruction of Bcyclin.

In order to prevent deleteriousnondisjunctionevents, a keycell cycle checkpoint,thespindle checkpoint,verifies this evenly balanced alignment and ensures that every kinetochore is properly attached to a bundle of microtubules and is under balanced bipolar tension. Sister chromatids require activeseparaseto hydrolyze thecohesinthat bind them together prior to progression toanaphase.Any unattached or improperly attached kinetochores generate signals that prevent the activation of theanaphase promoting complex(cyclosome or APC/C), aubiquitin ligasewhich targetssecurinandcyclin Bfor degradation via theproteosome.As long as securin and cyclin B remain active, separase remains inactive, preventing premature progression to anaphase.

Metaphase in cytogenetics and cancer studies

Human metaphase chromosomes (normal malekaryotype)

The analysis of metaphasechromosomesis one of the main tools of classicalcytogeneticsandcancerstudies. Chromosomes are condensed (thickened) and highly coiled in metaphase, which makes them most suitable for visual analysis. Metaphase chromosomes make the classical picture of chromosomes (karyotype). For classical cytogenetic analyses, cells are grown in short term culture and arrested in metaphase usingmitotic inhibitor.Further they are used for slide preparation and banding (staining) of chromosomes to be visualised under microscope to study structure and number of chromosomes (karyotype).Stainingof the slides, often withGiemsa(G banding) orQuinacrine,produces a pattern of in total up to several hundred bands. Normal metaphase spreads are used in methods likeFISHand as a hybridization matrix forcomparative genomic hybridization(CGH) experiments.

Malignant cellsfrom solidtumorsorleukemiasamples can also be used for cytogenetic analysis to generate metaphase preparations. Inspection of the stained metaphase chromosomes allows the determination of numerical and structural changes in the tumor cell genome, for example, losses of chromosomal segments ortranslocations,which may lead to chimericoncogenes,such asbcr-ablinchronic myelogenous leukemia.

External links

• Media related toMetaphaseat Wikimedia Commons