Source: Onion Root Tip Cell Division Lab, Florida Virtual School
Tags: cell cycle, mitosis, interphase, prophase, metaphase, anaphase, telophase, cytokinesis, onion root tip, cell division stages, mitotic index
Difficulty: Introductory | Prerequisites: Basic cell structure (organelles, nucleus, chromosomes).
Cell division is how organisms grow, repair tissue, and reproduce. Before a cell can divide, it needs to copy its DNA and organelles, then distribute them evenly into two new daughter cells. This lab uses the tip of an onion root, where cells divide rapidly, to observe each stage of mitosis under a microscope. The proportion of cells you find in each stage tells you how long that stage takes relative to the others.
Cells spend most of their time in interphase, preparing to divide. The actual division (mitosis) moves through prophase, metaphase, anaphase, and telophase, finishing with cytokinesis. The more cells you spot in a given stage, the longer that stage lasts.
Cell cycle
The ordered sequence of events a cell goes through from one division to the next, including growth, DNA replication, and division. In simple terms, it is the full life of a cell from birth to the point it splits in two.
Interphase
The longest phase of the cell cycle, during which the cell grows, copies its DNA, and prepares for division. Subdivided into G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis). Think of it as the cell's "getting ready" period. The cell looks normal under the microscope, with no visible chromosomes, just diffuse chromatin.
Mitosis
The process of nuclear division that produces two genetically identical daughter nuclei. Comprises prophase, metaphase, anaphase, and telophase. In simple terms, this is the part where the duplicated chromosomes are physically sorted into two sets.
Prophase
The first stage of mitosis. Chromatin condenses into visible chromosomes (X-shaped because each consists of two sister chromatids). The nuclear envelope begins to break down. Think of it as the chromosomes becoming visible for the first time, though they are not yet lined up.
Metaphase
The stage where chromosomes align along the cell's equator (the metaphase plate). Spindle fibres attach to the centromere of each chromosome. In simple terms, the chromosomes line up single-file in the middle of the cell.
Anaphase
The stage where sister chromatids are pulled apart to opposite poles of the cell by the shortening spindle fibres. Think of it as the moment the "X" splits into two and each half moves to a different end of the cell.
Telophase
The final stage of mitosis. Chromosomes de-condense, nuclear envelopes re-form around each set, and the cell has two distinct nuclei. In simple terms, the cell is rebuilding two nuclei, one at each end.
Cytokinesis
The physical division of the cytoplasm that follows mitosis, producing two separate daughter cells. In plant cells (like onion root), a cell plate forms between the two new cells. Think of it as the final pinch (or wall-building, in plants) that actually separates the two cells.
Chromatin
The loosely organised form of DNA and protein found in the nucleus during interphase. When the cell prepares to divide, chromatin condenses into chromosomes. In simple terms, chromatin is the "unwound" version of chromosomes.
Mitotic index
The ratio of cells in mitosis to the total number of cells observed in a sample. A higher mitotic index indicates more active cell division. Think of it as a quick measure of how busy a tissue is with dividing.
The cell cycle has two major parts: interphase (G1, S, G2) and the mitotic phase (mitosis + cytokinesis).
Interphase accounts for roughly 90% of the cycle. The cell grows, duplicates its DNA, and prepares its organelles.
The mitotic phase is comparatively short: the chromosomes are divided (mitosis) and then the cell itself splits (cytokinesis).
Interphase: The nucleus is intact and clearly visible. Chromatin appears as a diffuse, granular mass with no distinct chromosome shapes. No X-shaped structures.
Prophase: Chromatin begins condensing into visible, X-shaped chromosomes (sister chromatids joined at the centromere). They appear scattered, not yet organised. The nuclear envelope starts to break down.
Metaphase: Chromosomes are lined up along the middle of the cell (the metaphase plate). This is the stage where chromosomes are most clearly visible and countable.
Anaphase: Sister chromatids have separated and are being pulled toward opposite poles. You can see two clusters of chromosomes moving apart.
Telophase: Chromosomes have arrived at each pole, a nuclear envelope is re-forming around each set, and the chromosomes begin to de-condense.
Cytokinesis: The cytoplasm divides. In plant cells, a cell plate forms across the middle. You can see two distinct daughter cells forming.
Stage | Cells Observed | Percentage of Cycle |
|---|---|---|
Interphase | 34 | 68% |
Prophase | 8 | 16% |
Metaphase | 3 | 6% |
Anaphase | 2 | 4% |
Telophase | 2 | 4% |
Cytokinesis | 1 | 2% |
Total | 50 | 100% |
Interphase is the longest stage because the cell is doing the most work: growing, duplicating all its DNA, and producing the proteins and organelles needed for division.
The later mitotic stages are mechanically simpler (sorting and separating chromosomes) and therefore faster.
The number of cells caught in each stage directly reflects the time spent there. More cells in interphase means cells spend more time in interphase.
The root tip is a region of active growth called the apical meristem.
Cells here divide rapidly, so you can observe many stages of mitosis in a single sample.
If you observed cells further from the tip (in the zone of elongation or maturation), most cells would be in interphase or fully differentiated and no longer dividing. You would see far fewer mitotic cells.
Percentage of cell cycle spent in a stage:
(Number of cells in that stage / Total cells counted) x 100 = % of cycle
Example: 34 interphase cells / 50 total cells x 100 = 68%
Mitotic index:
Number of cells in mitosis / Total number of cells observed
Example: (8 + 3 + 2 + 2) / 50 = 15 / 50 = 0.30, or 30%
(Note: whether cytokinesis is included in the mitotic index depends on the definition your course uses. Some definitions count only prophase through telophase.)
Understanding the cell cycle is central to cancer biology. Cancer cells divide uncontrollably because the normal checkpoints in the cell cycle fail. Chemotherapy drugs often target specific stages of mitosis (for example, taxol blocks cells in metaphase by stabilising microtubules). The mitotic index is used clinically to assess how aggressively a tumour is growing.
"Mitosis is the whole cell cycle." Mitosis is only the nuclear division phase. The cell cycle also includes interphase (G1, S, G2) and cytokinesis. Most of the cell's life is spent in interphase.
"Cells are resting during interphase." Interphase is the busiest period. The cell is growing, replicating its entire genome, and synthesising proteins. "Resting" is misleading.
"Cytokinesis and telophase are the same thing." Telophase is the re-formation of the nuclear envelopes. Cytokinesis is the splitting of the cytoplasm. They overlap in timing but are distinct processes.
"A dividing cell has no nucleus." During prophase through anaphase, the nuclear envelope is absent, but it re-forms in telophase. A cell is only briefly without a nuclear envelope.
⚠️ You will likely be asked to calculate the percentage of cells in each stage and explain what it tells you about relative time spent in that stage.
⚠️ Know the visual features of each stage. Exam questions often show microscope images and ask you to identify the stage.
⚠️ Be ready to explain why the root tip (meristematic tissue) is used rather than mature tissue.
⚠️ Understand the difference between a dividing cell and a non-dividing cell under the microscope: the key distinction is the presence or absence of visible, condensed chromosomes and whether the nuclear envelope is intact.
True or false: Interphase is the shortest stage of the cell cycle. False. Interphase is the longest.
Fill in the blank: During ______, chromosomes line up along the middle of the cell. Metaphase.
True or false: Cytokinesis happens before telophase. False. Cytokinesis follows telophase.
Fill in the blank: The percentage of cells in a stage reflects the relative ______ spent in that stage. Time.
True or false: During interphase, chromosomes are visible as distinct X-shapes. False. DNA exists as diffuse chromatin during interphase.
Q: Why is the onion root tip used to study mitosis?
A: The root tip contains the apical meristem, a region of rapid cell division. This provides many cells in various stages of the cell cycle in a single sample.
Q: Based on the data, which stage of the cell cycle takes the longest? Explain how you know.
A: Interphase takes the longest. 68% of the cells observed were in interphase, which means cells spend the greatest proportion of their cycle in this stage. The number of cells in a stage is proportional to the time spent there.
Q: What is the difference between the nucleus of a dividing cell and a non-dividing cell?
A: In a non-dividing (interphase) cell, the nucleus is intact with a clear nuclear envelope and diffuse chromatin. In a dividing cell (prophase through anaphase), the nuclear envelope breaks down and chromosomes become condensed and visible.
Q: If you observed cells further from the root tip, how would your results differ?
A: You would see far more cells in interphase and very few (if any) in mitosis. Cells further from the tip are in the zone of elongation or maturation and have largely stopped dividing.
Q: Calculate the mitotic index from the lab data and explain what it tells you.
A: Mitotic index = cells in mitosis / total cells = (8 + 3 + 2 + 2) / 50 = 0.30, or 30%. This means 30% of the cells in the root tip were actively dividing at the time of observation, indicating a high rate of cell division.
This material connects to DNA replication (which happens during the S phase of interphase, before mitosis can begin) and to meiosis (the specialised cell division that produces gametes, which shares several stage names but has two rounds of division and produces non-identical cells). Understanding the normal cell cycle is also foundational for studying cancer biology, where checkpoints fail and division becomes unregulated.
cell cycle, mitosis, meiosis, interphase, prophase, metaphase, anaphase, telophase, cytokinesis, cell division, mitotic phase, mitotic index, sister chromatids, centromere, spindle fibres, metaphase plate, nuclear envelope, chromatin, chromosomes, apical meristem, onion root tip, cell plate, daughter cells, G1 phase, S phase, G2 phase, cell cycle stages, dividing vs non-dividing cells, plant cell division