Meiosis is a specialized form of cell division that plays a fundamental role in sexual reproduction. This process reduces the number of chromosomes in a parent cell by half, creating four gamete cells, which are sperm or egg cells in humans. Meiosis also introduces genetic variation, ensuring that offspring are genetically distinct from their parents. This reduction in chromosome number and the generation of diversity are both important for maintaining genetic stability and for species adaptation.
The Chromosomal Starting Line
Before the process of meiosis can begin, a cell is in a state called interphase, specifically the G1 phase. During this period, the cell prepares for division, and its chromosomes are long, thread-like structures within the nucleus. Most cells destined for meiosis are diploid, meaning they contain two sets of chromosomes. One set is inherited from the maternal parent, and the other from the paternal parent, forming homologous pairs. At this stage, each chromosome consists of a single, unduplicated DNA molecule.
DNA Replication: Doubling the Genetic Material
A key event for meiosis occurs during the S (synthesis) phase of interphase. In this phase, the cell’s DNA is replicated. Each chromosome, which previously contained a single DNA molecule, is duplicated to form two identical copies called sister chromatids. These sister chromatids remain tightly attached to each other at a constricted region known as the centromere.
Although the amount of DNA doubles during this replication, the chromosome number itself remains unchanged. This is because the two sister chromatids are still considered part of a single chromosome until they separate later in the division process. Following the S phase, the cell enters a G2 phase, which acts as a checkpoint to ensure DNA replication is complete and to prepare for meiotic divisions. This preparation ensures each chromosome is ready for segregation.
First Steps of Meiosis I: Chromosome Pairing and Exchange
Meiosis begins with Prophase I, an extended stage of Meiosis I. During this phase, the duplicated chromosomes, each composed of two sister chromatids, condense, becoming shorter and thicker. A defining event of Prophase I is synapsis, where homologous chromosomes pair up. This close association forms structures known as bivalents or tetrads, each containing four chromatids.
Within these paired homologous chromosomes, a process called crossing over occurs, which is a source of genetic variation. During crossing over, segments of DNA are exchanged between non-sister chromatids of the homologous chromosomes. This reciprocal exchange creates new combinations of alleles on each chromatid, ensuring that the resulting gametes are genetically unique. As Prophase I progresses, the nuclear envelope, which encloses the genetic material, begins to break down, and the spindle apparatus starts to form, preparing the cell for chromosome movements.