Is Meiosis Considered Part of the Cell Cycle?

Meiosis shares so much molecular machinery with the standard mitotic cell cycle that many researchers refer to it as the “meiotic cell cycle,” a specialized variant rather than an entirely separate process. Textbooks usually present the cell cycle as the repeating loop of growth, DNA replication, and division that produces two identical daughter cells, and then treat meiosis in its own chapter. But at the level of the proteins actually running the show, the boundary is blurry. Meiosis uses the same cyclin-dependent kinase system, a similar DNA-replication phase, and overlapping checkpoint machinery, all re-tuned to produce four genetically unique haploid cells instead of two identical diploid ones.

Why the Question Is Harder Than It Looks

The confusion is understandable. If you ask whether meiosis is “part of” the cell cycle, the answer hinges on which cell cycle you mean. The mitotic cell cycle is the default loop most cells go through: they grow, copy their DNA, and divide. Germ cells break out of that loop. They undergo one round of DNA replication followed by two rounds of division, producing cells with half the original chromosome number. That two-division structure is fundamentally different from standard mitosis, and it only happens in cells destined to become eggs or sperm (or spores, in organisms like yeast and plants).

Yet the phrase “meiotic cell cycle” appears throughout the research literature. One widely cited description calls it a cell cycle “comprised of two consecutive M-phases” that is “crucial for the production of haploid germ cells.”¹ The framing matters: researchers treat meiosis not as something that exists outside cell-cycle biology, but as a modified version of it. The core regulatory engine is the same. What changes is how that engine is tuned, paused, and restarted.

The Shared Regulatory Engine

Both mitosis and meiosis are driven by cyclin-dependent kinases, the enzymes that act as master timekeepers of cell division. In both programs, the order and timing of events are controlled by these kinases and their cyclin partners, which rise and fall in concentration to push the cell through each phase.² This is a big part of why biologists view meiosis as a cell-cycle variant rather than something wholly distinct. The kinases are the same families, the cyclins are closely related, and many of the downstream targets overlap.

Where the regulation diverges is in the specific cyclins expressed, the timing of their activation, and the checkpoints layered on top. Meiosis has to accomplish things mitosis never does: pairing up homologous chromosomes, swapping DNA segments through recombination, and then holding sister chromatids together through a first division that separates homologs before finally pulling sisters apart in a second division. Each of those steps demands tweaks to the standard cell-cycle controls, but the underlying architecture remains recognizable.

A Different Kind of S Phase

Before meiosis begins, the cell copies its DNA in what is called the pre-meiotic S phase. This mirrors the S phase of the mitotic cell cycle, but it is not a carbon copy. Research in yeast has shown that the pre-meiotic S phase takes roughly twice as long as the mitotic one, with replication origins firing less efficiently and in a slightly different order.³ In mammals, similar differences have been documented: fewer replication origins fire during meiotic DNA replication, slowing the process and creating a distinctive replication pattern that influences where recombination later occurs.⁴

Why would the cell bother replicating its DNA more slowly before meiosis? One likely reason is that the slower pace gives the cell more time to set up the protein structures needed for recombination, which must be loaded onto chromosomes around the same time as replication. The pre-meiotic S phase is doing double duty: copying the genome and laying the groundwork for the chromosome gymnastics that follow. That dual role has no equivalent in the mitotic cell cycle, where S phase is strictly about getting the DNA copied.

The Critical Gap Between the Two Divisions

One of the most striking differences between the meiotic and mitotic programs is what happens between the first and second divisions. In a normal mitotic cycle, after the cell divides, there is a gap phase (G1) during which the cell can grow and eventually replicate its DNA again. In meiosis, after the first division separates homologous chromosomes, the cell proceeds almost immediately into the second division without replicating its DNA. This is the whole point: skipping a second round of replication is what reduces the chromosome number by half.

The cell actively enforces this skip. Experiments in fission yeast have shown that even when researchers artificially supply the proteins normally needed to initiate DNA replication during the interval between the two meiotic divisions, only a partial and inefficient round of replication occurs. Both the initiation and the elongation steps of DNA copying are inhibited.⁵ In starfish oocytes, the mechanism involves cyclin B assembling with its kinase partner right after the first division. When researchers blocked that assembly, premature DNA replication kicked in, showing that the kinase activity acts as a barrier to unwanted replication between the two divisions.⁶

This is a genuinely elegant piece of biology. The cell uses the same kinase system it relies on for normal cell-cycle progression to prevent a step (DNA replication) that it normally promotes. It is the cell-cycle machinery being repurposed in an almost paradoxical way, and it highlights how meiosis is less a separate program than a creative re-wiring of the existing one.

Pauses That Can Last Decades

If you are looking for the most dramatic way meiosis departs from the standard cell cycle, it is the pauses. In human females, oocytes enter meiosis during fetal development and then arrest in the first meiotic prophase. They stay there for years, sometimes decades, until ovulation. No mitotic cell cycle arrest comes close to this duration.

The arrest depends on keeping CDK1 activity low through a combination of inhibitory signals. Cyclic AMP produced within the oocyte activates protein kinase A, which in turn keeps a kinase called WEE2 active and a phosphatase called CDC25B locked in the cytoplasm where it cannot do its job. The net effect is that CDK1, the enzyme needed to push the cell into division, stays inactive.⁷ The regulatory landscape is complex, involving signals from surrounding follicle cells and hormonal cues from the pituitary gland that coordinate when arrest is maintained and when it is released.⁸

Male germ cells have their own timing quirks. During fetal development, male germ cells do not enter meiosis at all; instead, they are held in a G0/G1 arrest state, kept out of the meiotic program by specific factors. Female germ cells in the same embryo exit the mitotic cycle and enter meiosis. The divergence is coordinated by signals like retinoic acid and its antagonists, which determine whether a germ cell takes the meiotic or the arrest path.⁹ Once males reach puberty, spermatocytes proceed through meiosis relatively quickly and continuously, without the years-long pause that characterizes oogenesis.

Waiting for Fertilization at Metaphase II

In most mammals, oocytes face a second arrest point after completing the first meiotic division. They pause at metaphase of meiosis II and wait there until a sperm arrives. This pause is maintained by what researchers call Cytostatic Factor, an activity that keeps the enzyme complex MPF (essentially CDK1 bound to cyclin B) stable and active, preventing the cell from finishing division. The arrest is ultimately broken by the surge of calcium triggered by the fertilizing sperm, which activates the protein-destruction machinery that degrades cyclin B and lets the cell complete meiosis II.¹⁰

Nothing like this exists in the mitotic cell cycle. A dividing skin cell or liver cell does not pause mid-division and wait for an external signal to finish. The fact that meiosis incorporates these long, regulated pauses is one of the strongest arguments for treating it as a distinct program, even though the molecular players involved in maintaining and breaking the arrest are standard cell-cycle components.

A Looser Safety Net for Chromosome Errors

Both mitosis and meiosis use the spindle assembly checkpoint, a surveillance system that is supposed to delay cell division until all chromosomes are properly attached to the spindle fibers. In mitosis, this checkpoint is quite stringent: even a single misattached chromosome can hold up the process. In female meiosis, the checkpoint is noticeably weaker.

Research in mouse oocytes has shown that the spindle assembly checkpoint is normally silenced and division proceeds on schedule even when one or a few chromosomes are misaligned or improperly attached. The checkpoint simply cannot sustain arrest under those conditions.¹¹ This helps explain why chromosome segregation errors are much more common in eggs than in most other dividing cells, and why the risk of conditions caused by an extra or missing chromosome rises with maternal age.

Male meiosis appears to be somewhat more checkpoint-competent, but even here the picture is nuanced. Reducing the levels of certain checkpoint proteins that cause problems in mitotically dividing cells does not always lead to errors in spermatocytes. Only specific checkpoint components, when reduced, have been shown to compromise the fidelity of male meiosis.¹² The checkpoint machinery is shared between mitosis and meiosis, but meiosis dials it down, particularly in oocytes.

How Chromosomes Are Held and Released in Stages

Another way meiosis modifies standard cell-cycle behavior involves the glue that holds chromosomes together. In mitosis, the protein complexes called cohesins hold sister chromatids together until the cell is ready to divide, at which point they are all cleaved at once. In meiosis, cohesion must be removed in two stages: first from the chromosome arms during meiosis I (so homologs can separate), and then from the centromere regions during meiosis II (so sisters can separate). A protein called shugoshin protects centromeric cohesion during the first division, ensuring that sisters stay together until they are supposed to part.¹³

This stepwise loss of cohesion is a fundamentally meiotic invention. Standard mitotic division has no need for it, because sisters are simply held together until one moment of release. The machinery involved, cohesins and their regulators, is shared, but the logic of how and when it is deployed is unique to meiosis.

The Switch Back to Mitosis at Fertilization

Once meiosis finishes and an egg is fertilized, the newly formed embryo must transition back to the mitotic cell cycle. This switch is not trivial. The linkages between sister chromatids are remodeled to support equal (mitotic) rather than reductional (meiotic) divisions, the centrosome is reintroduced from the sperm side, and the cell shifts from relying heavily on pre-stored maternal messenger RNA to transcribing its own genes.¹⁴

Even the physical structure of the cell-division apparatus changes. The small, barrel-shaped meiotic spindle is replaced by a larger spindle with prominent astral microtubules at its poles. In sea squirt and frog eggs, this shift has been linked to a decrease in the activity of an enzyme called CK2 at fertilization, which in turn unleashes factors that reshape the spindle into its mitotic form.¹⁵ The transition underscores that meiosis and mitosis are not just conceptually related: the cell literally converts from one program to the other using overlapping molecular components.

Meiosis Evolved from the Mitotic Cell Cycle

The deep kinship between meiosis and mitosis is not coincidental. Evolutionary analysis indicates that the meiotic cell cycle evolved by modifying the existing mitotic machinery. The core components, the CDKs, cyclins, cohesins, and spindle-checkpoint proteins, were already present for mitosis. Evolution layered on new functions: homolog pairing, recombination, and the two-division logic. The result is that meiosis uses many of the core components of the mitotic cell cycle, adapted through evolutionary changes to perform specialized meiotic functions.¹⁶

This evolutionary perspective is probably the most satisfying way to resolve the question in the title. Meiosis is not a phase of the standard cell cycle the way S phase or mitosis is. But it is not an independent invention either. It is a derivative, a modified program that borrows heavily from its mitotic ancestor and then adds layers of new regulation to accomplish something the mitotic cycle cannot: halving the chromosome number while generating genetic diversity.

How Yeast Enter Meiosis

In multicellular organisms, the decision to enter meiosis is coordinated by hormones and signals from surrounding cells. In single-celled organisms like budding yeast, the trigger is environmental: nutritional starvation pushes diploid cells out of the mitotic cell cycle and into the meiotic program.¹⁷ In fission yeast, nitrogen starvation triggers a regulatory cascade in which a kinase that normally keeps cells in mitosis is inactivated, allowing meiotic entry.¹⁸

Yeast are useful here because they make the relationship between the mitotic and meiotic programs especially visible. A yeast cell cycling mitotically can, under the right conditions, switch into the meiotic program. The switch involves flipping regulatory states, not building an entirely new division apparatus from scratch. This reinforces the view that meiosis and mitosis are two modes of the same underlying system rather than two separate systems.

When Meiosis Runs in Reverse

The standard meiotic sequence, separate homologs first and then sisters, is so ubiquitous that it is easy to assume it is the only option. But some organisms with holocentric chromosomes (where spindle fibers attach along the entire length of the chromosome rather than at a single centromere) run the divisions in the opposite order: sisters separate first, and homologs separate second. This inverted meiosis has been documented across several species as an adaptation to the structural challenges posed by their chromosome architecture.¹⁹

Even more surprisingly, inverted meiosis can coexist with canonical meiosis inside the same cell. When centromere positions are experimentally repositioned in fission yeast, chromosomes with the new centromere position undergo inverted meiosis while other chromosomes in the same cell proceed in the standard order.²⁰ The finding reveals a flexibility in meiotic division that the textbook version does not capture. The two-division structure of meiosis is conserved, but the order in which chromosomes are parceled out can vary depending on their physical architecture, even within a single dividing cell.