Prophase Anaphase Metaphase Telophase

Reading Mitosis Under the Microscope

You will spend most of your time trying to catch a cell caught between phases. That is the reality of working with mitosis. The textbook diagrams show clean, perfect cells lining up like soldiers. Real biology does not cooperate on schedule. Cell preparations from onion root tips or whitefish blastula slides are your usual materials, and even with fresh samples, you are looking at a small fraction of cells actually dividing at any given moment. Most cells in your field of view will be in interphase, sitting quietly with their nuclei intact. The trick is knowing what each stage actually looks like before you start scanning. Here is the breakdown as it presents on a slide.

Prophase Anaphase Metaphase Telophase

Prophase

Chromosomes condense from diffuse chromatin into visible, distinct structures. Each chromosome now consists of two sister chromatids joined at the centromere. The nuclear envelope breaks down somewhere during this phase, though on a static slide you might catch it mid-disassembly or already gone. The mitotic spindle begins forming from the centrosomes, which move toward opposite poles. You will see the spindle fibers radiating but not fully organized yet. Under 400x magnification, the chromosomes look like dark, tangled threads that are gradually thickening and separating from one another. If your stain is poor, they will just look like a blob of purple with no internal structure visible. That is usually a staining time issue, not a biological one. Ten to fifteen minutes in aceto-orcein or acetocarmine is typical for root tip squash prep. Go longer and you get clumps that look like every chromosome has merged into one dark mass. This is the stage most students skip because it is short and messy. The nuclear envelope is fully fragmented. Spindle microtubules reach into the former nuclear region and attach to kinetochores at the centromeres. Chromosomes are highly condensed and jiggling as they get pulled and reeled by microtubule dynamics. On a slide, you will see chromosomes scattered throughout the cell area with no clear organization. If you try to label a cell like this as metaphase, you will be wrong. The key difference is that metaphase chromosomes are aligned along the equator. Prometaphase chromosomes are anywhere and everywhere. Chromosomes align at the metaphase plate, which is the equatorial plane of the cell. This is the classic X-shaped chromosome image everyone recognizes. The spindle assembly checkpoint is active here, ensuring every chromosome has proper bipolar attachment before anaphase can proceed. Under the microscope, you should see roughly eight distinct chromosomes in an onion root tip cell spread across a line through the middle of the cell. They are not all in the same exact plane, so some may appear slightly above or below the line depending on the focal plane. Adjusting fine focus helps. If the chromosomes are clustered at one end of the cell or floating randomly in the cytoplasm, that cell is not in metaphase. It might be in anaphase already, or it could be undergoing a spindle disruption artifact from rough handling during slide preparation.

Sister chromatids separate and move toward opposite poles. This is the point of no return. The cohesin proteins holding the chromatids together are cleaved by separase, and the chromosomes are pulled apart by shortening kinetochore microtubules. Under the microscope, anaphase cells look like two groups of chromosomes moving away from each other. Sometimes the chromatids are clearly separated with a visible gap in the middle of the cell. Sometimes they are so close together you can barely tell they have separated yet. V-shaped or J-shaped chromosomes indicate the centromere is leading as the rest of the arm trails behind. If you see chromosomes being pulled into two distinct masses with the cell elongating, that is anaphase. If the masses are still converging toward the center, you are probably still in late metaphase or early anaphase transition, which is hard to call with confidence on a fixed slide. Chromosomes arrive at the poles and begin decondensing. Nuclear envelopes reform around each set. The spindle disassembles. Cytokinesis usually begins during late anaphase or telophase and creates the physical separation between daughter cells. In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms across the middle. On your onion root tip slide, telophase cells will show two distinct nuclei forming at opposite ends, sometimes with a faint line appearing between them where the cell plate is developing. If you see a clear wall forming down the center with two condensed chromosome masses on either side, you have a telophase cell. The trick is distinguishing early telophase from late anaphase. In telophase, the chromosome masses are decondensing and becoming less dense. In late anaphase, they are still tightly packed and clearly separating. I once spent an entire lab period trying to count metaphase cells in a prepared slide that turned out to be over-fixed. The formalin had been sitting too long, and the chromosomes were so over-preserved they appeared shrunken and fragmented, making metaphase plates impossible to distinguish from late anaphase pull-apart. I ended up switching to a fresh root tip prep and had clear metaphase cells within twenty minutes. Over-fixation is one of those silent killers in mitosis labs. If your chromosomes look crispy or shattered rather than smooth and solid, the fixation is the problem, not your eyesight.

Another issue is overlapping cells in the squash. Root tip squashes that are too thick will have multiple cell layers stacked on top of each other. You cannot accurately determine phase when chromosomes from one cell are visually superimposed on another. A properly squashed slide should have most cells as single layers. If you cannot trace individual cell boundaries clearly at 400x, you need to remount and apply more pressure with the coverslip, or start with a younger, more actively dividing root tip.

Counter-Intuitive Details

Not all cells in a tissue divide at the same rate. In an onion root tip, the meristematic zone just behind the root cap is where the action is. Move two or three millimeters back toward the mature region and you will barely find any dividing cells. The phase distribution is also not equal. Interphase dominates, usually accounting for eighty to ninety percent of observed cells. Metaphase is one of the shortest phases, which means you will see far fewer cells in that stage simply because cells spend less time there. This is why some students think their slide is defective when they only count two or three metaphase cells out of a hundred. It is normal. A second point that trips people up: chromosomes are not always evenly spaced in metaphase. In some species, and sometimes in allium, chromosomes cluster tightly together at the plate rather than spacing out like a neat row. Do not assume a messy metaphase plate means the cell is not actually in metaphase. Spindle forces can push chromosomes into compact groups, especially if the cell is under mild stress from preparation.

What This Method Cannot Do

Fixed slide observation gives you a snapshot, not a movie. You cannot determine the exact duration of each phase from a static sample. You can estimate relative timing by counting the percentage of cells in each phase across a large sample, but those estimates depend heavily on sampling enough cells and assuming the population is asynchronously dividing. If your culture was synchronized by a drug block, the phase distribution will be artificially skewed. Standard root tip preps are asynchronous, so the counting method works reasonably well, but the estimates are rough at best, usually within a factor of two of the true duration. You also cannot observe dynamic processes like kinetochore-microtubule attachment correction or the actual moment of cohesin cleavage. Those require live-cell imaging with fluorescent tags, which is a different level of equipment entirely. For a standard undergraduate or diagnostic lab context, fixed-stain microscopy of mitotic phases is adequate, but you should understand its limits before drawing conclusions about what the cell was doing moment to moment.

Quick Reference for Phase Identification

Condensed chromosomes but scattered randomly: prophase or prometaphase. Look for a remnant of nuclear envelope to distinguish between them, though that is often not visible. Chromosomes aligned at the center: metaphase. Chromosome groups moving apart: anaphase. Two nuclei reforming with possible cell plate: telophase. Chromosomes still compact but no clear separation or alignment: likely prophase. Chromosomes pulled into two masses with the cell visibly elongating: anaphase. The boundaries between late prophase, prometaphase, and early anaphase are the fuzzy ones, and accepting that uncertainty is part of reading these slides.