Meiosis is a specialized form of cell division that plays a central role in the reproduction of sexually reproducing organisms. It generates gametes, such as sperm and egg cells, which are essential for creating new life.
Chromosome Replication Before Meiosis
Chromosomes do undergo replication, but this event happens before meiosis commences, not during the meiotic divisions themselves. This preparatory phase, interphase, involves cell growth and DNA synthesis. During the S (synthesis) phase of interphase, the cell’s entire genome is duplicated.
This replication process ensures that each chromosome, which originally consists of a single DNA molecule, is duplicated into two identical copies. These identical copies are known as sister chromatids. They remain physically joined together at a constricted region called the centromere, forming a single replicated chromosome. After duplication, the cell is prepared to enter the stages of meiosis. No additional DNA replication takes place between the first meiotic division (Meiosis I) and the second meiotic division (Meiosis II).
The First Meiotic Division
The first meiotic division, known as Meiosis I, is often referred to as the reductional division because it halves the chromosome number of the parent cell. During this stage, homologous chromosomes, which are pairs of chromosomes inherited one from each parent, find and pair with each other. Each of these homologous chromosomes consists of two sister chromatids, formed during the preceding S phase.
Crossing over, also termed genetic recombination, occurs within Prophase I of meiosis. During crossing over, non-sister chromatids of homologous chromosomes exchange segments of genetic material. This physical exchange creates new combinations of alleles on the chromosomes, contributing to genetic diversity. Following this pairing and exchange, the homologous chromosome pairs align at the cell’s center.
During Anaphase I, the homologous chromosomes separate and move to opposite poles of the cell. Each chromosome still retains its two sister chromatids. This separation is what reduces the chromosome number by half in each forming daughter cell. By the end of Meiosis I, two haploid daughter cells are formed, each containing a set of replicated chromosomes.
The Second Meiotic Division
Following Meiosis I, the two haploid cells enter the second meiotic division, Meiosis II, without any intervening DNA replication. This division shares many similarities with mitosis and is often referred to as an equational division because the chromosome number per cell does not change during this phase. The primary event in Meiosis II is the separation of sister chromatids.
In each of the two cells from Meiosis I, the chromosomes, still composed of two sister chromatids, condense and align individually at the center of the cell. Spindle fibers attach to the centromeres of these sister chromatids. During Anaphase II, these sister chromatids are pulled apart, becoming individual, unreplicated chromosomes that move towards opposite poles of the cell.
Meiosis II results in the formation of four haploid daughter cells from the original single diploid cell. Each of these four resulting cells contains a single set of unreplicated chromosomes.
The Purpose of Meiosis
Meiosis serves two main biological purposes. The first purpose is the reduction of the chromosome number. By reducing the chromosome count from diploid (two sets) to haploid (one set) in gametes, meiosis ensures that when two gametes fuse during fertilization, the resulting offspring has the correct, stable number of chromosomes characteristic of the species. Without this reduction, the chromosome number would double with each generation, leading to an unsustainable increase.
The second purpose of meiosis is to generate genetic diversity. This diversity arises from two main mechanisms: crossing over and independent assortment. Crossing over, occurring in Meiosis I, shuffles genetic material between homologous chromosomes, creating new combinations of genes. Independent assortment of homologous chromosomes during Metaphase I also contributes significantly. This random alignment and subsequent separation of maternal and paternal chromosomes results in a vast number of possible chromosome combinations in the gametes. Genetic variation is crucial for populations to adapt to changing environments and promotes species survival.