Mitosis is the process by which a cell divides its nucleus to produce two genetically identical daughter cells. For decades, the onion root tip has been the standard specimen used in biology laboratories to observe this complex process. The specimen offers a unique combination of biological characteristics and practical advantages that make it an ideal model for visualizing the dynamic stages of cell division under a light microscope.
Locating the Region of Rapid Growth
The primary reason for using the root tip is that it contains the apical meristem, a specialized region of plant tissue where active cell division occurs continuously. This meristematic zone, situated just behind the protective root cap, is responsible for the downward growth of the root as it pushes through the soil. The cells in this area are perpetually cycling through the stages of division at a high rate.
Examining tissue from most other parts of a plant, such as a mature leaf or stem, would reveal very few cells actively undergoing mitosis. Excising the tip of an onion root guarantees a high mitotic index, which is the ratio of dividing cells to the total number of cells observed. This high concentration ensures that all four stages of mitosis—prophase, metaphase, anaphase, and telophase—are visible on a single prepared slide.
Optimal Cell Structure for Visualization
The onion cell possesses structural features that enhance the visibility of the mitotic process. Most notably, the chromosomes within the onion cell nucleus are relatively large and distinct compared to those in many other organisms. The onion plant, Allium cepa, has a diploid number of 16 chromosomes (2n=16), a manageable number to clearly observe and track during separation.
The large size of these chromosomes means that the movements of the genetic material, such as condensation in prophase and migration during anaphase, are easily resolved with a standard student light microscope. The onion root cell structure is relatively simple, lacking the dense accumulation of complex plastids, such as chloroplasts, found in tissues like leaves. The clear cytoplasm and prominent nucleus provide an unobstructed field of view for the dividing genetic material.
Accessibility and Ease of Preparation
Beyond the biological advantages, practical considerations for a laboratory setting favor the use of the onion root. Onions are inexpensive, universally available year-round, and easily manipulated to grow fresh roots simply by suspending the bulb in water. This accessibility ensures that labs can procure the necessary fresh material quickly and affordably for any class size.
Preparing the tissue for microscopic analysis is also straightforward due to a technique known as the squash method. The root tip tissue is naturally soft and pliable, which allows for easy fixation and staining. A brief treatment with a mild acid, such as hydrochloric acid, works to break down the pectin in the cell walls and dissolve the middle lamella that holds the cells together. This softening process is followed by physically pressing the tissue under a coverslip, which spreads the cells out into a single, thin layer. Creating this monolayer prevents the overlapping of nuclei, ensuring that each phase of mitosis is clearly distinguishable.