Meiosis is a fundamental biological process for sexually reproducing organisms. It plays a significant role in generating genetic diversity within a species and maintaining the correct number of chromosomes across generations. This specialized cell division involves two distinct rounds: Meiosis I and Meiosis II.
Meiosis I
Meiosis I is the first major division, focusing on the separation of homologous chromosomes. Before meiosis, the cell’s DNA replicates during interphase, ensuring each chromosome consists of two identical sister chromatids. Meiosis I reduces the chromosome number by half, transitioning cells from a diploid to a haploid state. This reduction differs from Meiosis II, which separates sister chromatids, similar to mitosis.
Prophase I
Prophase I initiates Meiosis I, a complex stage subdivided into five sequential phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. During leptotene, chromosomes begin to condense, becoming gradually visible as long, thin threads. Though each chromosome consists of two sister chromatids, they are not yet clearly discernible.
As the cell progresses into zygotene, homologous chromosomes begin to pair up precisely along their lengths, a process called synapsis. This pairing is facilitated by the synaptonemal complex, a protein structure. The paired homologous chromosomes, now tightly associated, are referred to as bivalents or tetrads.
Pachytene follows, marked by the completion of synapsis and the full condensation of chromosomes. During this phase, crossing over occurs between non-sister chromatids of the homologous chromosomes. This exchange generates new combinations of alleles, significantly increasing genetic diversity. Recombination nodules appear on the synaptonemal complex, facilitating these exchanges.
In diplotene, the synaptonemal complex begins to disassemble, and the homologous chromosomes start to separate from each other. They remain connected at specific points called chiasmata, which are the visible manifestations of where crossing over occurred. These chiasmata initially form during pachytene but become apparent as the chromosomes desynapse in diplotene.
Diakinesis is the final phase of Prophase I. Chromosomes condense further, becoming shorter and thicker. The chiasmata move towards the ends of the chromatids, a process known as terminalization. Concurrently, the nuclear envelope breaks down, and spindle fibers begin to form in the cytoplasm.
Metaphase I and Anaphase I
In Metaphase I, homologous chromosome pairs align along the metaphase plate at the cell’s equator. Independent assortment is a key event, where the orientation of each homologous pair at the metaphase plate is random. This shuffles paternal and maternal chromosomes independently, contributing substantially to genetic variation in the resulting gametes. For humans with 23 pairs of chromosomes, this random assortment alone can lead to over 8 million different combinations of chromosomes in the gametes.
In Anaphase I, homologous chromosomes separate and are pulled towards opposite poles by the spindle fibers. Sister chromatids remain attached at their centromeres, which differs from mitosis and Meiosis II. Each pole receives one chromosome from each homologous pair.
Telophase I and Cytokinesis
In Telophase I, separated homologous chromosomes arrive at opposite poles. At each pole, a nuclear envelope reforms around the chromosomes. The chromosomes may also begin to decondense.
Cytokinesis occurs concurrently with Telophase I, dividing the cytoplasm of the parent cell and forming two daughter cells. Each resulting cell is haploid, as they contain one chromosome from each original homologous pair. Each chromosome still consists of two sister chromatids. These two haploid daughter cells are now prepared to enter Meiosis II, a division that separates sister chromatids.