Describe the Stages of Mitosis

The Quest to Understand Cell Division

Every second, your body generates millions of new cells to replace old ones, heal wounds, and sustain growth. How does a single cell become two, each with the same genetic instructions as the original? This question puzzled biologists for centuries, because early microscopes lacked the resolution to observe chromosomes in action. The discovery of mitosis — the orderly division of a cell's nucleus — stands as one of the great achievements of cell biology. Understanding mitosis also provides the foundation for explaining how organisms grow, repair tissue, and how errors in division can lead to diseases like cancer.

From Flemming's hand-drawn sketches to fluorescent live-cell videos, the central question has remained the same: how does a cell distribute its chromosomes with such precision that virtually every new cell is genetically identical to the original? Answering this question requires a stage-by-stage analysis of mitosis, which is exactly what the following sections provide.

Foundational Ideas

Before examining each stage, you need to understand several key principles that govern how cells divide. Mitosis is specifically the division of the nucleus and its chromosomes; it is usually followed by cytokinesis, the physical splitting of the cytoplasm into two daughter cells. Together, the two processes ensure that one parent cell becomes two genetically identical offspring cells. Mitosis is a form of asexual reproduction at the cellular level and should not be confused with meiosis, which produces gametes with half the chromosome number.

The Stages at a Glance

The diagram below illustrates the five classical stages of mitosis — prophase, prometaphase, metaphase, anaphase, and telophase — followed by cytokinesis. Each stage represents a distinct set of chromosome and spindle behaviors visible under a microscope. Trace the path from left to right to see how the duplicated chromosomes inside a single nucleus are systematically separated into two identical nuclei.

This diagram illustrates the Crosscutting Concept of Structure and Function: the spindle apparatus (structure) enables the precise sorting of chromosomes (function). Each stage can be identified by the position and state of the chromosomes, the integrity of the nuclear envelope, and the arrangement of the spindle fibers. In the next sections, we examine each stage in molecular detail.

A Molecular Walkthrough of Each Phase

Mitosis is a continuous process, but biologists divide it into distinct stages based on observable changes in chromosome behavior and cell architecture. Understanding the molecular cause-and-effect at each transition is essential. Below, each phase is described with attention to the Science and Engineering Practice of constructing explanations — we explain not just what happens but why it happens at the molecular level.

1. Prophase

During prophase, chromatin fibers coil tightly around histone proteins, condensing into visible chromosomes. Each chromosome appears as an X-shaped structure because it consists of two sister chromatids joined at the centromere. Meanwhile, the two centrosomes (each containing a pair of centrioles in animal cells) begin migrating toward opposite poles of the cell. As they move, they extend microtubules, forming the early mitotic spindle. The nucleolus fades from view as its ribosomal RNA genes are packed into condensed chromosomes.

2. Prometaphase

The nuclear envelope fragments during prometaphase, dissolving the boundary between the nucleus and cytoplasm. Spindle microtubules now penetrate the nuclear region and attach to protein complexes called kinetochores — specialized structures assembled on each chromatid's centromere. Each chromosome has two kinetochores, one per sister chromatid, and each kinetochore captures microtubules from the nearest spindle pole. The chromosomes begin to jerk back and forth as opposing motor proteins tug them in different directions, a behavior sometimes called congression.

3. Metaphase

At metaphase, all chromosomes are aligned along the metaphase plate, an imaginary plane equidistant between the two spindle poles. This alignment is critical: the spindle assembly checkpoint (SAC) verifies that every kinetochore is attached to spindle fibers from opposite poles. If even one kinetochore is unattached, the checkpoint halts progression, preventing premature separation of sister chromatids. Only when every chromosome passes this inspection does the cell advance to anaphase. This checkpoint is a key example of feedback regulation — a systems-level control.

4. Anaphase

Once the checkpoint is satisfied, the enzyme separase cleaves the cohesin proteins holding sister chromatids together. Anaphase proceeds in two overlapping sub-stages. In anaphase A, motor proteins at the kinetochores walk along shortening microtubules, dragging chromatids toward their respective poles. In anaphase B, the spindle poles themselves move apart as non-kinetochore microtubules slide against each other and the cell elongates. By the end of anaphase, each pole has a complete set of chromosomes.

5. Telophase

During telophase, two new nuclear envelopes assemble around each cluster of chromosomes from fragments of the parent cell's endoplasmic reticulum. The chromosomes begin to decondense back into diffuse chromatin, restoring access for gene transcription. Nucleoli reappear, and the mitotic spindle disassembles. At this point, the cell effectively contains two nuclei. Cytokinesis, which typically overlaps with telophase, completes division by pinching the cytoplasm in animal cells (via a cleavage furrow) or building a cell plate in plant cells.

Key Structures in Action

To truly understand mitosis, you must connect the structures involved with their specific functions at each stage. The table below provides a concise reference, and the diagram that follows visualizes the cell cycle clock, showing where mitosis fits within the larger cycle.

The cell cycle diagram above emphasizes a key quantitative pattern: mitosis is remarkably fast relative to the rest of the cycle. For a typical human cell dividing every 24 hours, mitosis may last only about one hour. The bulk of the cell's time is spent in G₁ (growth), S (DNA replication), and G₂ (preparation for division). This Crosscutting Concept of Scale, Proportion, and Quantity reminds us that even though mitosis is the most visually dramatic part of the cell's life, it represents a small fraction of the overall cycle.

Analyzing a Microscope Image of Dividing Cells

One of the most common tasks in a biology lab is examining a stained slide of an onion root tip (or another actively dividing tissue) and identifying which stage of mitosis each cell is in. The worked example below walks through the reasoning process step by step.

Two Types of Nuclear Division

Students often confuse mitosis with meiosis, the other form of nuclear division. While both involve chromosome condensation and spindle-based separation, they serve fundamentally different purposes and produce different outcomes. The table below highlights the critical distinctions, which also connect to the NGSS Disciplinary Core Idea LS3.A (Inheritance and Variation of Traits).

When Mitosis Goes Wrong

The precision of mitosis depends on a network of regulatory proteins. When these regulators malfunction, cells may divide uncontrollably, forming tumors. This connection between normal mitosis and cancer is one of the most important applications of cell biology and represents the NGSS performance expectation LS1.B (Growth and Development of Organisms). Two major classes of genes govern the cell cycle.

The protein p53 — often called the 'guardian of the genome' — normally monitors DNA integrity at the G₁ checkpoint. If it detects damage, it halts the cell cycle and activates repair enzymes. If the damage is irreparable, p53 triggers apoptosis (programmed cell death). When both copies of the TP53 gene are mutated, the cell loses this critical checkpoint, accumulates mutations, and may become cancerous. In fact, TP53 mutations are found in over 50% of all human cancers.

Many modern cancer therapies target specific stages of mitosis. For example, taxol (paclitaxel) stabilizes microtubules so they cannot shorten during anaphase, freezing cancer cells in metaphase and triggering apoptosis. Vincristine prevents microtubule assembly, blocking spindle formation entirely. These drugs exploit the structure-function relationship of the mitotic spindle — disrupting the structure halts the function. This is also a powerful example of the SEP of Designing Solutions based on an understanding of biological mechanisms.

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