PROPHASE: IN MITOSIS AND MEIOSIS - BIOLOGY - 2025

The prophase is the first stage of cell division by mitosis and meiosis. It is the phase after the DNA synthesis stage (S phase of the cell cycle). At this stage, the chromosomes reach a high degree of condensation and individuality.

In meiosis there are two prophases, which are very different from each other and from mitosis. Only in meiotic prophase I, for example, does recombination occur. This phase is divided into different stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

Prophase. Leomonaci98, from Wikimedia Commons

During prophase, in addition to the condensation reached by the duplicated chromosomes, change processes take place at the extra-nuclear level. The most important cytoplasmic event during prophase is the formation of the achromatic spindle at each cell pole. This allows the chromosomes to be mobilized in successive stages of cell division to ensure their correct segregation.

There are important differences between cell divisions in animal cells and plant cells. Some will be mentioned later. In all, however, there is a complete reorganization of the cell.

Therefore, mitosis and meiosis focus on the fate of the DNA and the nucleus. But the truth is that when a cell divides, it divides everything and everything participates in the process.

Thus all cellular components undergo radical changes during the prophases of mitosis and meiosis. The endoplasmic reticulum and the Golgi complex seem to disappear: however, they only change their structure. Mitochondria and chloroplasts also divide, giving rise to new organelles.

Prophase in mitosis

Cell division by mitosis. Taken from es.wikipedia.org

Animal prophase

Animal cells have a single centriole. Upon completion of DNA synthesis in preparation for mitosis, the centriole also prepares to divide.

Centrioles are made up of a pair of identical structures called diplosomes, perpendicular to each other. These separate, and each one will be a mold for the genesis of a new one. The synthesis of the new diplosome occurs as each old diplosome migrates to opposite poles of the cell.

The other defining event of prophase, and one that is shared with plant cells, is chromatin compaction. This is perhaps the most notable cytological element of prophase during cell division.

DNA reaches a high degree of compaction, and for the first time it is observed as a morphologically individualized chromosome.

The compacted chromosomes involve the sister chromatids of each of them, still united by the same centromere. Although this centromere is really double, it behaves like a single one.

The chromosomes will be seen as an X, because they are two copy chromatids attached to the same center. Therefore, each cell in prophase will have a double number of chromatids, compared to a number of centromeres equal to the number '2n' of the species.

That is, a prophase mitotic cell is diploid by number of centromeres, but tetraploid (4n) by number of chromatids.

Vegetable prophase

In plant cells there is a phase prior to prophase called preprophase. In preparation for cell division, the large cell vacuole disintegrates.

Thanks to this, a free or unoccupied cytoplasmic band is formed, called a fragmosome. This allows the plant cell nucleus to position itself towards the equator of the cell.

Additionally, the cortical organization of the microtubules collapses towards the same site. This will give rise to what is known as the preprophase band (BPP).

The preprophasic band of plant cell division. Taken from en.wikipedia.org

The preprophasic band will appear first as a ring, but will end up covering the nucleus. That is, the microtubules that line the cell membrane internally will all mobilize towards the fragmosome.

Then, the preprophase band that surrounds the equatorial nucleus will allow to organize locally the site where the fragmoplast that will replace it will eventually appear.

Dynamically speaking, the microtubules of the plant cell will pass from one phase to another without obvious transitions. That is, from cortical arrangement to fragmosome and from there to fragmoplast.

The site of all these structural changes in the plant cell is the same where the deposition of the cell plate will occur. And therefore, it represents the plane where the cell will divide.

For everything else, the plant prophase is identical to that observed in the prophase of animal cells

Prophase in meiosis

The meiotic division. Taken from es.wikipedia.or

Only in Prophase I of meiosis does genetic recombination occur. Therefore, the formation of complex structures between chromosomes requires two divisions in meiosis.

With previous DNA synthesis, the sister chromatids were produced on each chromosome. With their compaction we have double chromosomes that in meiosis, in addition, pair between homologues.

This leads to the generation of bivalents (two interacting homologous chromosomes). Since each one is duplicated, we are actually talking about tetrads. That is to say, of chromatid tetrads united in a structure that must be resolved by means of two cell divisions.

In the first, the homologous chromosomes will be separated, while in the second, the sister chromatids must be separated.

Prophase I

In meiotic prophase I, sister chromatids are organized on compact proteinaceous structures that constitute the central chromosomal axis.

The synaptonemic complex (CS) will form on this axis, which will keep the homologous mating chromosomes together. During Prophase I, the synaptonemic complex will allow homologous chromosomes to enter synapses.

In these stages, intercrossing points can be formed, visible as chiasmas, where the genetic recombination process will take place. That is, the physical exchange between the participating DNA molecules that defines the pachytene.

Prophase II

Prophase II is not preceded by previous DNA synthesis. Here double chromosomes joined by the same (double) centromere were inherited. This is so because DNA synthesis, both in mitosis and meiosis, occurs only in the S (synthesis) phase of the cell cycle.

In this second division we will have four meiocytes. A meiocyte is a cell that is the product of a meiotic division.

Prophase II, therefore, will be in charge of the separation of sister chromatids from chromosomes inherited from prophase I. Therefore, at the end of the meiotic process each meiocyte will have the haploid set of chromosomes of the species.

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. Ishiguro, K.-I. (2018) The cohesin complex in mammalian meiosis. Genes to Cells, doi: 10.1111 / gtc.12652
5. Rasmussen, CG, Wright, AJ Müller, S. (2013) The role of the cytoskeleton and associated proteins in determination of the plant cell division plane. The Plant Journal, 75: 258-269.