Mitosis is the process of cell division where a single parent cell divides into two genetically identical daughter cells. This process is fundamental for growth, tissue repair, and the replacement of old cells. Mitosis ensures that the daughter cells receive the same number and type of chromosomes as the original parent cell, maintaining an exact copy of the genetic material.
Establishing the Parent Cell Starting Point
A chromosome is a tightly packaged structure of deoxyribonucleic acid (DNA) and associated proteins that carries genetic information. Most human cells are diploid (2n), meaning they contain two complete sets of chromosomes, one inherited from each parent.
In human somatic (non-sex) cells, the diploid number is 46 chromosomes, arranged in 23 pairs. Before division, the cell must prepare by duplicating its entire DNA content during the S-phase of the cell cycle. This replication results in each chromosome consisting of two identical strands, called sister chromatids, joined at a central region called the centromere.
Although the cell now contains 92 chromatids, the chromosome count remains 46. This is because the two sister chromatids are physically considered a single chromosome as long as they are connected by the centromere. The cell is now ready to enter mitosis with a duplicated set of 46 chromosomes.
The Critical Phase: Ensuring Identical Copies
The successful partitioning of chromosomes relies on precise movements orchestrated by the cell’s internal machinery. During metaphase, the 46 duplicated chromosomes align along the cell’s equatorial plane, forming the metaphase plate. The sister chromatids of each chromosome are positioned to face opposite poles, ready for separation.
The transition to anaphase determines the final chromosome count for the daughter cells. Specialized proteins holding the sister chromatids together at the centromere are cleaved, allowing the identical copies to separate. Once separated, each former sister chromatid is recognized as an independent chromosome.
This separation temporarily doubles the total chromosome count within the dividing cell, from 46 to 92 individual chromosomes. These 92 newly separated chromosomes are pulled toward opposite ends of the cell by shortening spindle fibers. This simultaneous movement ensures that an exact set of 46 chromosomes moves to each pole.
The Final Chromosome Count
Following the migration of chromosomes to opposite poles, the cell enters telophase and undergoes cytokinesis, the physical division of the cytoplasm. This results in the formation of two distinct daughter cells, each containing a complete nucleus. Each resulting daughter cell is an exact genetic replica of the parent cell.
Each daughter cell receives 46 chromosomes, which is the full diploid (2n) number characteristic of human somatic cells. These chromosomes, once separated sister chromatids, are now single-stranded and ready to function within the new cell. Maintaining this consistent chromosome number preserves genetic stability across generations of cells.
Mitosis facilitates the repair of damaged tissues and enables organism growth through the addition of new cells. By producing two daughter cells that are genetically and numerically identical to the parent, the functional integrity of the tissue is preserved.
Mitosis Compared to Meiosis
While mitosis is responsible for cell maintenance and growth, another form of cell division, meiosis, serves the function of sexual reproduction. The key distinction between the two processes lies in the final chromosome number of the resulting cells. Mitosis maintains the diploid (2n) count, ensuring daughter cells are numerically identical to the parent.
Meiosis, in contrast, involves two rounds of division and results in four daughter cells with a reduced chromosome number. These cells, known as gametes (sperm or egg), are haploid (1n), meaning they contain only one set of chromosomes. In humans, this means each gamete has 23 chromosomes.
The halving of the chromosome number in meiosis is necessary because the gametes will later fuse during fertilization, restoring the full diploid number of 46 chromosomes in the newly formed organism. The 46-chromosome outcome of mitosis ensures somatic cell fidelity, while the 23-chromosome outcome of meiosis supports reproductive diversity.