METAPHASE: IN MITOSIS AND IN MEIOSIS - BIOLOGY - 2025

The metaphase is the second stage of mitosis and meiosis. It is characterized by the alignment of chromosomes at the equator of the cell. After the key events of prophase that led to the condensation of the chromosomes, they must be mobilized.

To achieve efficient segregation, the chromosomes must be located on the equatorial plate. After being correctly positioned, they will be able to migrate towards the poles of the cell during anaphase.

Chromosomes aligned on the equatorial plate of the cell during mitotic metaphase. Taken from commons.wikimedia.org

It is not an exaggeration to say that metaphase is one of the most important checkpoints of mitosis and meiosis. In both cases, it is essential that the chromosomes are on the equatorial plate and with the kinetochores properly oriented.

In mitosis the chromosomes orient themselves on the equatorial plate in such a way as to secrete sister chromatids. In meiosis we find two metaphases. In metaphase I, the orientation of the bivalents leads to the segregation of homologous chromosomes. In meiosis II, segregation of sister chromatids is achieved.

In all cases, the efficient mobilization of chromosms is achieved thanks to the microtubule organization centers (COM). In animal cells they are organized in centrosomes, while in plants they act in a slightly more complex way, but without centrioles.

In general, the metaphase guarantees a symmetrical division of cells. But the metaphase can also determine an asymmetric division, when it is the need of the organism. Asymmetric division is a fundamental part of the acquisition of cellular identity in metazoans.

The metaphase in mitosis

In both the animal and plant cells there are mechanisms that guarantee that the chromosomes are located on the equatorial plate. Although it was previously conceived as an imaginary line equidistant between cell poles, it appears to be "real".

Silvia3, from Wikimedia Commons

That is, there are mechanisms in the cell that ensure that the chromosomes in a dividing cell reach that point. Except in controlled asymmetric splits, this is always the case, and the same point.

Equatorial plate and alignment

Reaching the equatorial plate and lining up to split are two independent processes. Both are controlled by a set of different proteins.

In fact, the "spindle assembly check" system prevents entry into anaphase unless all chromosomes are linked with some fiber of the spindle. In the chromosome the binding site is the kinetochore.

In metaphase the kinetochores must assume a bipolar orientation. That is, in an apparent single centromere, there will be two kinetochores. Each one will be oriented towards a pole opposite the other.

In addition to the separation force exerted by the microtubule organization centers, the binding force between chromatids and chromosomes must also be considered.

The chromatids remain attached by the action of mitotic cohesins. Therefore, in the metaphase one begins with closely united sister chromatids that must be located in the equator of the cell.

As they all reach the equatorial plate and orient themselves bipolarly attached to their respective spindle fibers, the metaphase ends.

Once at the cell's equator, the spindle fibers will hold the kinetochores attached to the centrioles at opposite poles of the animal cell. Traction forces will subsequently separate the sister chromatids of each chromosome, so that a complete set of these will migrate to each pole.

Sister chromatids cohesive and attached to microtubules. Modified from

This can only be achieved if all the chromosomes are located on the equatorial plate of the cell. It has been shown that if any chromosome takes time to be located, the fibers of the spindle perceive it and it is expected that all of them are located to proceed to their segregation.

The metaphase in meiosis

The meiotic division. Taken from es.wikipedia.org

Analogous to mitosis, the meiotic sister chromatids are also attached. But in this case by meiotic cohesins. Some are specific to metaphase I, and others to metaphase II.

In addition, homologous chromosomes have been part of the alignment, synapse and crossover processes. That is, they are inseparable from the synaptonemic complexes that have allowed the recombination and correct segregation of the DNA molecules involved. You also have to separate them.

Unlike mitosis, in meiosis you have to separate four strands of DNA instead of two. This is achieved by first separating the homologous chromosomes (metaphase I), and then the sister chromatids (metaphase II).

Metaphase I

The correct position of the chromosomes in the equatorial plate of metaphase I is achieved by the chiasms. Chiasms expose homologous chromosomes so that it is these that migrate towards the poles.

Furthermore, although homologous chromosomes must have a bipolar orientation, sister chromatids must not. That is, in metaphase I, contrary to II, the sister chromatids of each homologous chromosome must be monopolar (and opposite that of the homologous pair).

This is achieved by specific proteins that bind to the kinetochores of sister chromatids during metaphase I.

Metaphase II

During metaphase II the chromosomes line up on the equatorial plate with the kinetochore of each sister chromatid facing opposite poles. That is, now your orientation is bipolar. This arrangement of chromosomes is protein-specific.

The controlled meiotic metaphases guarantee the production of gametes with the correct number and identity of chromosomes. Otherwise, the appearance of individuals with significant chromosomal aberrations can be promoted.

References

1. Alberts, B., Johnson, AD, Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014) Molecular Biology of the Cell (6th Edition). WW Norton & Company, New York, NY, USA.
2. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Philadelphia, PA, USA.
3. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11th ed.). New York: WH Freeman, New York, NY, USA.
4. Maiato, H., Gomes, AM, Sousa, F., Barisic, M. (2017) Mechanisms of chromosome congression during mitosis. Biology 13, doi: 10.3390 / biology6010013
5. Ishiguro, KI (2018) The cohesin complex in mammalian meiosis. Genes to Cells, doi: 10.1111 / gtc.12652
6. Tan, CH, Gasic, I., Huber-Reggi, SP, Dudka, D., Barisic, M., Maiato, H., Meraldi, P. (2015) The equatorial position of the metaphase plate ensures symmetric cell divisions. elife, 4: e05124. doi: 10.7554 / eLife.05124.