An image of a hand placing an onion root tip of a plastic slide with tweezers

Biology A level revision resource: Observing mitosis and meiosis

Mitosis and meiosis are different types of cell division, and although they are first encountered at GCSE level, they both remain important topics of research in universities. Microscopy remains the most powerful tool that we have to help us understand these processes.

Observing mitosis with the 'onion root tip squash'

One could be forgiven for the assumption that the classic 'onion root tip squash' experiment is a bit of a dated and tired experiment - after all, it has been performed in schools for decades. However, the reason it is a favourite of teachers and exam boards alike is because it requires you to demonstrate some key laboratory skills whilst enabling you to see cell division with your own eyes - a fundamental part of the curriculum.

The use of microscopes, including light microscopes, remains an important part of biology both for undergraduate students and researchers. Although the equipment is often a little more sophisticated at university, the concepts remain the same. You will still need to work out the overall magnification using the eyepiece and objective lens, you will need to be able to quickly and effectively prepare a slide for use in the microscope and you will also need to have some way of recording your observations down the microscope - whether that is through producing a scientific drawing or using a digital camera or imaging system.

Understanding cell division is also something that attracts huge effort from researchers. Cancer research is a vital area of the work of universities, and uncontrolled mitosis is part of what happens in cells when tumours develop. Meiosis is also a huge topic for researchers, with work at Birmingham including how meiotic pairing and recombination is controlled in plants, as well as investigations into telomere biology.

Is the onion root tip experiment relevant to research?

An image of a man standing in a greenhouse
Is the onion root tip experiment relevant to research?

The case of Henrietta Lacks

Henrietta Lacks, from Baltimore, Maryland, died because of complications from her adenocarcinoma in 1951, aged 31. Before her death, cancer cells were harvested from Henrietta and passed on to Dr. George Gey for research purposes. At the time, one of the greatest challenges in medicine was to create human cells that could be grown in culture in a laboratory, rather than needing a whole human patient; cancer cells were used to accomplish this because of the unregulated mitosis that meant the cell line could be maintained. 

Until that point, Gey, and many other researchers, had never managed to sustain human cells in the laboratory, but Henrietta's cells were different. The cells harvested from her not only survived outside of her body, but thrived. They replicated continually and remained viable, with hundreds of thousands of cells produced in the first few weeks alone. Henrietta's cancer cells were 'immortal'. Gey named the strain HeLa, and sent cells to researchers around the world. HeLa cells initially proved useful to test the polio vaccine, and this success lead to the cells being used in everything from chemotherapy, to cloning, and were even taken into space. It is estimated that more than 20 tons of HeLa cells have been produced since 1951, and more than 11,000 patents have been filed that involve HeLa cells.

It was not until the cell line began to contaminate other cell lines that anybody thought to ask where they had come from. Henrietta's family began to be asked by researchers in the early 1970's for blood samples so they could create genetic markers to help contain the contamination. No consent had been sought from Henrietta to take the cells originally, and subsequently family medical records were also published without consent. The family had no idea about how the cells were being used, and had been given no explanation, leaving them to try to piece together why some of Henrietta's cells were still living more than 20 years after her death.

HeLa cells have undoubtedly contributed enormously to our understanding of human health and disease, but this case is equally interesting from an ethical and 'how science works' perspective, thinking about the the way Henrietta and her family were treated. Does the ethical treatment of humans extend to the cells of a human that can be grown in the laboratory? Should the family have been properly informed about the use of Henrietta's cells, and given the commercialisation of the cell line, should they also have been compensated? Finally, what should we do with a technology that has been acquired in an unethical way? Is it wrong to go on using such a technology, or would it be worse to discard it and knowingly lose all of the potential good that it could create?

What should I be able to see under the microscope?

If you have managed to prepare a section of root tip that is actively growing then it should be possible to observe all of the phases of cell division. Be aware that if your plant cells are not actively growing then you are unlikely to see much cell division, so select your subject carefully.

Even in a growing root tip, the majority of the cells will likely be in interphase. In interphase they will look like 'normal' plant cells, with a dark and well defined nucleus . Remember that although interphase is the phase cells spend the most time in, they are not inactive. Cells will be copying their DNA in preparation for cell division (referred to as Synthesis or S) . Before and after Synthesis, the cell will be growing (referred to as Gap 1 before Synthesis and Gap 2 after) which involves the production of proteins and organelles, including mitochondria.

Cells undergoing mitosis will look different depending on the stage they have reached. Cells in prophase look similar to those in interphase but the nucleus will appear less well defined as the nuclear membrane begins to break down. In metaphase, the chromosomes will appear lined up along the middle of the cell, so individual chromosomes should be visible. In anaphase chromosomes will be migrating to opposite ends of the cell, which you may be able to see, or the cell may just look 'smudgy'. Finally, in telophase the cell will appear to have two nuclei as the two sets of chromosomes are fully separated and the nuclear membrane reforms. This can be tricky to spot as you need to make sure there is no cell wall separating the two nuclei.

Cytokinesis is a separate stage of cell division from mitosis, where (in plants) a cell plate forms across the middle of the cell, which eventually becomes a new cell wall. Cytokinesis in plants is significantly different from cytokinesis in animal cells because of the presence of the cell wall. When observing this under the microscope, plant cells may appear slightly 'pinched' in the middle, but will also have a hazy or wavy line separating the two new daughter cells.