Mitosis is the process that allows a single parent cell to divide and produce two genetically identical daughter cells. It serves as the fundamental mechanism for growth, tissue repair, and asexual reproduction in many organisms. The result of this division is that each new daughter cell maintains the exact same number of chromosomes as the original parent cell. For example, if the parent cell begins with 46 chromosomes, the two resulting cells will also each contain 46 chromosomes.
Defining the Starting Chromosome Count
The chromosome count of a cell is determined by the number of distinct DNA packages it contains. In human somatic (body) cells, the characteristic number of chromosomes is 46. These 46 chromosomes are organized into 23 pairs, with one member of each pair inherited from each biological parent.
A cell containing two complete sets of chromosomes is referred to as diploid, or \(2n\). The 22 non-sex pairs are called autosomes, and the final pair consists of the sex chromosomes (XX or XY). This \(2n=46\) count for humans sets the baseline that mitosis must reproduce during cell division.
The Essential Preparation Phase
Before a cell can enter the active phases of mitosis, it must first undergo a preparatory period known as interphase. The most significant event is the synthesis (S) phase, where the cell replicates its entire genome. Every DNA strand is duplicated, resulting in two identical copies of each chromosome.
These identical copies are known as sister chromatids. They remain physically linked together, most tightly at the centromere. Although the genetic material has doubled, the cell’s official chromosome count does not change. A single chromosome composed of two sister chromatids is still counted as one chromosome because it possesses only one centromere. Therefore, the cell starts mitosis with 46 replicated chromosomes.
How Mitosis Ensures Identical Counts
The main purpose of mitosis is to separate these sister chromatids, ensuring each new cell receives a complete, identical set of genetic information. This separation occurs primarily during the metaphase and anaphase stages. During metaphase, the 46 duplicated chromosomes align precisely along the cell’s equatorial plane, forming the metaphase plate.
This alignment is verified by the spindle assembly checkpoint, which ensures that microtubules from opposite poles are correctly attached to the centromere of each chromatid pair. The transition into anaphase is triggered by the cleavage of cohesin proteins, the molecular glue holding the sister chromatids together. This separation is mediated by the enzyme separase.
Once the cohesin is dissolved, the two sister chromatids of each original chromosome are pulled apart toward opposite poles of the cell. The moment they separate, they are considered individual chromosomes. As the cell elongates and the two sets of 46 newly separated chromosomes arrive at the poles during telophase, the division process is complete. Cytokinesis then divides the cytoplasm, resulting in two genetically identical cells, each containing 46 chromosomes, matching the parent cell.
Mitosis Versus Meiosis
The maintenance of the chromosome number distinguishes mitosis from meiosis. Mitosis occurs in somatic cells for maintenance and growth, producing two diploid (\(2n\)) daughter cells from a single diploid parent cell. The goal is to create genetic clones, preserving the \(2n=46\) count in humans.
Meiosis, in contrast, is a specialized two-step division process that occurs only in germ cells to produce sex cells, or gametes. The outcome is four genetically distinct cells, each containing only half the original number of chromosomes, referred to as haploid (\(1n\)). In humans, this process reduces the chromosome count from 46 to 23, preparing the egg and sperm cells for fusion during fertilization.