Meiosis is a specialized type of cell division that creates reproductive cells, known as gametes (sperm and eggs). This process is necessary for sexual reproduction and involves two sequential rounds of division. The primary function of meiosis is to ensure that the resulting daughter cells contain only half the number of chromosomes present in the original cell. This reduction is fundamental to maintaining the correct chromosome count across generations when two gametes combine.
Setting the Stage: Chromosome Counts Before Division
The cells that undergo meiosis, called germline cells, begin as diploid cells, meaning they contain two full sets of chromosomes. In humans, this diploid state is represented by 2n, which corresponds to 46 total chromosomes. These 46 chromosomes exist as 23 homologous pairs, with one chromosome in each pair inherited from each parent.
Before the division process begins, the cell goes through a preparatory phase called interphase. During this phase, specifically the S phase, the cell’s DNA is replicated. Each of the 46 chromosomes is duplicated, resulting in two identical copies called sister chromatids, which remain tightly joined. Although the amount of DNA has doubled, the cell is still counted as having 46 chromosomes because the chromosome number is determined by the number of centromeres.
The Reduction Phase: Meiosis I
Meiosis I is often called the reduction division because it is during this stage that the chromosome number is halved. The central event is the separation of the homologous chromosome pairs. During Prophase I, homologous chromosomes pair up and exchange genetic material in a process known as crossing over.
In Anaphase I, the homologous chromosomes, each still consisting of two sister chromatids, are pulled apart toward opposite ends of the cell. The sister chromatids do not separate from each other during this stage. The cell then divides, resulting in two intermediate cells.
Each of these two new cells contains 23 chromosomes, reducing the count from 46 to 23. While the chromosome number is now haploid (n=23), each of these 23 chromosomes is still duplicated, containing two sister chromatids. These cells are haploid in number but still contain excess DNA material.
The Final Split: Meiosis II and the Daughter Cells
The two cells produced after Meiosis I enter the second division, Meiosis II, which resembles mitosis. Meiosis II is an equational division, meaning it does not further reduce the chromosome number. The purpose of this stage is solely to separate the remaining sister chromatids.
During Anaphase II, the centromere connecting the sister chromatids dissolves, allowing the individual chromatids to separate and move to opposite poles of the cell. Once separated, each chromatid is considered a full, distinct chromosome. The two intermediate cells then divide, resulting in a total of four daughter cells from the original parent cell.
Each of these four final daughter cells is truly haploid (n) and contains 23 chromosomes. Each of these 23 chromosomes now consists of a single chromatid, representing the single set of genetic instructions required for a gamete. The daughter cells produced at the end of meiosis have exactly half the number of chromosomes as the starting germline cell.
The Biological Purpose of Halving the Count
Halving the chromosome number is a requirement for sexual reproduction and the maintenance of genetic stability. The final products of meiosis—sperm and egg cells—must each carry only a single set of chromosomes. If gametes were not haploid, their fusion during fertilization would result in a doubling of the chromosome number with every successive generation.
For example, if a human sperm with 46 chromosomes fused with an egg with 46 chromosomes, the offspring would have 92 chromosomes. By ensuring each gamete has exactly 23 chromosomes, the fusion of sperm and egg restores the full diploid number of 46 in the fertilized cell, or zygote. This mechanism guarantees that the characteristic chromosome count for the species is preserved across generations.