Mitosis is the fundamental process of cell division that ensures a parent cell creates two daughter cells that are genetically identical to itself. This replication is the mechanism for growth, tissue repair, and replacing old cells throughout the body. A healthy human cell must maintain a stable number of chromosomes, typically 46. The question of how many chromosomes are present at the beginning of mitosis is complicated because the cell must first double its genetic material before dividing it equally.
The Baseline Diploid Number
The vast majority of cells in the human body, known as somatic cells, contain 46 chromosomes. This count is referred to as the diploid number (2n), meaning the cell holds two complete sets of chromosomes. One set of 23 chromosomes is inherited from each parent, forming 23 homologous pairs. Before division, each of these 46 chromosomes exists as a single, long strand of DNA.
Each chromosome in this unreplicated state has one centromere, which is the constricted region of the DNA. The term “haploid” (n) refers to cells, like sex cells, that contain only one set of 23 chromosomes. Maintaining the diploid number (2n=46) is the primary goal of the entire mitotic process.
Preparing for Division (Interphase)
Before mitosis begins, the cell must go through a preparatory phase called Interphase, which includes the S phase, or Synthesis phase. During the S phase, the cell replicates its entire DNA content. This process results in a doubling of the genetic material, moving the cell from a 2n DNA content to a 4n DNA content.
Despite the doubling of the DNA, the number of chromosomes remains unchanged at 46. This is because the newly synthesized DNA strand remains physically attached to the original strand at the single centromere. The two identical strands joined together are called sister chromatids, and as long as they share a centromere, they are still counted as one chromosome.
Counting Chromosomes at the Start of Mitosis
Mitosis begins with Prophase, where the duplicated chromosomes condense, making them visible under a microscope. At this point, the cell contains 46 chromosomes. Each of these 46 chromosomes is composed of two identical sister chromatids.
The fundamental rule for counting chromosomes is to count the number of centromeres. Since the 46 replicated structures each possess only one centromere holding the two chromatids together, the chromosome count remains 46. Although the cell contains 92 chromatids (46 chromosomes multiplied by two), the centromere rule dictates the chromosome number. The highly condensed state of these chromosomes ensures they can be neatly moved and separated without tangling.
The Outcome of Mitosis
The goal of mitosis is to distribute these 46 replicated chromosomes equally between the two forming daughter cells. This separation occurs during Anaphase, where the centromeres finally divide. Once the centromere splits, each sister chromatid is instantly considered an individual chromosome.
During Anaphase, the cell temporarily holds 92 individual, unreplicated chromosomes as they are pulled toward opposite poles. Telophase and Cytokinesis complete the division, resulting in two distinct daughter cells. Each new cell receives 46 unreplicated chromosomes, restoring the original 2n=46 count and ensuring genetic identity with the parent cell.