Cell division is fundamental for growth, development, and reproduction. This mechanism ensures that genetic information is accurately passed from one cell to the next. Chromosomes, thread-like structures found within the nucleus of cells, carry this genetic information in the form of DNA. Chromosomes become tightly coiled and visible under a microscope when cells prepare to divide.
Chromosomes in Body Cells
Most cells in the human body, known as somatic cells, are diploid, containing two complete sets of chromosomes—one inherited from the mother and one from the father. In humans, the diploid number is 46 chromosomes, organized into 23 pairs. These pairs are known as homologous chromosomes, which are similar in length, gene sequence, and centromere position. While homologous chromosomes carry the same genes, they may have different versions of those genes, called alleles. The 22 pairs of non-sex chromosomes are called autosomes, and the 23rd pair consists of sex chromosomes (XX for females and XY for males).
Meiosis I: The Reduction Division
Meiosis is a specialized type of cell division that produces gametes, which are sex cells like sperm and eggs. This process occurs in two main stages: Meiosis I and Meiosis II. Meiosis I is often referred to as the “reduction division” because it halves the number of chromosome sets.
Before Meiosis I begins, the cell’s DNA replicates, so each chromosome consists of two identical sister chromatids joined at a centromere. During Meiosis I, homologous chromosomes pair up and exchange genetic material through a process called crossing over, which contributes to genetic diversity. These paired homologous chromosomes then separate, with one chromosome from each pair moving to opposite poles of the cell. This separation results in two daughter cells, each containing 23 chromosomes in humans. Although the chromosome number is halved, each chromosome still consists of two sister chromatids.
Meiosis II: Separating Duplicated Chromosomes
Following Meiosis I, the two cells proceed to Meiosis II, which is similar to mitosis. There is no further DNA replication between Meiosis I and Meiosis II.
In this stage, the sister chromatids of each chromosome separate and move to opposite poles of the cell, leading to the formation of four new cells. Meiosis II does not reduce the chromosome number further; it separates the duplicated sister chromatids. Each of the four resulting cells now contains single, unduplicated chromosomes.
The Resulting Chromosome Count
The entire process of meiosis ultimately produces four haploid cells from a single diploid parent cell. Haploid cells contain only one set of chromosomes, represented as ‘n’. In humans, this means each gamete (sperm or egg cell) ends up with 23 chromosomes. These 23 chromosomes consist of 22 autosomes and one sex chromosome.
The reduction of chromosome number in gametes is important for sexual reproduction. When a sperm and an egg fuse during fertilization, their single sets of chromosomes combine, restoring the full diploid number of 46 chromosomes in the offspring. This mechanism ensures that the characteristic chromosome number for a species is maintained across generations.