Mitosis is the process by which a single cell divides into two identical daughter cells, driving growth and tissue repair. This cellular division relies on organic compounds, which are complex structures built around a carbon backbone. These molecules, including proteins and nucleic acids, actively direct and execute every step of mitosis with precision.
The Regulatory Proteins: Cyclins and CDKs
The timing of the cell cycle is controlled by regulatory proteins to ensure each phase completes correctly. This system is built around proteins called cyclins and their partner enzymes, Cyclin-Dependent Kinases (CDKs). CDKs are the engines of cell cycle progression but remain inactive on their own. They require binding to a specific cyclin protein to switch on their enzymatic function.
Different types of cyclins are produced at various stages of the cell cycle, and each type binds to a specific CDK partner. For instance, the rise of mitotic cyclins like Cyclin B during the G2 phase leads to the activation of its partner, CDK1. This Cyclin B-CDK1 complex then phosphorylates other proteins, triggering events of mitosis like chromosome condensation and the breakdown of the nuclear envelope.
The concentration of each cyclin rises and falls in a predictable wave, ensuring CDK activity is present only when needed. Once a stage is complete, the corresponding cyclin is tagged for destruction. This degradation inactivates the CDK, preventing it from pushing the cell forward prematurely and ensuring mitosis progresses in an orderly fashion.
The Genetic Material: DNA and Histones
Mitosis must accurately partition the cell’s genetic blueprint, Deoxyribonucleic acid (DNA), into two new cells. Before mitosis begins, the cell’s DNA is replicated, resulting in two identical copies of every chromosome. These copies, known as sister chromatids, are initially joined together.
A single human cell contains about two meters of DNA, which is incredibly long. To manage this material without tangles, the cell packages it with histone proteins. Histones act as molecular spools around which the DNA is tightly wound, forming a complex known as chromatin.
During the prophase stage of mitosis, chromatin undergoes condensation. The DNA and histones are looped and coiled into progressively thicker fibers, forming the dense, compact chromosomes visible under a microscope. This organization reduces the DNA molecule’s length thousands of times, making it a manageable package for sorting.
The Structural Components: The Mitotic Spindle
After the genetic material is condensed, the mitotic spindle forms to physically separate the sister chromatids. This structure is a scaffold constructed from microtubules, which are made of the protein tubulin. Individual tubulin molecules polymerize to form the long, hollow filaments that make up the spindle fibers.
The mitotic spindle originates from two centrosomes, which move to opposite poles of the cell. Microtubules extend from each centrosome, creating a framework that spans the cell. Some spindle fibers attach to a protein structure on the chromosomes called the kinetochore, linking each chromosome to the spindle.
The microtubules then shorten and pull on the chromosomes, aligning them at the cell’s equator during metaphase. During anaphase, the connection between sister chromatids is severed, and the microtubules pull them apart toward opposing ends of the cell. This action ensures each new daughter cell receives a complete set of chromosomes.
The Energy Currency: Adenosine Triphosphate (ATP)
The processes of mitosis are energy-dependent, and the power for these activities is supplied by Adenosine Triphosphate (ATP). ATP functions as the primary energy currency for all living cells, capturing chemical energy and releasing it to fuel cellular work.
Nearly every step of mitosis consumes ATP. This includes the enzymatic activity of CDKs, the condensation of DNA, and the assembly of the mitotic spindle. The movement of chromosomes is also an energy-intensive process.
Motor proteins that walk along the spindle’s microtubule tracks use ATP as direct fuel. They bind to an ATP molecule and break it down to release a burst of energy. This energy allows the motor proteins to change shape and generate the force needed to pull the chromosomes apart.