Mitosis is a fundamental process of cell division, allowing a single parent cell to create two identical daughter cells. This biological event is a precise and highly regulated sequence of changes within a cell. Recognizing and differentiating the various phases of mitosis is a foundational skill in biology.
What Mitosis Is
Mitosis serves several purposes in living organisms, including growth, repair, and asexual reproduction in single-celled organisms. It is the process by which a parent cell duplicates its chromosomes and then distributes them equally into two new, genetically identical daughter cells. For example, mitosis allows a fertilized egg to develop into a complex organism or enables skin cells to replace old and damaged ones. This equitable distribution of genetic material is achieved through a series of distinct stages.
Preparatory Phase for Cell Division
Before a cell undergoes mitosis, it completes a preparatory period known as interphase. Although not part of mitosis, interphase is a lengthy stage where the cell grows and replicates its DNA, making up over 95% of the cell cycle. During this phase, the cell synthesizes proteins, lipids, and other cellular components.
Interphase is subdivided into three phases: G1, S, and G2. In the G1 phase, the cell grows and accumulates building blocks for DNA and energy reserves. The S phase (synthesis phase) is when DNA replication occurs, resulting in two identical sister chromatids for each chromosome, which remain attached at the centromere. The G2 phase involves further cell growth and the synthesis of proteins, including those needed for the mitotic apparatus. During interphase, chromosomes are not individually visible under a microscope; instead, the genetic material appears as diffuse chromatin within the nucleus, and the nucleolus is present.
Identifying the Stages
Prophase
Prophase marks the beginning of visible changes as the cell commits to division. During this stage, the diffuse chromatin within the nucleus undergoes condensation, forming distinct, rod-like chromosomes visible under a microscope. As prophase progresses, the nucleolus shrinks and eventually disappears, and the nuclear envelope starts to break down into small vesicles. Concurrently, the mitotic spindle, composed of microtubules, begins to form outside the nucleus, extending from opposite ends of the cell.
Metaphase
Following prophase, the cell enters metaphase, characterized by the precise alignment of chromosomes. The condensed chromosomes move and align along the metaphase plate, a plane at the cell’s equator. This alignment is guided by spindle fibers (microtubules) that attach to specialized protein complexes called kinetochores located at the centromere of each sister chromatid. These microtubules exert tension, pulling the chromosomes back and forth until they are positioned in the middle of the cell.
Anaphase
Anaphase is a rapid stage where sister chromatids separate and move to opposite poles of the cell. This separation begins with the splitting of the centromeres that held them together. Once separated, each chromatid is considered an individual chromosome. Kinetochore microtubules shorten, pulling these newly independent chromosomes towards the spindle poles, often causing them to appear V-shaped or Y-shaped as they are dragged through the cytoplasm. Concurrently, interpolar microtubules lengthen, contributing to the elongation of the entire cell, stretching it into an oval shape.
Telophase
Telophase represents the final stage of nuclear division, essentially reversing the events of prophase. As the separated chromosomes arrive at opposite poles of the elongated cell, they begin to decondense and uncoil, returning to their diffuse chromatin state. A new nuclear envelope forms around each set of chromosomes at both poles, creating two distinct daughter nuclei. The nucleoli reappear within these newly formed nuclei, and the mitotic spindle fibers disassemble and disappear.
Cytokinesis
Cytokinesis is the physical division of the cytoplasm, organelles, and cell membrane, which typically overlaps with telophase. In animal cells, a contractile ring of actin and myosin microfilaments forms around the cell’s equator, pinching the cell membrane inward to create a cleavage furrow. This furrow deepens until the cell is completely divided into two daughter cells. In plant cells, which have a rigid cell wall, Golgi vesicles carrying cell wall components fuse along the metaphase plate, forming a cell plate that grows outward to divide the cell into two.
Tools and Techniques for Observation
Observing the stages of mitosis requires a light microscope. Samples where cells are actively dividing are ideal for observation, such as onion root tips or whitefish blastula. These tissues contain meristematic cells that undergo frequent mitosis.
To enhance visibility of chromosomes and other cellular components, prepared slides are stained with dyes that bind to DNA. Common stains like orcein ethanoic or toluidine blue O are used. Proper slide preparation, including thin sections of tissue and careful application of a coverslip, ensures light passes through the specimen effectively for clear microscopic viewing.