When a fungus undergoes mitosis, its chromosomes replicate and separate into two identical sets, just like in animal or plant cells. The critical difference is that most fungi do this inside an intact nuclear envelope, a process called “closed mitosis.” In animals and plants, the nuclear membrane breaks apart before chromosomes separate. In fungi, it stays whole throughout division.
The Nuclear Envelope Stays Intact
This is the single most important distinction between fungal mitosis and the version you typically learn about in textbook diagrams. In animal and plant cells, the membrane surrounding the nucleus completely disassembles early in mitosis so that the chromosome-pulling machinery can access the DNA. Fungi skip that step entirely. The nuclear envelope remains continuous while everything else, chromosome condensation, separation, and redistribution, happens inside it.
The nucleus essentially elongates and then pinches into two daughter nuclei, each with a complete copy of the genome, all while wrapped in its original membrane. This means the nuclear contents never mix freely with the cytoplasm during division.
Spindle Pole Bodies Replace Centrosomes
Animal cells use structures called centrosomes to organize the network of protein fibers (called microtubules) that pull chromosomes apart. Fungi don’t have centrosomes. Instead, they use spindle pole bodies, or SPBs, which serve the same purpose but look completely different.
SPBs are large, layered protein structures embedded directly in the nuclear envelope rather than floating in the cytoplasm. Because the nuclear membrane never breaks down, the SPBs sit right in the envelope and organize microtubules on both sides of it simultaneously. On the nuclear side, microtubules attach to chromosomes and pull them apart. On the cytoplasmic side, they help position the nucleus within the cell. Unlike centrosomes, SPBs contain no centrioles, the small cylindrical components that define centrosomes in animal cells.
Like centrosomes, each SPB duplicates exactly once per cell cycle, producing a new “daughter” SPB next to the original. The two SPBs then move to opposite sides of the nucleus and anchor the spindle that will separate the chromosomes. SPBs also act as signaling platforms that help coordinate the final stages of division, triggering the cell to physically split once the chromosomes are safely sorted.
The Same Phases, Different Packaging
Fungal mitosis still proceeds through the familiar stages: DNA condenses into visible chromosomes, the spindle forms between the two SPBs, chromosomes line up, sister copies are pulled to opposite poles, and the nucleus divides. The molecular machinery driving each transition is highly conserved. Research on baker’s yeast (Saccharomyces cerevisiae) identified a key gene originally called “START,” now known as CDK1, that triggers the shift from the growth phase into active mitosis. This same type of protein, a cyclin-dependent kinase paired with its partner cyclin, controls entry into mitosis across nearly all eukaryotic life.
The main visual difference is that in fungi, the entire process looks like a nucleus stretching and splitting rather than chromosomes floating freely through the cell before being repackaged into new nuclei.
Mitosis Powers Asexual Reproduction
Mitosis is the engine behind most of the reproduction fungi do day to day. Asexual spores, the primary way fungi spread through an environment, are produced through mitotic division followed by separation of the cytoplasm. In molds, for example, a specialized structure buds off from a hypha (a thread-like filament), mitosis occurs as the bud develops, and both the new spore and the parent cell end up with a single nucleus containing identical DNA. In other fungi, barrel-shaped spore cells swell and undergo additional rounds of mitosis to generate extra nuclei before maturing.
This is distinct from sexual reproduction, which involves meiosis and the fusion of genetically different nuclei. But for rapid colonization of a food source or spread across a surface, mitotic spore production is far more common.
Nuclear Movement in Filamentous Fungi
In filamentous fungi, the ones that grow as branching networks of hyphae, mitosis comes with an extra logistical challenge. Hyphae grow at their tips, and nuclei need to keep pace with that growth. After mitosis produces new nuclei, motor proteins physically transport them along microtubule tracks toward the advancing tip. Mutations in the genes controlling this transport cause nuclei to cluster in one spot, leaving the growing tip empty and stunting the organism. So mitosis in filamentous fungi isn’t just about copying DNA; it’s tightly linked to a transport system that distributes nuclei throughout an expanding network.
Keeping Two Nuclei in Sync
Some fungi, particularly mushroom-forming species in the Basidiomycota group, spend much of their life with two genetically distinct nuclei in every cell, a state called dikaryotic. Maintaining this arrangement during mitosis requires remarkable coordination. Both nuclei must divide at the same time, and the resulting four nuclei must be sorted so that each daughter cell gets one of each type.
These fungi accomplish this with structures called clamp connections: small bridge-like projections that form near the site where a new cell wall (septum) will be built. One nucleus enters the clamp and divides there, while the other divides in the main hypha. The clamp then fuses back with the adjacent cell, delivering the right nucleus to the right compartment. This synchronized mitosis in two separate spaces ensures the dikaryotic state is faithfully maintained through every round of growth.
Why Fungi Evolved Closed Mitosis
One hypothesis ties the difference to genome size. DNA density inside the nucleus is roughly constant across eukaryotes, so organisms with larger genomes need larger nuclei. The chromosome-pulling spindle also scales with genome size, but it grows faster than the nucleus does. In organisms with very large genomes, like plants and animals, the spindle would eventually be too long to fit inside the nucleus, making it necessary to remove the nuclear envelope. Fungi, with their comparatively compact genomes, never hit that limit, so there was no pressure to evolve open mitosis.
A more surprising hypothesis involves transposable elements, segments of DNA that copy and paste themselves throughout a genome. Both plants and animals have genomes bloated with these elements, and both use open mitosis. A closed nuclear envelope may act as a physical barrier that limits transposable elements from shuttling between the cytoplasm and the genome. If a transposable element evolved the ability to punch holes in the envelope, it would gain better access to the DNA, potentially driving the transition to open mitosis as those elements became permanent fixtures of the genome. Fungi, with fewer transposable elements and smaller genomes, may have simply never faced that evolutionary pressure.