Meiosis is a specialized type of cell division responsible for creating sex cells, or gametes, such as sperm and eggs. Unlike most cell division, which creates exact copies, meiosis is necessary for sexual reproduction. This process involves a parent cell splitting into four descendant cells. The cells produced by meiosis are genetically different from the original parent cell and unique from one another, which is central to inheritance and biological variation.
Meiosis vs. Mitosis: The Fundamental Difference in Outcome
Meiosis and mitosis are the two main types of cell division, but they have entirely different outcomes. Mitosis is an equational division where a single parent cell divides once to produce two daughter cells. These cells are genetically identical to the parent and contain the same number of chromosomes, used for growth, repair, and replacing somatic cells.
Meiosis, in contrast, is a reductional division designed to halve the chromosome number. It begins with a diploid parent cell, containing two complete sets of chromosomes. The meiotic process involves two successive rounds of division, resulting in four haploid daughter cells. Haploid cells contain only a single set of chromosomes, half the number of the original parent cell.
For example, in humans, a diploid cell starts with 46 chromosomes, and the resulting gametes each contain 23 chromosomes. This halving is necessary so that when gametes fuse during fertilization, the offspring cell is once again diploid with the correct total of 46 chromosomes. The daughter cells produced by meiosis are genetically distinct from the parent cell and from each other.
Mechanisms Generating Genetic Diversity
The reason meiotic cells are genetically different is due to two mechanisms that occur during the first meiotic division. These processes ensure that the final four haploid cells contain a unique mix of genetic information.
Crossing Over
The first mechanism is crossing over, which takes place early in Prophase I when homologous chromosomes pair up closely. Homologous chromosomes are the pairs—one maternal and one paternal—that carry the same genes. While paired, segments of the non-sister chromatids physically overlap and exchange sections of DNA, known as genetic recombination. This exchange creates chromosomes that are mosaics, containing a mix of genetic material not present in the original parent chromosome.
Independent Assortment
The second mechanism is independent assortment, which occurs later in Metaphase I. During this stage, the homologous chromosome pairs line up randomly along the center of the cell. The orientation of one pair is independent of how any other pair lines up, meaning the maternal and paternal chromosomes are sorted randomly into the two dividing cells. For humans with 23 pairs of chromosomes, this random alignment alone can create over 8 million possible combinations of chromosomes in the resulting gametes. The combined effects of crossing over and independent assortment guarantee that the four cells produced by meiosis are genetically unique.
The Distinct Roles of Meiosis I and Meiosis II
Meiosis consists of two separate divisions, Meiosis I and Meiosis II, which perform distinct tasks.
Meiosis I: Reductional Division
Meiosis I is termed the reductional division because its primary role is to separate the homologous chromosome pairs. This separation reduces the chromosome number from diploid to haploid. The two cells resulting from Meiosis I each contain one chromosome from every original homologous pair.
Meiosis II: Equational Division
The cells from Meiosis I immediately enter Meiosis II, which is classified as an equational division. Meiosis II is similar to mitosis because it involves the separation of sister chromatids. Since Meiosis II begins with haploid cells, the separation of sister chromatids yields four final cells. Each of these four cells possesses a single, non-duplicated set of chromosomes. The two-step process of Meiosis I’s reduction and recombination, followed by Meiosis II’s final chromatid separation, ensures a high degree of genetic variability. This variation-generating process is the reason the four descendant cells are genetically different from the parent cell and from one another.