In the world of cell biology understanding cell division is crucial. Cell division is driven by the cell cycle which consists of interphase, mitosis, and cytokinesis. Interphase is where a cell will spend the majority of its life until it is signaled to divide. Once a cell is signaled to divide it will proceed through the G1, S, and G2 phases of interphase which composes 80% of the cell cycle. The purpose of interphase is to make sure the cell has all the necessary parts needed to divide. In order to check that no errors have been made, there are checkpoints after each phase of interphase. A critical checkpoint follows the S phase and its function is to make sure all 46 chromosomes have been duplicated. Having two complete sets of chromosomes will ensure the formation of two functional daughter cells. It is important to note that not all animal cells have the same number of chromosomes. Zebrafish for example are a diploid species with 50 chromosomes (25 pairs)(Postlethwait et al, 2008). The process of DNA replication occurs in the nucleus. DNA replication is carried out by DNA polymerase, which requires a RNA primer, unwinding the double helix and running antiparallel with the template strand of DNA to create the leading strand. The coding strand will make the lagging strand which will contain okazaki fragments that will be replaced with DNA later on. The unzipping of the double helix is done by the enzyme helicase. Once the cell has successfully passed each check point, meaning it has copied all required organelles and DNA, the cell can enter mitosis. 

     Mitosis consists of four stages: Prophase, Metaphase, Anaphase, and Telophase. In multicellular organisms mitosis is used for growth from the time you are a zygote and to replace old or damaged cells (Gillespie, 2018). In prophase the chromosomes condense tightly and the nuclear envelope begins to dissolve. It is sort of like a rest period before the cell actually divides. Next is metaphase where the chromosomes line up in the center of the cell and the mitotic spindles attach at the centromere of each chromosome. Before the cell can advance to anaphase it must proceed through a checkpoint to ensure that all chromosomes have attached to a spindle properly. This is crucial because if one chromosome is not properly attached, then one of the daughter cells will not have all of its required chromosomes. The last stage in mitosis is Anaphase and this is where the chromosomes separate and each chromatid begins moving towards the poles of the cell. The last phase in mitosis is telophase, and in this step the nuclear envelope begins to reform around each set of chromosomes. The chromosomes will also decompact into euchromatin. 

     Cytokinesis is the final phase in cell division. This is when the cell membrane pinches off and two separate identical daughter cells are formed. The separation of the daughter cells is driven by protein filaments (Robinson, 2021). This separation completes cell division. 

      In a recent study done by Keat Ying Chan and his colleagues it was found that Zebrafish undergo a form of cell division that is unlike anything we have seen before. Their research data contained four images to help highlight the key points of their findings. In Figure 1, each cell on the zebrafish larvae was colored so they could track the progression of each cell individually. The coloring was done by injecting either red, blue, or green fluorescence into H2B-BFP2, a histone protein, which creates up to 60 different colors for the cells. The superficial epithelial cells (SEC) in the larvae were the targets of this injection. When looking at the overall stained image there are some small black holes present which represent parts of the cell that did not express the gene for the fluorescence. This was a crucial step because the color differences made it easy to track each individual cell, and it ensured that they were not accidentally studying basal epithelial cells (BEC). Figure 2 looked at the surface area of the zebrafish larvae compared to the SECs of the larvae at 8 and 10 d.p.f and a clear increase was observed. Figure 3 demonstrated how the rapid cell division created major margins for error. These errors include not splitting the DNA evenly and leaving a piece(s) of DNA behind. Despite the many errors made, each cell did still have a nucleus. One important detail from these images is that with each division the fluorescence of the cell would decrease. Finally in Figure 4, it was stated that the presumed reason for the new method of division was to compensate for increased demand of skin cells during a rapid period of growth.

     When considering all of the data presented in the four figures it is clear that this form of cell division is not exactly mitosis. The first and most obvious reason why this is not true mitosis is because the cells are not replicating their DNA before they divide. In fact, they are able to divide with unequal or missing pieces of DNA in each daughter cell (Chan et al., 2022). This creates cells of different sizes and genomes which is also not consistent with true mitosis. One similarity is that one division creates two total daughter cells, each containing a nucleus.

     Another commonly known form of cell division to compare this form of division to is meiosis. Meosis forms four daughter cells after two successive rounds of division while the division seen here only forms two. However, each of these cells is able to replicate up to two times without replicating their DNA (Chan et al., 2022). Much like with mitosis it is problematic that these cells are able to divide without replicating their DNA because that does not align with any other form of division. It is also easily noted that this form of division cannot be mitosis because mitosis occurs in germ cells and not skin cells. The main similarity is that the daughter cells in this new form of division and meiosis have genetic variation. After considering all of the data it is obvious that the biological reason for this irregular division is an urgent need for skin cells as the zebrafish larvae are growing. While this is not a normal process because it differs from both mitosis and meiosis drastically, the cells that divide irregularly are only a temporary measure and are later replaced with new cells formed from true mitosis (Chan et al., 2022). This astounding discovery has led researchers to wonder if this form of new division is present in other vertebrate species as well.

References:

Postlethwait, J. et al., 2008. Chapter 8 the zebrafish genome. Methods in Cell Biology. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0091679X08618981 [Accessed June 10, 2022]. 

Gillespie, C., 2019. At which stage of life does mitosis occur more rapidly? Sciencing. Available at: https://sciencing.com/three-primary-purposes-mitosis-8374117.html [Accessed June 10, 2022]. 

Robinson, A., 2019. The 4 Mitosis Phases: Prophase, Metaphase, Anaphase, Telophase. Prepscholar.com. Available at: https://blog.prepscholar.com/mitosis-phases-prophase-metaphase-anaphase-telophase [Accessed June 10, 2022].

Chan, K., Yan, C., Roan, H., Hsu, S., Tseng, T., Hsiao, C., Hsu, C. and Chen, C., 2022. Skin cells undergo asynthetic fission to expand body surfaces in zebrafish. nature. Available at: https://www.nature.com/articles/s41586-022-04641-0 [Accessed June 19, 2022].

‌