metaphase

Meaning: Metaphase comes after prometaphase and before anaphase. The chromosomes are at their most condensed and line up along the cell's equator, the metaphase plate. Each sister chromatid is attached at its kinetochore to microtubules from one pole, so the two chromatids of a chromosome face opposite poles. The spindle assembly checkpoint delays anaphase until the kinetochores are correctly attached.

On the MCAT: You may be asked to name the stage from a micrograph or a description of chromosome position. A common passage setup uses a microtubule poison. Cells cannot form a working spindle, so they stall before anaphase and accumulate in M phase. Cells arrested in metaphase are also used to prepare karyotypes. In meiosis, metaphase I lines up homologous pairs (tetrads). Their random orientation produces independent assortment. Metaphase II lines up individual chromosomes, as in mitosis. Another favorite is DNA content. A mitotic metaphase cell has twice the DNA of a G1 cell, but its chromosome number is the same as in G1, because chromosomes are counted by centromeres.

Common confusion: Metaphase is the stage when chromosomes align. Sister chromatids separate in anaphase. The metaphase plate is an imaginary plane, not a physical structure. In metaphase I the aligned units are homologous pairs, and sister chromatids stay together until anaphase II.