The Significance Of Allium Root Tips When Investigating Mitosis

Mitosis is a period of cell division in the cell cycle, which takes place when new cells are needed for growth or to replace damaged tissue.1 This process of mitosis consists of 4 stages: prophase, metaphase, anaphase and telophase. These individual stages of mitosis can be observed by looking at allium root tips under a microscope.

Allium is a genus of plants, including onion, garlic, leek and chive.2 These plants are particularly useful for investigating mitosis because their roots are continuously growing as they search for water and nutrients.3 Cell division by mitosis takes place in specific tissues, called meristems, that are found at the growing root and shoot tips and in the cambium between the xylem and phloem of the vascular bundle. The cells in the meristems are undifferentiated4, so you can find a high proportion of cells that are undergoing mitosis in this region, at any given time.

Furthermore, allium plants usually have large chromosomes and a low chromosome number.5 This means that when a thin layer of the root tip is squashed and stained on a slide and placed under a microscope, the chromosomes of individual cells are very clear to observe, as they are relatively large. Also, the advantage of a low chromosome number is that it is easier to count the cells that are undergoing mitosis and identify which stage of mitosis they are undergoing. For example: if you see that the chromosomes are lined up along the middle of the cell, then you can identify this stage of mitosis as metaphase.1

Garlic cloves are most often suggested as a source of root tips. This is because it is easy to obtain, as fresh garlic will sprout overnight if a clove is supported so that it just touches the surface of some water in a vessel – for example: by simply inserting the clove firmly into a test tube of suitable diameter.6

Although onion root tips are also from an allium plant, it is better to use garlic root tips because their cells are more compact, which would usually give better staining as more cells are likely to be covered with the stain.7 As a result, using garlic root tips would give consistently better results than those obtained with onion root tips, as a better stain coverage would mean that more cells containing visible chromosomes would be more easily identified. Not only this, but many onions that are obtained commercially have been heat-treated8 to prevent sprouting, and will therefore produce few roots. It is due to these reasons, that I used garlic root tips in my practice, rather than onion root tips.

In addition, the cell cycle is thought to be influenced by the plant’s circadian clock.9 Circadian clocks are time-keeping mechanisms that allow plants to anticipate and prepare for daily and seasonal changes in surrounding environments10, so plants may prepare for these changes by reproducing through mitosis. It is suggested that active cell division is most likely to be observed in the morning or around midday.9 Therefore, I carried out the practical during this time, as it would increase the likelihood of finding cells undergoing mitosis and therefore it would be easier to count the number of cells containing visible chromosomes. This can then be used to work out the mitotic index: number of cells containing visible chromosomes/total number of cells observed.

Bibliography:

1. McGlade C., Rogers R. and Thompson H. (2016). CGP A-Level Biology AQA Student Book. Glasgow: Coordination Group Publications Ltd. (CGP), p.86-87
2. Biology Online, (2005). Biology Dictionary ‘Allium’. [online] Available at: https://www.biology-online.org/dictionary/Allium [Accessed 18/10/19]
3. Warren, D (2004). Online Onion Root Tips. The Biology Project. [online] Available at: http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html [Accessed 18/10/19]
4. Science & Plants for Schools, (Year unknown). A-level set practicals – microscopy of root tip mitosis. [online] Available at: https://www.saps.org.uk/secondary/teaching-resources/1358-a-level-set-practicals-microscopy-of-root-tip-mitosis [Accessed 19/10/19]
5. Nefic H, Musanovic J, Metovic A and Kurteshi K (2013). Chromosomal and Nuclear Alterations in Root Tip Cells of Allium Cepa L. Induced by Alprazolam. NCBI. [online] Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4272485/ [Accessed 19/10/19]
6. Nuffield Foundation, (2011). Investigating mitosis in allium root tip squash. [online] Available at: https://www.nuffieldfoundation.org/practical-biology/investigating-mitosis-allium-root-tip-squash [Accessed 19/10/19]
7. GTAC, (2016). Mitosis in growing root tips. [online] Available at: https://www.gtac.edu.au/wp-content/uploads/2016/01/Mitosis_Roottips_LabPreparation.pdf [Accessed 19/10/19]
8. BBC, (2014). Growing onions and garlic. [online] Available at: http://www.bbc.co.uk/gardening/basics/techniques/growfruitandveg_growingonions1.shtml [Accessed 19/10/19]
9. Future Learn, (Year unknown). Root tip mitosis. [online] Available at: https://www.futurelearn.com/courses/teaching-biology-inspiring-students-with-plants-in-science/1/steps/553539 [Accessed 19/10/19]
10. Inoue K, Araki T and Endo M (2018). Circadian clock during plant development. Springer Link. [online] Available at: https://link.springer.com/article/10.1007/s10265-017-0991-8 [Accessed 19/10/19]