In humans, the daughter cells produced at the end of meiosis each contain 23 chromosomes, exactly half the 46 found in the original cell. More generally, meiosis takes any cell with two complete sets of chromosomes and produces cells with just one set. But that simple answer hides a detail that trips up almost everyone who encounters meiosis for the first time: the chromosome count drops to 23 after the first division, not the second, and the two rounds of division accomplish fundamentally different things.
What Happens in the First Division
Before meiosis begins, the cell duplicates all of its DNA, so each of its 46 chromosomes is made up of two identical copies called sister chromatids, joined at a region called the centromere. In meiosis I, the cell does not pull those sister chromatids apart. Instead, it separates the paired homologous chromosomes, the matching set you inherited from each parent. One copy of chromosome 1 goes to one daughter cell; the other copy goes to the opposite cell. The same happens for every pair.
The result after meiosis I is two cells, each containing 23 chromosomes. Each of those chromosomes, however, still consists of two sister chromatids glued together at their centromeres. This is a critical detail: the chromosome number has already been halved, but the total amount of DNA per cell has not yet reached its final level. The sister chromatids within each chromosome are held together by a protein complex called cohesin. During meiosis I, the cohesin along the chromosome arms is dissolved so that homologs can separate, but cohesin at the centromere is deliberately protected so that the sister chromatids stay paired.1The Journal of Cell Biology. The Reduction of Chromosome Number in Meiosis Is Determined by Properties Built into the Chromosomes
What Happens in the Second Division
Meiosis II follows immediately, with no new round of DNA copying in between.2PubMed Central. Sister chromatid segregation in meiosis II: deprotection through phosphorylation In this round, the remaining centromere cohesin is released, and the sister chromatids are finally pulled apart, much like what happens during ordinary cell division. Each of the two cells from meiosis I divides into two more, yielding four daughter cells in total. Each one still has 23 chromosomes, but now each chromosome is a single chromatid rather than a joined pair. The DNA content per cell is now truly halved compared to where things started.
So if someone asks “how many chromosomes after meiosis I?” and “how many after meiosis II?”, the numerical answer is the same: 23 in humans. The difference is in what those chromosomes look like internally. After meiosis I, each is a double structure. After meiosis II, each is a single strand. This distinction matters because the chromosome count and the DNA content are not the same measurement, and confusing them is one of the most common stumbling blocks when learning about cell division.
The Chromosome-Versus-Chromatid Confusion
A chromosome is counted as one chromosome regardless of whether it is made of one chromatid or two joined chromatids. A cell that has just duplicated its DNA still has 46 chromosomes, not 92, even though there are 92 chromatid strands in it. This convention is the source of enormous confusion, because it means the chromosome number drops at meiosis I (from 46 to 23) while the DNA amount drops at both divisions.
Think of it this way: before meiosis, you have 46 pairs of shoes tied together at the laces. After meiosis I, you have 23 tied-together pairs in each cell. After meiosis II, you have 23 individual shoes in each cell. The “shoe count” (chromosome number) went from 46 to 23 at the first step. The “how much leather is in the room” question (DNA content) changed at both steps. If you keep this mental picture, the numbers stop being contradictory.
The Molecular Glue That Controls the Process
The entire reduction in chromosome number depends on a remarkably precise molecular trick: cohesin must be stripped away in two stages, not all at once. During meiosis I, the enzyme separase cuts the cohesin along chromosome arms, letting homologs drift apart. But at the centromere, cohesin is shielded by a protein called shugoshin, a name borrowed from the Japanese for “guardian spirit.”3PubMed Central. Shugoshin protects cohesin complexes at centromeres Shugoshin recruits another protein (PP2A) that chemically reverses the signal telling separase to cut, keeping the centromeric cohesin intact while everything else around it is being dismantled.4Nature Cell Biology. Unified mode of centromeric protection by shugoshin in mammalian oocytes and somatic cells
This mechanism was first worked out in yeast, where the protein is called Sgo1.5Nature. The conserved kinetochore protein shugoshin protects centromeric cohesion during meiosis Later research confirmed that mammals use a closely related version, Sgo2, for the same purpose in egg cells. The conservation of this system across such distantly related organisms underscores how fundamental it is: if shugoshin fails, sister chromatids separate too early, and the daughter cells end up with the wrong number of chromosomes.
When Chromosome Counts Go Wrong
A cell that finishes meiosis with 24 or 22 chromosomes instead of 23 is called aneuploid. If that cell happens to be a sperm or egg that goes on to form an embryo, the resulting baby will have 47 or 45 chromosomes instead of the expected 46. About one in every 300 live births involves an aneuploid infant, most commonly with an extra or missing sex chromosome, or with an extra copy of chromosome 21, which causes Down syndrome.6Nature Reviews Genetics. To err (meiotically) is human: the genesis of human aneuploidy
The errors can happen at either meiotic division, but they are not equally likely for every chromosome. For trisomy 13 (Patau syndrome), over 91% of cases originate from errors in the mother’s egg cells, and a surprisingly large share of those, roughly 37%, trace to problems during meiosis II rather than meiosis I.7PubMed. The origin of trisomy 13 Both classical non-disjunction, where a whole chromosome goes to the wrong cell, and premature separation of a single chromatid contribute to aneuploidy. In a study of nearly 1,400 unfertilized human egg cells, about 11% were aneuploid, and abnormalities from single chromatids separating at the wrong time outnumbered whole-chromosome errors.8Oxford Academic (Human Reproduction). Mechanisms of non-disjunction in human female meiosis: the co-existence of two modes of malsegregation evidenced by the karyotyping of 1397 in-vitro unfertilized oocytes
Why Maternal Age Matters
Women are born with their egg cells already partway through meiosis I, arrested at a stage before the first division completes. Those cells can sit in that suspended state for decades until ovulation. During all that time, the cohesin proteins holding chromosomes together gradually degrade. By the time a woman is in her late thirties or forties, the cohesin has been weakening for decades, and the chromosomes are more likely to separate incorrectly when meiosis finally resumes.
Multiple lines of evidence now point to this cohesin deterioration as a leading cause of the well-known rise in chromosomal abnormalities with maternal age.9PubMed Central. Age-Related Loss of Cohesion: Causes and Effects 10Biology of Reproduction. Meiotic Origins of Maternal Age-Related Aneuploidy The shugoshin protein discussed earlier plays a direct role here as well. Research on human egg cells has shown that when shugoshin (SGO2) fails to localize properly at the centromere, cohesin integrity weakens and chromosomes are more likely to mis-segregate, a problem that worsens as a woman ages.11Current Biology. Human SGO2 protects cohesin and prevents chromosome mis-segregation in oocytes
Sperm cells, by contrast, are produced continuously from puberty onward, so their cohesin never sits around for decades. While sperm can accumulate other types of DNA damage with age, the specific cohesin-decay problem that drives age-related aneuploidy is far more prominent in eggs.
Meiosis in Sperm Versus Egg Production
The chromosome arithmetic is the same for both sexes: 46 goes in, 23 comes out per daughter cell. But the physical division of the cell happens very differently. In sperm production, meiosis generates four roughly equal-sized cells, each of which matures into a functional sperm. In egg production, the cell divides its chromosomes evenly but parcels almost all of its cytoplasm into just one of the daughter cells. The smaller cells, called polar bodies, get the correct number of chromosomes but almost none of the cellular machinery needed to support an embryo. They typically degenerate. The net result is one large, nutrient-packed egg cell and two or three tiny polar bodies, all with 23 chromosomes.
This asymmetric division is not a quirk of human biology; it is nearly universal among animals that produce eggs. The strategy preserves the stockpile of proteins, mitochondria, and signaling molecules that a fertilized egg will need during its earliest cell divisions, long before its own genes kick in.
Safety Mechanisms That Catch Errors
Cells do not blindly race through meiosis. A surveillance system called the spindle assembly checkpoint monitors whether every chromosome is properly attached to the spindle fibers that pull them apart. If a chromosome is not correctly connected, the checkpoint stalls the process to give the cell time to fix the attachment. A protein called Zwint-1 is one key component of this system in egg cells. When researchers knocked down Zwint-1 in mouse oocytes, the first meiotic division sped up abnormally, chromosomes failed to line up correctly, and aneuploidy rates soared.12PubMed Central. Zwint-1 is required for spindle assembly checkpoint function and kinetochore-microtubule attachment during oocyte meiosis
Despite this checkpoint, errors still slip through in human egg cells at a rate considerably higher than in most other species. The checkpoint in oocytes appears to be somewhat more permissive than in other cell types, which may explain why human reproduction involves such a high proportion of chromosomally abnormal embryos, many of which are lost before a pregnancy is even detected.
What About Organisms With Different Chromosome Numbers
The halving principle works the same regardless of species: whatever the starting number, meiosis cuts it in half. A dog cell starts with 78 chromosomes and produces gametes with 39. A fruit fly starts with 8 and ends with 4. A fern can start with several hundred and still divide them faithfully.
Things get more complicated in polyploid organisms, which carry more than two complete sets of chromosomes. Many crop plants, including wheat, strawberries, and potatoes, are polyploid. Their cells must sort through three, four, or even six copies of each chromosome rather than just two. In newly formed polyploids, related chromosomes can form tangled multi-chromosome structures called multivalents instead of tidy pairs, and these are more error-prone during segregation. Over evolutionary time, polyploid species develop mechanisms to suppress multivalent formation and force chromosomes into orderly pairs, enabling a clean halving of the chromosome number.13PubMed Central. Meiosis in Polyploids and Implications for Genetic Mapping: A Review
Inverted Meiosis Flips the Script
In the standard textbook version of meiosis, homologous chromosomes separate first, then sister chromatids separate second. But some organisms do it backwards. In plants with holocentric chromosomes, where the spindle-attachment sites are spread along the entire length of the chromosome rather than concentrated at a single centromere, sister chromatids can separate during meiosis I while homologous non-sister chromatids stay linked until meiosis II.14Nature Communications. Chiasmatic and achiasmatic inverted meiosis of plants with holocentric chromosomes
The plant Luzula elegans provides a vivid example: its sister chromatids behave as two independent units during the first division, separating to opposite poles, while homologous non-sister chromatids remain connected by threads of repetitive DNA until they finally separate in the second division.15PubMed Central. Alternative meiotic chromatid segregation in the holocentric plant Luzula elegans The end result is still four haploid cells with the correct reduced chromosome number, just achieved by reversing the order of events. This inverted sequence demonstrates that halving the chromosome count is the essential outcome of meiosis; the particular route to get there can vary.
Parthenogenesis and Meiotic Workarounds
Some animals reproduce without fertilization, a process called parthenogenesis. You might expect these species to skip meiosis entirely, but most parthenogenetic animals actually retain it. They go through meiosis and produce haploid cells, then restore the full chromosome number through various tricks: fusing two products of the same meiosis back together, or doubling the chromosome set after meiosis is complete.16PubMed Central. The evolution of meiotic sex and its alternatives This form of reproduction, called automixis, allows the organism to benefit from some aspects of meiotic recombination while still producing offspring without a mate.
In the parasitic wasp Venturia canescens, for instance, the egg undergoes a normal reduction division and then restores chromosome number through a restitution step, producing diploid offspring from unfertilized eggs.17PubMed. Automictic parthenogenesis in the parasitoid Venturia canescens (Hymenoptera: Ichneumonidae) revisited The daughter cells technically went through a halving event, but the system then stitched the number back together. These workarounds highlight that meiosis is a flexible toolkit, not a rigid assembly line.
Environmental Chemicals and Chromosome Segregation
The precision of meiosis can be disrupted by chemical exposure. Bisphenol A (BPA), a compound found in many plastics and food-container linings, has been shown in laboratory studies to interfere with the spindle apparatus that pulls chromosomes apart during egg-cell meiosis. In mouse oocytes treated with BPA, the spindle fibers became elongated and disorganized, and the structures that anchor them were degraded. At moderate doses, cells could still complete meiosis I after BPA was removed, but at reduced rates compared to untreated cells, and with a higher risk of chromosome misalignment.18Molecular Human Reproduction. Bisphenol-A induces cell cycle delay and alters centrosome and spindle microtubular organization in oocytes during meiosis
BPA is far from the only compound under scrutiny. Various pesticides, heavy metals, and industrial chemicals have been investigated for their effects on meiotic fidelity, though the human relevance of many laboratory findings remains debated. What the research makes clear is that the molecular machinery controlling chromosome segregation in meiosis is sensitive to external disruption, and that even temporary interference can increase the chance of daughter cells ending up with the wrong number of chromosomes.
Why Halving Matters Beyond Reproduction
The reduction from diploid to haploid is not just a numerical formality needed so that fertilization restores the full count. It has consequences for how natural selection acts on a population. In the haploid state, every gene is present in a single copy, meaning there is no second copy to mask a harmful version. Deleterious mutations that could hide behind a functional partner in a diploid cell are fully exposed in haploid gametes and in any haploid life stage. This exposure makes selection against harmful mutations more efficient, effectively purging genetic damage that would otherwise accumulate silently.19Heredity. A combinational theory for maintenance of sex The cycle of doubling and halving chromosome numbers across generations is one reason sexual reproduction persists despite its obvious costs, like needing to find a mate in the first place.