Several organic compounds work together to drive mitosis, but the ones most central to the process are proteins and DNA. Cyclins, tubulin, actin, myosin, and histones each play distinct roles in ensuring a cell divides correctly, from the initial trigger to enter mitosis through the physical splitting of one cell into two. Phospholipids also contribute by forming and reforming the nuclear envelope. No single molecule runs the show alone.
Cyclins: The Trigger Proteins
Mitosis doesn’t start on its own. It requires a buildup of a protein called cyclin B1, which pairs with an enzyme called CDK1 to form a complex known as the Mitosis Promoting Factor, or MPF. During the late G2 phase (the period just before mitosis begins), cyclin B1 gradually accumulates in the cytoplasm. Once enough cyclin B1 has built up and the complex is activated through a series of chemical switches, the cell commits to dividing.
Cyclins are organic compounds, specifically proteins, and they’re among the most important regulators of cell division. Different cyclins handle different phases. Cyclin A and cyclin E can also partner with CDK1, but cyclin B1 is the primary driver of entry into mitosis. When the cell finishes dividing, cyclins are rapidly destroyed by a protein-recycling machine called the anaphase-promoting complex (APC/C), which ensures the cell doesn’t try to divide again immediately.
DNA and Histone Proteins in Chromosome Packaging
DNA is the organic molecule that must be copied and divided equally between two daughter cells. But DNA on its own is far too long and tangled to sort neatly. During prophase, the first stage of mitosis, chromatin (the loose form of DNA wrapped around histone proteins) condenses into tightly packed, rod-shaped chromosomes. This compaction is essential for accurate segregation.
The compaction process relies on protein complexes called condensins, which bind to chromosomes starting in early prophase and use energy from ATP to coil the DNA into progressively tighter structures. Condensins interact with histone proteins, specifically the tail regions of histones H2A and H4, to anchor themselves along the chromosome. Another protein complex, cohesin, holds the two identical copies of each chromosome (sister chromatids) together like a ring until the cell is ready to pull them apart at anaphase. The cleavage of cohesin’s ring structure is what finally allows the sister chromatids to separate.
Tubulin: Building the Mitotic Spindle
Once chromosomes are condensed and lined up, the cell needs a physical structure to pull them apart. That structure is the mitotic spindle, built from microtubules. Microtubules are hollow tubes approximately 25 nanometers in diameter, assembled from repeating units of two proteins: alpha-tubulin and beta-tubulin. These two proteins lock together into pairs called heterodimers, which then stack end to end into long filaments. Thirteen of these filaments wrap around each other to form a single hollow microtubule.
Microtubules have a built-in polarity. Beta-tubulin faces one end (the plus end, which grows quickly) and alpha-tubulin faces the other (the minus end, anchored at the spindle pole). This polarity is what allows the spindle to grab chromosomes at their centers and pull sister chromatids toward opposite ends of the cell. Without tubulin, there would be no mechanism to physically separate genetic material.
Motor Proteins That Move Chromosomes
Tubulin builds the tracks, but motor proteins are the vehicles. Two families of motor proteins, kinesin and dynein, travel along microtubules to power nearly every mechanical aspect of mitosis. They establish the spindle’s two-poled shape, attach chromosomes to spindle fibers, align chromosomes at the cell’s center, and then drive the separation of sister chromatids toward opposite poles.
Kinesins generally move toward the plus end of microtubules, while dyneins move toward the minus end. Together, they create the pushing and pulling forces that position chromosomes precisely. Dyneins are present in all cell types and contribute to both mitosis and meiosis, transporting not just chromosomes but also organelles and other cellular cargo along the microtubule network.
Actin and Myosin in Cytokinesis
After the chromosomes have been separated, the cell still needs to physically split in two. In animal and fungal cells, this final step, called cytokinesis, depends on two more organic compounds: actin and myosin. These are the same proteins responsible for muscle contraction, repurposed here to form a contractile ring around the cell’s equator.
Actin filaments grow in initially random directions but are organized into a tight bundle by myosin motor proteins. Myosin clusters grab actin filaments and rotate them into alignment, then pull on them to generate tension. This self-organizing process draws the ring into a narrow ribbon roughly 0.1 micrometers wide, which progressively tightens like a drawstring until it pinches the cell membrane inward and divides the cell into two daughter cells. A crosslinking protein called alpha-actinin helps bundle neighboring actin filaments together, adding structural stability to the ring as it constricts.
Phospholipids and the Nuclear Envelope
The nuclear envelope, which surrounds and protects DNA during normal cell life, is made of phospholipids, a class of organic fat molecules. During mitosis, the nuclear envelope breaks down so that spindle fibers can access the chromosomes. Studies in amoeba cells show that when the envelope disassembles, its phospholipid components disperse throughout the cytoplasm rather than being destroyed. No new phospholipid production occurs during mitosis.
At telophase, the final stage of mitosis, those same phospholipids return to reassemble around each set of daughter chromosomes, forming two new nuclear envelopes. The mitotic cytoplasm contains a factor that stimulates this exchange of phospholipids between the envelope and the surrounding cell, essentially recycling the membrane material rather than building it from scratch.
How These Compounds Work in Sequence
The organic compounds involved in mitosis don’t act independently. They follow a tightly choreographed sequence. Cyclins accumulate and activate CDK1, which triggers the cell to enter mitosis. Condensins and histones compact DNA into chromosomes while the nuclear envelope’s phospholipids scatter into the cytoplasm. Tubulin assembles into spindle fibers, and kinesin and dynein motor proteins use those fibers to align and then separate chromosomes. Cohesin is cleaved to release sister chromatids. The APC/C destroys cyclins to signal the end of mitosis. Phospholipids reassemble into new nuclear envelopes. Finally, actin and myosin constrict to split the cell in two.
Each of these molecules is an organic compound, meaning it contains carbon and is produced by living cells. Proteins (cyclins, tubulin, histones, actin, myosin, kinesin, dynein, condensin, cohesin) make up the majority. DNA is a nucleic acid. Phospholipids are lipids. Together, they represent three of the four major classes of biological organic molecules, all coordinating to ensure that one cell becomes two with exactly the right genetic material in each.