Mitosis is the biological process by which a cell divides to create two genetically identical daughter cells. This form of cell division is essential for growth, development, and the replacement of damaged or old cells. The primary goal of mitosis is to ensure that the exact genetic blueprint is passed from the original cell to the new cells. The key to this successful duplication lies in maintaining the same number of chromosomes in the resulting daughter cells as were present in the parent cell.
The Starting Point: Parent Cell Chromosome Count
The genetic material within a cell nucleus is organized into structures called chromosomes, which are long strands of DNA tightly wound around proteins. Each species has a characteristic number of chromosomes, and in humans, non-sex cells, known as somatic cells, begin the division process with a specific count.
A human parent cell starts with 46 chromosomes, organized into 23 pairs. One set of 23 chromosomes is inherited from each biological parent. This full complement of chromosomes is the baseline number that must be faithfully copied and distributed to the two new cells.
Duplicating Genetic Material
Before the cell can physically divide, it must ensure it has enough genetic material for two separate cells. This preparation happens during the synthesis, or S phase, of the cell cycle, which precedes mitosis. During the S phase, the cell replicates all of its DNA, creating a duplicate of every chromosome.
This replication results in each chromosome temporarily consisting of two identical DNA copies, which are physically joined together at a central region called the centromere. These two attached copies are known as sister chromatids. At this stage, the cell contains twice the amount of DNA, but the chromosome count is still considered to be 46, because the two identical strands are still linked and counted as a single unit.
Achieving Identical Chromosome Counts
The mechanism for maintaining the chromosome count occurs in the later stages of mitosis, specifically during anaphase. Before this phase, during metaphase, all 46 duplicated chromosomes line up neatly along the center of the cell. This alignment sets the stage for the equal distribution of the genetic material.
In anaphase, the centromere connecting the two sister chromatids simultaneously splits for all 46 chromosomes. As a result, each former sister chromatid is instantly recognized as an independent, full chromosome. These newly separated chromosomes are then pulled by spindle fibers toward opposite ends of the cell.
Since the 46 original duplicated chromosomes split into two, there are temporarily 92 individual chromosomes moving to the poles of the dividing cell. When the cell physically divides during cytokinesis, the two new clusters of chromosomes become the nuclei of the two daughter cells. Each daughter cell receives one complete group of 46 chromosomes, ensuring they are genetically identical to the parent cell. Therefore, the daughter cells produced through mitosis contain 46 chromosomes.