Mitosis produces cells with the same chromosome count as the parent cell. In humans and most animals you hear about in biology class, that parent cell is diploid, so the two daughter cells are also diploid. But the deeper answer is that mitosis preserves whatever ploidy the cell already has. A haploid cell that divides by mitosis yields two haploid daughters; a polyploid cell yields two polyploid daughters. The question most people are really asking is how mitosis compares with meiosis, and the distinction is simpler than it sounds once you see how each process handles chromosomes differently.
How Mitosis Keeps the Chromosome Count the Same
Before a cell divides by mitosis, it copies every chromosome. Each chromosome and its copy stay physically connected, forming an X-shaped structure held together by a protein ring called cohesin. During the division itself, that cohesin is cleaved so the two copies can be pulled to opposite ends of the cell. The cell then pinches in half, and each new cell ends up with one complete set of the original chromosomes. The entire goal of the process is to partition those duplicated chromosomes into daughter cells accurately, so that no genetic information is gained or lost.
When the cohesin cleavage step is disrupted, chromosomes can lag behind and end up in the wrong daughter cell. Research in fission yeast has shown that interfering with the molecular machinery responsible for cohesin cleavage causes chromosomes and their attachment points to lag at anaphase, the stage where the two halves of each chromosome are supposed to separate cleanly.1PubMed Central. Fission yeast neddylation ligase Dcn1 facilitates cohesin cleavage and chromosome segregation at anaphase That kind of error is exactly what the cell works hard to prevent, and it highlights why the mechanics of mitosis are built around chromosome fidelity rather than chromosome reduction.
How Meiosis Differs Without Getting into Ratios
The confusion between mitosis and meiosis is the reason most people search this question in the first place. Meiosis is the division process reserved for making sex cells, and it cuts the chromosome number in half. A diploid human cell with 46 chromosomes goes through meiosis to produce sperm or egg cells with 23 chromosomes each. When a sperm and egg fuse at fertilization, the count returns to 46.
Mitosis does not do any of that halving. It is the workhorse division behind growth, tissue repair, and the replacement of worn-out cells. Every time your skin heals a cut or your bone marrow churns out new blood cell precursors, mitosis is doing the work and faithfully copying the full chromosome set each time. The two processes share some of the same molecular hardware, but their outcomes are fundamentally different: meiosis changes ploidy, mitosis preserves it.
Mitosis in Haploid Organisms
If mitosis always preserves ploidy, then a haploid organism that grows by mitosis will produce haploid daughter cells. This is not hypothetical. Male jewel wasps (Nasonia vitripennis) develop from unfertilized eggs and are haploid throughout their bodies. Their cells divide by mitosis the same way diploid cells do, except every daughter cell has just one set of chromosomes instead of two.
What gets interesting is how these haploid males make sperm. Normally, sperm production involves meiosis, which halves the chromosome count. But a haploid male cannot halve its already-single set. Research on jewel wasps found that their germ cells express several genes whose functions are normally restricted to meiosis, including well-known meiotic genes, at levels comparable to those seen in the female ovary.2Nature Publishing Group. Spermatogenesis in haploid males of the jewel wasp Nasonia vitripennis The males appear to go through something resembling an abortive meiosis, entering the process but not completing a true chromosome reduction. The end result is haploid sperm produced from a haploid body, with mitosis having handled all the growth leading up to that point.
Many fungi, algae, and some protists also spend most of their life cycle as haploid organisms, dividing by mitosis to grow and only becoming briefly diploid after two cells fuse. In mosses and liverworts, the green leafy plant you see is actually the haploid generation, called the gametophyte. It grows entirely through haploid mitosis. The diploid stage (the sporophyte) develops while still attached to the haploid mother plant.3PubMed Central. Sexual conflict and the alternation of haploid and diploid generations So the answer to whether mitosis makes diploid or haploid cells depends on the organism. Both are perfectly normal outcomes of mitosis in different biological contexts.
When Mitosis Produces Polyploid Cells on Purpose
Some cells in your body are not diploid at all, and they got that way through a modified version of mitosis. Megakaryocytes, the giant cells in bone marrow that produce blood platelets, routinely become polyploid by entering mitosis but never finishing it. They duplicate their chromosomes, start to divide, but abort the process in late anaphase and skip the final step where the cell physically splits in two. The result is one cell with double the DNA content. When this cycle repeats several times, a single megakaryocyte can end up with 16, 32, or even 64 copies of each chromosome.4PubMed. Megakaryocytes express functional Aurora-B kinase in endomitosis
This process, called endomitosis, involves a successful entry into mitosis but an exit without complete cytokinesis (the splitting step).5PubMed Central. Endomitosis: a new cell fate in the cell cycle leading to polyploidy in megakaryocytes and hepatocytes Hepatocytes, the main working cells of the liver, do the same thing. A healthy adult liver contains a substantial fraction of polyploid cells, and the polyploidy appears to serve a functional purpose. Polyploid cells can also act as a reservoir for generating new diploid cells during regeneration, which may help tissues bounce back after injury.6Europe PMC. Polyploidy in Tissue Repair and Regeneration
So even in your own body, not every mitotic division produces a neat pair of diploid daughters. Some divisions are deliberately incomplete, and the resulting polyploid cells are a normal, healthy part of tissue function.
The Checkpoint That Keeps Mitosis Honest
Given how important it is for mitosis to distribute chromosomes accurately, cells have a built-in quality-control system called the spindle assembly checkpoint. This checkpoint acts during both mitosis and meiosis to delay cell division until every chromosome is correctly attached to the fibers that will pull them apart.7Current Biology. The Spindle Assembly Checkpoint In practical terms, the checkpoint prevents the cell from progressing past the metaphase stage, where chromosomes line up at the cell’s equator, until each one has a proper connection to opposite poles of the dividing cell.8eLife. Spindle assembly checkpoint-dependent mitotic delay is required for cell division in absence of centrosomes
Think of it as a gate that will not open until the cell can confirm that every chromosome will go to the right place. If a chromosome is dangling unattached or connected to the wrong pole, the checkpoint holds the process in pause. Only when the attachment is corrected does division proceed. This is one reason mitosis is so reliable in healthy tissue. Billions of cell divisions happen in your body every day, and the vast majority produce perfectly accurate copies.
What Happens When Mitosis Gets the Count Wrong
The checkpoint is good but not perfect. Occasionally, a chromosome ends up in the wrong daughter cell, producing one cell with an extra chromosome and another missing one. This is called aneuploidy, and when it happens during mitotic divisions in an embryo or growing tissue, it can create a patchwork pattern known as mosaicism, where some cells in the body have the normal chromosome number and others do not.
A study using genome-wide analysis to investigate patients with chromosome abnormalities found that 16 cases of mosaic aneuploidy originated from mitotic errors rather than meiotic ones. These mitotic-origin cases included several rare trisomies and all of the monosomies studied, consistent with the idea that selective pressures influence which aneuploid cells survive in the body.9Human Molecular Genetics. Mechanisms of mosaicism, chimerism and uniparental disomy identified by single nucleotide polymorphism array analysis In other words, some types of chromosome imbalance that would be lethal if present in every cell can persist in a mosaic pattern because only a fraction of cells carry the error.
Mosaic conditions can range from clinically silent to medically significant depending on how many cells are affected, which tissues they are in, and which chromosome is involved. Some people carry mosaic aneuploidy their entire lives without knowing it. Others develop symptoms that may be milder versions of what you would expect if the full trisomy were present in every cell. The point is that mitotic errors can change the ploidy of individual cells even though the process is supposed to preserve it.
Polyploidy, Mitosis, and Cancer
In cancer biology, the relationship between mitosis and ploidy becomes especially tangled. Tumor cells frequently have abnormal chromosome numbers, and up to about 80% of human cancers, particularly solid tumors, contain aneuploid cells.10PubMed Central. Chromosomal instability, tolerance of mitotic errors and multidrug resistance are promoted by tetraploidization in human cells Some of this aneuploidy arises one chromosome at a time through individual missegregation events. But a substantial portion comes from a more dramatic route: whole-genome doubling, where a diploid cell becomes tetraploid (four copies of everything) before subsequent messy divisions scatter chromosomes unevenly.
When tetraploid cells attempt mitosis, the extra chromosomes create logistical problems. There are more attachment sites that need to connect correctly, more opportunities for errors, and a higher tolerance for mistakes because the cell has enough spare copies to survive the loss of a few. Research has shown that chromosomal instability is a frequent, though not inevitable, outcome of whole-genome doubling.10PubMed Central. Chromosomal instability, tolerance of mitotic errors and multidrug resistance are promoted by tetraploidization in human cells The resulting chromosomal variation can give tumor cells a wider range of genetic configurations to draw from, accelerating the evolution of drug resistance and other dangerous traits.
In a Drosophila tumor model, researchers found that both polyploid mitosis and a process called depolyploidization, where polyploid cells attempt to return to a lower ploidy, are error-prone steps that generate chromosomal copy-number variation and widespread aneuploidy.11PubMed Central. Polyploid mitosis and depolyploidization promote chromosomal instability and tumor progression in a Notch-induced tumor model Tumor cells in this model also underwent additional rounds of DNA replication without division, pushing their ploidy even higher. In 7-day-old tumors, roughly 40% of cells had a DNA content exceeding the maximum initially present, indicating that these cells had stacked extra rounds of replication on top of an already abnormal chromosome count.12Developmental Cell. Polyploid mitosis, endoreplication, and ploidy-reduction drive tumor progression and enhance tumor cell heterogeneity
Cancer, then, represents a situation where the normal rules of mitosis break down. The process still copies and distributes chromosomes, but it does so sloppily, and the daughter cells end up with wildly variable chromosome counts. The cells are still dividing by mitosis in a mechanical sense, but the outcome no longer fits the clean narrative of “same ploidy in, same ploidy out.”
Why the Textbook Answer Is Incomplete
Most biology courses teach that mitosis produces two identical diploid cells, and for human somatic cells under normal conditions, that is correct. But framing mitosis as inherently diploid misses a lot. Haploid organisms grow by mitosis and stay haploid. Polyploid crop plants, which include wheat, strawberries, and many others bred for agriculture, divide their massive chromosome sets faithfully through mitosis. Megakaryocytes and hepatocytes in your own body deliberately use modified mitosis to become polyploid.
The common thread is not diploidy. It is fidelity to whatever chromosome count the cell started with. Mitosis is a copying process, and like a photocopier, it does not care what is on the page. If the original is haploid, the copy is haploid. If the original is hexaploid, the copy is hexaploid. The answer to the title question, then, is that mitosis produces whatever the parent cell already was, and calling it “diploid” only works as a shortcut for talking about the most familiar case in human biology.
Practical Contexts Where This Distinction Matters
Understanding that mitosis preserves ploidy rather than enforcing diploidy has real implications in several fields. In agriculture, plant breeders who work with polyploid species need mitosis to faithfully replicate complex genomes across billions of cell divisions as a crop grows. Errors during mitosis in a polyploid plant can produce sectors of tissue with different chromosome counts, which sometimes shows up as odd-colored patches on fruit or uneven growth.
In medicine, the distinction matters for understanding genetic mosaicism. If you are told that a prenatal test found a chromosomal abnormality in some cells but not others, the explanation almost always traces back to a mitotic error in the early embryo rather than a problem inherited from the parents’ meiosis. Knowing that the error was mitotic can change the expected severity, because mosaic conditions tend to be less uniform and sometimes less severe than the same aneuploidy present in every cell.
In cancer treatment, the recognition that tumor cells cycle between polyploid states through error-prone mitosis has influenced how researchers think about drug resistance. A tumor that undergoes whole-genome doubling and then divides chaotically generates enormous genetic diversity in its cell population. That diversity gives the tumor more chances to stumble onto a configuration that resists whatever chemotherapy is being used. Some researchers are now exploring whether targeting the specific mitotic vulnerabilities of polyploid cells could be a way to slow that process down, though that work is still early.
In stem-cell and regenerative biology, the fact that polyploid cells can serve as a source for regenerating diploid cells adds another twist. Liver regeneration after injury appears to involve polyploid hepatocytes that can, under certain circumstances, undergo divisions that reduce their ploidy back toward diploid levels. Mitosis, in this context, is not just a maintenance process but a flexible tool the body uses to manage tissue health across a range of ploidy states.