Within each cell’s nucleus, chromosomes carry an organism’s genetic information. Meiosis is a specialized cell division in sexually reproducing organisms that produces reproductive cells, or gametes. This process involves two rounds of division, ultimately leading to cells with half the usual number of chromosomes.
The Starting Point: DNA Duplication
Before meiosis begins, a cell undergoes a preparatory phase called interphase. During interphase, specifically the S phase, the cell’s DNA is replicated. This means that while the overall number of chromosomes remains the same, each chromosome now consists of two identical copies, called sister chromatids, joined together.
For example, in human cells, after DNA replication, there are still 46 chromosomes, but each is duplicated, containing two sister chromatids. This effectively doubles the DNA content within the cell, even though the chromosome count itself has not changed. These duplicated chromosomes are ready for meiotic divisions.
Chromosomes in Meiosis I
Meiosis I is a “reductional division” because it significantly reduces the chromosome number. During this stage, homologous chromosomes—pairs of chromosomes, one inherited from each parent—come together and then separate. In human cells, there are 23 pairs of homologous chromosomes.
As Meiosis I progresses, these homologous pairs align and move to opposite poles of the cell. Each resulting daughter cell receives one chromosome from each homologous pair. At the conclusion of Meiosis I, the original diploid cell divides into two daughter cells, each containing 23 chromosomes. Each of these 23 chromosomes still consists of two sister chromatids.
The Significance of Meiosis I
The reduction in chromosome number during Meiosis I is important for maintaining a constant chromosome count across generations in sexually reproducing organisms. Without this reduction, the fusion of two gametes during fertilization would double chromosomes in each successive generation, leading to an unsustainable increase in genetic material.
Meiosis I also introduces genetic variation. This occurs through processes like crossing over, where homologous chromosomes exchange genetic material. Independent assortment of homologous chromosomes, where each pair aligns and separates randomly, further shuffles genetic information, contributing to the unique genetic makeup of each gamete.
Completing the Process: Meiosis II and Gamete Formation
Meiosis II follows Meiosis I, without another round of DNA replication. This second division is similar to mitosis and is an “equational division” because the chromosome number per cell does not change. During Meiosis II, the sister chromatids that still make up each chromosome separate.
This separation results in four haploid cells from the two cells produced in Meiosis I. Each of these four final cells contains a single set of unduplicated chromosomes. For humans, each gamete (sperm or egg cell) ends up with 23 single chromosomes. These haploid gametes are ready to combine during fertilization, restoring the full diploid set of chromosomes in the new organism.