# Mitosis – Definition, Stages, Process, and Importance > Learn what mitosis is, how chromosomes move through each stage, the role of the mitotic spindle and cytokinesis, and how mitosis differs from meiosis. Canonical URL: https://biologynotesonline.com/mitosis/ Author: Sourav Pan Last updated: August 29, 2026 ![Mitosis – Definition, Stages, Process, and Importance](https://biologynotesonline.com/wp-content/uploads/2022/12/Cell-Division-Cycle-Mitosis.webp) Mitosis is a process of nuclear division in eukaryotic cells where the duplicated chromosomes are separated and distributed into two daughter nuclei. It takes place in the M phase of cell cycle. Before mitosis, DNA has already been replicated during S phase, so the replicated chromosome at this time consists of two sister chromatids. The sister chromatids are arranged with the help of mitotic spindle. These are then separated and move towards the opposite poles. This gives an equivalent set of chromosomes to each daughter nucleus. Cytokinesis usually follows mitosis, or sometimes it overlaps with mitosis and the cytoplasm is divided to form two daughter cells. Cytokinesis is a separate process. Mitosis can therefore take place without cytokinesis also, which results in a cell having more than one nucleus. ## Where Does Mitosis Occur? Mitosis takes place in the dividing cells of eukaryotic organisms. In multicellular animals, it occurs mainly in somatic cells where new cells are required for growth and for replacement or repair of tissues. All body cells do not continue this division. Some differentiated cells remain outside active cell division. In plants, mitosis is particularly active in the [meristematic tissues](https://biologynotesonline.com/meristematic-tissue-definition-types-characteristics/). The apical meristems present at root and shoot tips continuously form new cells, and other meristematic regions can also retain the capacity for further division. In unicellular eukaryotes, mitotic division can form new individuals as a mode of reproduction. ![Circular eukaryotic cell cycle showing G1, S, G2 and M phases, with DNA replication during S phase and mitosis plus overlapping cytokinesis during M phase.](https://biologynotesonline.com/wp-content/uploads/2024/04/Mitosis-in-the-Eukaryotic-Cell-Cycle-1024x1024.png)Mitosis occurs after DNA replication in S phase and forms part of M phase, while cytokinesis is a distinct process that can overlap with late mitosis. ## What Is the Main Purpose of Mitosis? The main purpose of mitosis is the proper distribution of replicated chromosomes into two daughter nuclei. Chromosomes are copied before mitosis. These duplicated chromosomes are then separated, and one complete and equivalent chromosome set is received by each daughter nucleus. The genetic information and chromosome complement of the daughter cells thus remain same as the parent cell. In multicellular eukaryotes, mitotic cell division is mainly required for growth. Replacement or repair of cells and tissues also takes place by this division, and cell renewal depends on it. In some unicellular eukaryotes, cell division can form a new individual. It is used as a mode of reproduction. ![Mitosis - Definition, Phases, Significance, Functions](https://biologynotesonline.com/wp-content/uploads/2022/12/Cell-Division-Cycle-Mitosis.webp)Mitosis ## Where Does Mitosis Fit in the Cell Cycle? The [eukaryotic cell cycle](https://biologynotesonline.com/cell-cycle/) is divided into G1, S, G2 and M phases. G1, S and G2 together are referred to as interphase. Mitosis occurs in the M phase. - G1 phase- It comes after the previous M phase. This phase is before DNA replication. The cell remains metabolically active and growth takes place. - S phase- DNA replication occurs in this phase and the chromosomes are duplicated. Thus, chromosome duplication is completed before mitosis. - G2 phase- This follows the completion of DNA synthesis. Cell growth continues and proteins required for mitosis are produced. The last part of interphase before the cell enters M phase. - M phase- Mitosis takes place here. The replicated chromosomes are separated and distributed into the daughter nuclei. Cytokinesis usually follows mitosis and the cytoplasmic contents are divided between the daughter cells. Mitosis therefore lies after G2 phase and before the next G1 phase of cell cycle. ### Stages of Mitosis Mitosis takes place through five stages- prophase, prometaphase, metaphase, anaphase and telophase. These stages occur in sequence. It is a continuous process. ![Sequential diagram of prophase, prometaphase, metaphase, anaphase and telophase showing chromosome condensation, spindle attachment, alignment, separation and nuclear reformation.](https://biologynotesonline.com/wp-content/uploads/2024/04/Five-Stages-of-Mitosis-from-Prophase-to-Telophase-1024x341.png)The five stages of mitosis form a continuous process in which duplicated chromosomes condense, attach to the spindle, align, separate and become enclosed within two daughter nuclei. 1. Prophase- Chromosome condensation starts in this stage. The duplicated chromosomes become clearly recognizable and each chromosome contains two sister chromatids. Mitotic spindle formation also starts. In animal cells, [centrosomes](https://biologynotesonline.com/centrosome/) move towards the opposite sides of cell. 2. Prometaphase- The nuclear envelope breaks down in cells having open mitosis, after which spindle microtubules can reach the chromosomes. These microtubules become attached to the kinetochores of sister chromatids. Movement of chromosomes within spindle begins. 3. Metaphase- The chromosomes are now present at the middle region of spindle, the metaphase plate. Sister chromatids still remain joined. Attachment is with spindle microtubules coming from the opposite poles to their kinetochores. 4. Anaphase- In this stage, the connection between sister chromatids is broken. They separate. Each separated chromatid now becomes a daughter chromosome and moves towards opposite pole. The two spindle poles also move farther apart. 5. Telophase- Daughter chromosomes reach the opposite poles. These chromosomes begin to decondense. Around each chromosome group, nuclear envelope is again formed and two daughter nuclei are produced. The mitotic spindle is disassembled. Cytokinesis commonly starts before mitosis is completely ended. It continues through telophase. ### How Does the Mitotic Spindle Separate Chromosomes? Mitotic spindle separates the duplicated chromosomes through spindle microtubules, first by their attachment with chromosomes and later by movement of chromosomes towards opposite poles. The sister chromatids at this time remain joined. Proper spindle attachment is required before their separation starts. ![Mitotic spindle schematic showing kinetochore attachment, chromosome biorientation at metaphase, chromosome movement in anaphase A and spindle elongation in anaphase B.](https://biologynotesonline.com/wp-content/uploads/2024/04/Mitotic-Spindle-and-Chromosome-Separation-During-Mitosis-1024x341.png)Bioriented chromosomes attach to opposite spindle poles before sister chromatids separate; chromosome movement and spindle elongation then increase the distance between the two chromosome sets. - Attachment of spindle microtubules- The attachment occurs at kinetochores of chromosomes. These are protein structures formed at centromeric region. One kinetochore is present on each sister chromatid, and microtubules from opposite spindle poles attach with the two sister kinetochores. - Bi-orientation of chromosomes- Sister chromatids now have connection towards opposite poles and pulling force acts from the two sides. The chromosomes become arranged near the metaphase plate. At this point, cohesin still holds the sister chromatids together. - Spindle checkpoint- If a kinetochore is unattached or its attachment is improper, the spindle assembly checkpoint remains active. Entry into anaphase is prevented. No chromosome separation at this time. The required kinetochore-microtubule attachments must first be formed. - Removal of sister chromatid cohesion- With proper attachment, the checkpoint is switched off. The anaphase-promoting complex/cyclosome (APC/C) now becomes active and securin is degraded. Separase becomes active after this, which cleaves cohesin holding the sister chromatids. The connection between sister chromatids is then lost. - Anaphase A- Sister chromatids after separation are now daughter chromosomes. These move towards their respective spindle poles as kinetochore microtubules become shorter. Motor proteins and microtubule depolymerization are involved in the movement. - Anaphase B- Here the two spindle poles move farther away from each other. The overlapping spindle microtubules slide relative to one another, the spindle becomes elongated. Distance between the two groups of daughter chromosomes increases. ## How Does the Cell Prevent Premature Chromosome Separation? The sister chromatids formed after DNA replication need to remain connected until the beginning of anaphase. Their early separation is prevented by the action of cohesin, protection of centromeric cohesion and by controlling the activity of separase. The spindle checkpoint also has a major role in this process. ![Molecular pathway showing how an active spindle assembly checkpoint inhibits APC/C-Cdc20 and separase until correct chromosome attachment allows cohesin cleavage and anaphase.](https://biologynotesonline.com/wp-content/uploads/2024/04/Spindle-Assembly-Checkpoint-Control-of-Anaphase-1024x576.png)The spindle assembly checkpoint delays sister-chromatid separation until proper kinetochore attachment permits APC/C activation, separase activity and cohesin cleavage. The following are the mechanisms preventing premature chromosome separation- - Cohesin-mediated attachment- Cohesin is the protein complex which holds the newly replicated sister chromatids together. It remains associated with chromosomes after DNA replication, with strong cohesion maintained around the centromeric region. Due to this attachment, the pulling force produced by spindle microtubules cannot immediately separate the two chromatids. - Protection of centromeric cohesin- During prophase, much of the cohesin present on chromosome arms is removed by WAPL-dependent pathway. The centromeric cohesin, however, remains protected. Shugoshin 1 (SGO1) together with protein phosphatase 2A (PP2A) protects this cohesin from its early removal and the two sister chromatids therefore remain joined at the centromere. - Inactivation of separase- The enzyme separase is responsible for cleavage of the cohesin subunit RAD21/Scc1. Before anaphase it cannot perform this function. Securin binds with separase and maintains it in inactive condition, while Cyclin B-Cdk1 also takes part in the inhibition of separase. - Spindle Assembly Checkpoint (SAC)- Chromosome attachment with spindle microtubules is checked by the [Spindle Assembly Checkpoint](https://biologynotesonline.com/checkpoints-in-the-cell-cycle-g1-g2-metaphase-spindle-checkpoints/). An unattached or improperly attached kinetochore keeps this checkpoint active. Checkpoint proteins MAD2, BUBR1, BUB3 and CDC20 form the Mitotic Checkpoint Complex (MCC) which inhibits APC/C-Cdc20, and anaphase cannot begin at this condition. - Inhibition of APC/C- When the spindle checkpoint is active, Anaphase-Promoting Complex/Cyclosome (APC/C) remains inhibited. Therefore, securin and cyclin B are not degraded. Separase remains inactive and the cohesin present between sister chromatids is still maintained. - Removal of the block at anaphase- After proper attachment of chromosomes with spindle fibres, the checkpoint signal is removed and APC/C-Cdc20 becomes active. Securin and cyclin B are now degraded. This releases the inhibition of separase, cohesin is cleaved at the centromere and the sister chromatids become free from each other. They are then moved towards the opposite spindle poles. ## Cytokinesis and the Formation of Daughter Cells [Cytokinesis](https://biologynotesonline.com/cytokinesis/) is the process of division of cell cytoplasm, generally following chromosome separation during mitosis. It begins during anaphase in most animal cells and is completed after the two chromosome sets have moved apart. At the end, one parental cell is divided into two daughter cells. ![Comparison of animal-cell cytokinesis by actomyosin contractile ring and cleavage furrow with plant-cell cytokinesis by phragmoplast-guided cell-plate formation.](https://biologynotesonline.com/wp-content/uploads/2024/04/Animal-vs-Plant-Cell-Cytokinesis-1024x341.webp)Animal cells divide through cleavage-furrow constriction and abscission, whereas plant cells construct a new cell plate between the daughter nuclei. The following are the main events involved in cytokinesis and formation of daughter cells- - Determination of division site- The position of the division plane is controlled mainly by the mitotic spindle in animal cells. Signals coming from central and astral spindle microtubules determine the equatorial region where the cell will divide. The cleavage site is formed between the two separated chromosome sets. - Formation of contractile ring- At the equatorial cortex, a ring containing actin filaments and myosin II is assembled just below the plasma membrane. RhoA has an important role in this process. Its local activation promotes assembly and activity of the actomyosin ring, while proteins of the centralspindlin complex and Ect2 are involved in controlling this RhoA activity. - Formation of cleavage furrow- Contraction of the actin-myosin ring pulls the plasma membrane inward. A depression is first produced at the equatorial region, known as the cleavage furrow. With continued contraction, this furrow becomes deeper and the cytoplasm is progressively divided into two regions. - Intercellular bridge formation- The two cells are not immediately separated after furrow ingression. They remain connected by a narrow intercellular bridge, which contains the remains of central spindle microtubules. A dense structure called the midbody is present in this region. - Abscission- This is the final stage of animal cell cytokinesis. The narrow membrane bridge joining the daughter cells is cut and complete physical separation takes place. ESCRT-III proteins participate in this final membrane scission process. The two daughter cells are now separated from each other. - Distribution into daughter cells- During cell division, the separated chromosome sets are enclosed into the two newly formed nuclei, while cytoplasmic components and organelles are also distributed between the daughter cells. Usually each daughter cell receives one complete chromosome set. In asymmetric cell division, however, their size and some cytoplasmic components may not be distributed equally. - Cytokinesis in plant cells- Plant cells do not produce a cleavage furrow because of the presence of a rigid cell wall. Instead, a phragmoplast is formed between the two daughter nuclei. Golgi-derived vesicles are transported towards its central region where they fuse and begin formation of the cell plate. - Formation of new plant daughter cells- The cell plate grows outward until it reaches the parental plasma membrane and cell wall. It finally develops into the new separating wall and associated plasma membranes between the daughter cells. Hence the parental cytoplasm becomes divided into two individual plant cells. ### What Happens to Chromosome Number During Mitosis? ![Example with 2n equals 4 showing that DNA replication produces sister chromatids without changing chromosome number and that each daughter cell retains four chromosomes after mitosis.](https://biologynotesonline.com/wp-content/uploads/2024/04/Chromosome-Number-Before-and-After-Mitosis-1024x341.webp)DNA replication doubles the DNA content before mitosis but not the chromosome number; after sister chromatids separate, each daughter cell receives the original chromosome complement. The chromosome number is maintained during mitosis. Before this division, DNA replication has already occurred in S phase and each [chromosome](https://biologynotesonline.com/chromosome/) is made up of two sister chromatids. DNA amount increases at this point. Not the chromosome number. The two sister chromatids are still counted as parts of one replicated chromosome. At anaphase, sister chromatids separate and each separated chromatid is now considered as a daughter chromosome. These move towards opposite poles. For a short period in the single dividing cell, chromosome number therefore appears doubled. After nuclear division and cytokinesis, the chromosomes are divided between two daughter cells and each daughter cell receives the same chromosome number which was present in parent cell before DNA replication. A diploid parent cell therefore gives diploid daughter cells. Not haploid cells. ## Importance of Mitosis Mitosis is important for growth, development and maintenance of eukaryotic organisms. In multicellular organisms it increases the number of cells, while chromosome material is distributed between the newly formed cells. - Growth of organisms – Growth of multicellular organisms takes place largely by increase in cell number. One cell divides into two, these cells can again divide and a large number of cells are produced. Mitosis therefore contributes to the growth of tissues and organs. - Embryonic development – The fertilized egg is a single cell, which undergoes repeated cell divisions during development. These divisions produce the large number of cells required for formation of the embryo. Early embryonic cells can divide very rapidly. - Maintenance of chromosome number – During mitosis the replicated chromosomes are separated and distributed into the two daughter nuclei. Each daughter cell normally receives a complete chromosome set. This maintains chromosome number from one cell generation to another. - Replacement of cells – Many cells are continuously lost from the body and need to be replaced. New cells are formed by mitotic cell division, especially in tissues having regular cell turnover. It helps in maintaining the normal cell population of the tissue. - Repair of damaged tissues – After injury, cells present around the damaged region can increase in number. Cell proliferation is an important part of tissue repair, for example proliferation of epithelial cells, fibroblasts and other cells during wound healing. - Regeneration – Mitosis also takes part in regeneration of lost or damaged cells. New cells are produced and used for rebuilding the affected tissue, although the capacity for regeneration differs greatly among different tissues and organisms. - Genetic continuity – Accurate separation of chromosomes during mitosis allows the daughter cells to receive the replicated genetic material. Errors in this segregation can instead produce abnormal chromosome numbers and genomic instability. - Asexual reproduction – In some unicellular eukaryotes, mitotic division is also involved in production of new individuals. [Budding yeast](https://biologynotesonline.com/budding/), for example, forms a bud which grows and separates from the mother cell after mitosis. ## What Happens When Mitosis Goes Wrong? During mitosis, the replicated chromosomes have to be distributed correctly between the two daughter cells. Errors may occur in spindle attachment, chromosome movement, checkpoint control or during cytokinesis. Some defective cells are stopped or eliminated. Others may survive, but with abnormal chromosome number. ![Schematic of mitotic errors leading to chromosome missegregation, aneuploid daughter cells, lagging chromosomes and micronuclei, cytokinesis failure and chromosomal instability.](https://biologynotesonline.com/wp-content/uploads/2024/04/Mitotic-Errors-Aneuploidy-and-Chromosomal-Instability-1024x768.webp)Errors in chromosome attachment, segregation or cytokinesis can generate aneuploidy, micronuclei, tetraploid cells and continuing chromosomal instability, although these abnormalities do not inevitably cause cancer. The following are some of the major consequences when mitosis goes wrong- - Chromosome mis-segregation- A chromosome may become incorrectly attached with spindle microtubules. In other cases the sister chromatids do not separate properly. One daughter cell then receives a chromosome which is absent from the other cell. Failure in proper chromosome separation is one of the main mitotic errors. - Aneuploidy- Gain or loss of whole chromosomes produces an aneuploid daughter cell. It has an abnormal chromosome number. Changes in gene dosage are produced in such cells, and many of these cells show reduced growth or different cellular stresses. - Lagging chromosomes and micronuclei- Sometimes a chromosome does not move properly during anaphase and remains behind. This lagging chromosome may fail to enter the main daughter nucleus. A small separate nucleus is then formed, called a micronucleus. DNA present in micronuclei is more prone to replication problems and DNA damage. Extensive chromosome fragmentation and rearrangement can also occur in severe cases. - Failure of cytokinesis- Chromosome division may be completed but division of cytoplasm fails. The daughter cells do not separate. This can form a binucleated or tetraploid cell, with approximately twice the normal chromosome complement, and such cells can become unstable during later divisions. - Mitotic arrest- The spindle assembly checkpoint prevents anaphase until chromosome attachment is suitable. If serious spindle defects are still present, the cell can remain arrested in mitosis for a prolonged period. The cell may die. Sometimes it leaves mitosis without completing the normal division. - Mitotic catastrophe- Extensive DNA damage, abnormal spindle machinery or major checkpoint problems can result in mitotic catastrophe. It mainly acts as a protective response against propagation of severely defective cells. Cell death or permanent growth arrest may finally occur. Abnormal large nuclei, several nuclei or micronuclei are commonly associated with this condition. - Chromosomal instability- Chromosome segregation errors may continue over several cell divisions. Chromosomes are then repeatedly gained or lost. This condition is referred to as chromosomal instability (CIN), where the chromosome composition of cells continues changing from one generation to another. - Development of cancer cells- Mitotic errors and aneuploidy are commonly found in cancer. Cells which survive repeated chromosome segregation errors can acquire genomic changes and these changes may help tumor development. Very high levels of chromosome instability, however, can also decrease cell survival. Aneuploidy therefore does not always produce cancer but it has an important association with tumor development and progression. ![Mitosis infographic poster](https://biologynotesonline.com/wp-content/uploads/2024/04/Mitosis-infographic-poster-725x1024.png)Mitosis infographic poster ## References - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). An overview of M phase. In Molecular biology of the cell (4th ed.). Garland Science. [https://www.ncbi.nlm.nih.gov/books/NBK26931/](https://www.ncbi.nlm.nih.gov/books/NBK26931/) - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Mitosis. In Molecular biology of the cell (4th ed.). Garland Science. [https://www.ncbi.nlm.nih.gov/books/NBK26934/](https://www.ncbi.nlm.nih.gov/books/NBK26934/) - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Plant development. In Molecular biology of the cell (4th ed.). Garland Science. [https://www.ncbi.nlm.nih.gov/books/NBK26922/](https://www.ncbi.nlm.nih.gov/books/NBK26922/) - Andreeff, M., Goodrich, D. W., & Pardee, A. B. (2003). Proliferation. In D. W. Kufe, R. E. Pollock, R. R. Weichselbaum, R. C. Bast, Jr., T. S. Gansler, J. F. Holland, & E. Frei III (Eds.), Holland-Frei cancer medicine (6th ed.). BC Decker. [https://www.ncbi.nlm.nih.gov/books/NBK13035/](https://www.ncbi.nlm.nih.gov/books/NBK13035/) - Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022). Cell growth and division. In Anatomy and physiology 2e. OpenStax. [https://openstax.org/books/anatomy-and-physiology-2e/pages/3-5-cell-growth-and-division](https://openstax.org/books/anatomy-and-physiology-2e/pages/3-5-cell-growth-and-division) - Clark, M. A., Douglas, M., & Choi, J. (2018). Cell division. In Biology 2e. OpenStax. [https://openstax.org/books/biology-2e/pages/10-1-cell-division](https://openstax.org/books/biology-2e/pages/10-1-cell-division) - Clark, M. A., Douglas, M., & Choi, J. (2018). The cell cycle. In Biology 2e. OpenStax. [https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle](https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle) - Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. [https://openstax.org/books/biology-2e/pages/10-introduction](https://openstax.org/books/biology-2e/pages/10-introduction) - Cooper, G. M. (2000). The eukaryotic cell cycle. In The cell: A molecular approach (2nd ed.). Sinauer Associates. [https://www.ncbi.nlm.nih.gov/books/NBK9876/](https://www.ncbi.nlm.nih.gov/books/NBK9876/) - Cooper, G. M. (2000). The events of M phase. In The cell: A molecular approach (2nd ed.). Sinauer Associates. [https://www.ncbi.nlm.nih.gov/books/NBK9958/](https://www.ncbi.nlm.nih.gov/books/NBK9958/) - Dou, Z., Prifti, D. K., Gui, P., Liu, X., Elowe, S., & Yao, X. (2019). Recent progress on the localization of the spindle assembly checkpoint machinery to kinetochores. Cells, 8(3), 278. [https://doi.org/10.3390/cells8030278](https://doi.org/10.3390/cells8030278)