Cell division is the foundational process by which organisms grow, repair tissue, and reproduce. This process requires a precise mechanism to ensure that new cells receive a full set of genetic instructions. The duplication of chromosomes, the structures that hold the DNA, is a necessary step that precedes both Mitosis and Meiosis. Chromosome duplication does not happen during division, but rather occurs beforehand, during the preparatory phase of the cell cycle. Mitosis generates two identical body cells for growth, while Meiosis creates four genetically distinct sex cells for reproduction.
The Essential Precursor: Chromosome Duplication
Before a cell can commit to dividing, it must first replicate its entire genome. This takes place during the Synthesis (S-phase) of Interphase, the period between divisions where the cell grows and prepares. During this synthesis, each single-stranded chromosome is transformed into a duplicated chromosome consisting of two identical structures called sister chromatids.
These copies are physically joined together at a constricted region known as the centromere. A cell entering division has double the amount of DNA, but the overall chromosome number remains unchanged. For example, a human cell still has 46 chromosomes, but each is now composed of two sister chromatids instead of one. The cell then proceeds to the G2 phase, where it checks the replicated DNA for errors and synthesizes necessary proteins before entering the active division phase.
Mitosis: Division for Growth and Repair
Mitosis is a single division cycle designed to produce two daughter cells that are genetically identical to the original parent cell. This process occurs in somatic (body) cells and is the mechanism for organismal growth, tissue repair, and replacing old cells.
During the mitotic phase, the duplicated chromosomes first condense, making the two sister chromatids visible. These chromosomes then align along the cell’s central plane. The defining moment occurs in Anaphase, when the sister chromatids separate completely.
Once separated, each individual chromatid is considered a full, independent chromosome. These chromosomes are pulled to opposite ends of the cell, ensuring each daughter cell receives a complete and identical set. The outcome is two diploid cells, each containing the full set of chromosomes, just like the parent cell.
Meiosis: Reduction for Reproduction
Meiosis is a specialized form of cell division that occurs only in germ cells to produce gametes (sperm and egg cells) necessary for sexual reproduction. This process involves one round of chromosome duplication followed by two sequential rounds of cell division: Meiosis I and Meiosis II. The purpose is to reduce the chromosome number by half and introduce genetic variation.
Before Meiosis I, homologous chromosomes—the paternal and maternal copies—pair up tightly. While paired, these homologous chromosomes exchange segments of DNA in an event called crossing over, which shuffles the genetic information. During Meiosis I, the paired homologous chromosomes separate and move to opposite poles, while the sister chromatids remain attached.
This separation results in two haploid cells, meaning they contain only one set of chromosomes, though each chromosome is still duplicated. There is no further DNA replication before Meiosis II.
Meiosis II follows quickly and proceeds much like Mitosis. In this second division, the remaining sister chromatids finally separate. The end product is four genetically distinct haploid cells, each carrying a single, unduplicated set of chromosomes and half the chromosome number of the original parent cell.
Key Differences in Chromosome Behavior
The fundamental difference between the two processes lies in how they manage the duplicated chromosomes. Mitosis involves a single division where sister chromatids separate, resulting in two genetically identical, diploid (2n) cells. The goal is to maintain the original chromosome number for growth and repair.
Meiosis, in contrast, involves two divisions. Meiosis I separates homologous chromosomes, which are paired together, an action that does not occur in Mitosis. Meiosis II then separates the sister chromatids.
This two-step process achieves a reduction in chromosome number, yielding four genetically unique, haploid (n) cells. Furthermore, the crossing over that happens in Meiosis I introduces genetic variation, which is essential for sexual reproduction. Mitosis maintains genetic uniformity, while Meiosis ensures genetic diversity.