Mitosis begins with 46 chromosomes in human cells, and its purpose is to ensure that the two new daughter cells also finish with exactly 46 chromosomes. Mitosis is the form of cell division used by somatic cells to grow and repair tissue. It is often referred to as an equational division because the chromosome number is conserved from the parent cell to the daughter cells. The process precisely copies and then separates the genetic material to create two genetically identical cells.
The Starting Line: Understanding the Diploid State
The 46-chromosome count represents the diploid state of a human somatic cell, designated as 2n=46. This means the cell contains two complete sets of chromosomes, with one set of 23 inherited from each biological parent. These 46 chromosomes exist as 23 homologous pairs, where each pair consists of two chromosomes that carry genes for the same traits.
In the initial resting phase of the cell cycle, known as G1, each of the 46 chromosomes consists of a single, long strand of DNA. These single-stranded chromosomes are not yet tightly coiled into the familiar X-shape often depicted in diagrams. The DNA is instead loosely organized within the nucleus as chromatin. This state, with 46 single-chromatid chromosomes, is the true starting point for the division cycle.
The 23 pairs include 22 pairs of autosomes, which are the non-sex chromosomes. The final pair consists of the sex chromosomes, which are either two X chromosomes in females or an X and a Y chromosome in males. This arrangement of two complete sets of genetic information defines the cell as diploid. The cell must duplicate this genetic inventory before division, ensuring each new cell receives a full complement.
Preparing for Division: DNA Replication
Preparation for cell division occurs during the synthesis, or S, phase of the cell cycle. During this phase, the cell must replicate its entire genome so that each daughter cell receives a complete set of instructions. This replication process results in the doubling of the cell’s DNA content.
Every one of the 46 single-stranded chromosomes is duplicated to create an identical copy. This copy is called a sister chromatid, and it remains physically attached to the original strand. The two identical sister chromatids are held together by a single structure called the centromere.
Although the total amount of DNA has doubled, the cell’s official chromosome count remains at 46. This is because a chromosome is defined by the presence of its own centromere. Since the original chromosome and its newly synthesized sister chromatid are still connected at a single centromere, the structure is counted as only one chromosome.
At the end of the S phase, the cell contains 46 chromosomes, but each one is composed of two sister chromatids. This means the cell now has 92 chromatids in total, yet the chromosome number remains 46. Mitosis, which follows this preparation, is the mechanism for separating these 92 chromatids equally.
The Core Process: Tracking Chromosomes Through Separation
The visible stages of mitosis begin with prophase, where the duplicated chromosomes condense into the compact, X-shaped structures that are easily visible under a microscope. The 46 duplicated chromosomes then move to the center of the cell and align along the metaphase plate, ensuring an orderly separation. Each of the two sister chromatids in a chromosome is now attached to spindle fibers originating from opposite poles of the cell.
The most significant event for the chromosome count occurs during anaphase. This stage begins when the centromeres holding the sister chromatids together suddenly divide. Once separated, each former sister chromatid is considered an individual chromosome.
Because all 46 duplicated chromosomes split simultaneously, the cell temporarily contains 92 individual chromosomes. These 92 newly formed chromosomes are then pulled toward opposite poles of the cell by the shortening spindle fibers. This temporary doubling is a mechanical necessity to ensure that each half of the dividing cell receives a complete set of genetic material.
Following anaphase, telophase begins as the 46 chromosomes at each pole start to decondense, and a new nuclear membrane forms around each group. Cytokinesis, the division of the cytoplasm, then splits the single cell into two daughter cells. The result is two new cells, each containing a nucleus with 46 single-chromatid chromosomes, successfully maintaining the original diploid count.