Metaphase definition
Metaphase designates cell cycle phase in which chromosomes are in their most condensed state, aligned in cell equatorial plate. At this stage, chromosomes are attached to mitotic spindle, but have not yet segregated to cell opposite poles.
Cell Cycle
Cell basic function, which allows it to generate offspring exactly like itself, is the replication of its DNA and its division in order to give rise to two equal daughter cells. Cell performs this function through the cell cycle, which is composed by interphase (where duplication of all cellular material occurs) followed by M phase (where cell division and cytoplasm division occurs). The interphase has sub stages: G1 stage, S stage and G2 stage; and the same happens with M phase, which is subdivided into: prophase, metaphase, anaphase and telophase, which make mitosis, and cytokinesis.
Metaphase Characteristics
Mitotic Cdk complexes are produced during S and G2 stage, but are not activated until cell enters in mitosis. After activation, they induce, among other mechanisms, chromosomes condensation and their alignment in equatorial plate. In metaphase chromosomes are in their more condensed state, i.e., in fibers of 1400nm and are denominated ‘mitotic chromosomes’. At this stage, chromosomes are in its most known conformation of X (figure 1); conformation in which DNA transcription is impossible. Due to this conformation, it is possible to visualize the chromosomes through an optical microscope.
Figure 1 – Optical microscopy image of a cell in metaphase, with the chromosomes aligned in the cell equatorial plate, in its typical X conformation.
During this phase, sister chromatids are linked to one another by the centromere. In turn, centromere is associated with proteins to form two kinetochores, one for each sister chromatid. Through kinetochores, the microtubules (that form the mitotic spindle) attach to sister chromatids. Kinetochore microtubules of opposite cell poles connect with each other. Thus, due to constant forces exerted by mitotic spindle microtubules, sister chromatids are aligned and suspended in cell equatorial plate (figure 2), in a state of dynamic equilibrium, prepared to be separated and drawn to opposite spindle poles when signals arise.
Figure 2 – Schematic image of a cell in metaphase, with the chromosomes at its maximum condensation aligned in cell equatorial plate.
Regulation
An important checkpoint occurs at this cell cycle stage: confirmation that sister chromatids are attached to microtubules from centrioles of opposite cell poles. This binding of microtubules to sister chromatids kinetochores is denominated ‘bio orientation’ and generates a tension that is perceived by the kinetochores as being exerted in two opposite directions. Thus, cell has a control mechanism and, if the kinetochores do not perceive opposite tensions, they send inhibitory signals to the cell and cell cycle does not advance, at least until the microtubules are correctly attached to sister chromatids kinetochores.
Chromosome study
The study of chromosomes and gene organization within the chromosomes it is possible when chromosomes are in their more condensed conformation, i.e., during metaphase. One of the most commonly used techniques is fluorescence in situ hybridization (FISH), where specific DNA sequences are detected on chromosome. Another technique also widely used is G-banding, where chromosome regions with low gene content, and therefore transcriptionally inactive, are stained by Giemsa dye to become dark bands. Since chromosome regions rich in genes and, consequently, transcriptionally active, are not stained by dye, they become white bands. The latter technique is used to make karyotypes, usually performed to evaluate if there is any type of chromosome abnormality in the individual.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
