Synapsis, the precise pairing of homologous chromosomes, does not occur in mitosis. Mitosis is the form of cell division that creates two genetically identical daughter cells from one parent cell, primarily responsible for growth and repair. Synapsis is a specialized biological event necessary for sexual reproduction that takes place during a specific phase of meiosis.
Mitosis: Replication for Growth and Repair
Mitosis is the mechanism by which somatic cells divide, serving functions like tissue growth, repair, and asexual reproduction in some organisms. The goal of this process is to produce two daughter cells that are exact genetic duplicates of the parent cell. This cell division involves a single round of chromosome separation following DNA replication.
During the prophase of mitosis, chromosomes condense, and each replicated chromosome consists of two identical sister chromatids. When the cell enters metaphase, these individual chromosomes line up along the cell’s equatorial plane, known as the metaphase plate. Homologous chromosomes do not interact or pair with each other at any point during this division.
The absence of synapsis in mitosis is reflected in the independent behavior of the homologous chromosomes. They align separately, ensuring that when the sister chromatids pull apart in anaphase, each new cell receives a complete and identical set of chromosomes. This independent alignment prevents the side-by-side pairing required for synapsis and subsequent genetic exchange.
Synapsis: The Mechanism of Homologous Pairing
Synapsis is the lengthwise, intimate pairing of two homologous chromosomes. This pairing is precise, aligning the genes on one chromosome directly across from the corresponding genes on its partner. It is a regulated event that occurs during the prophase I stage of meiosis.
The physical connection between the paired chromosomes is mediated by a complex protein structure known as the synaptonemal complex (SC). This structure acts like a zipper, assembling between the homologous chromosomes to hold them in alignment. The SC is a tripartite structure, consisting of two lateral elements attached to the chromosomal axes and a central element that bridges the space between them.
Once synapsis is complete, the paired structure is referred to as a bivalent, because it consists of two homologous chromosomes. Since each homologous chromosome has already replicated, this structure contains four total chromatids, also called a tetrad. The purpose of this close association is to facilitate genetic recombination, or crossing over, which is the physical exchange of genetic material between non-sister chromatids.
Meiosis: The Context for Synapsis
Meiosis is a specialized cell division necessary for sexual reproduction, occurring only in germ cells to produce gametes (sperm and egg cells). This process involves two successive rounds of division but only one round of DNA replication. It ultimately results in four cells, each with half the number of chromosomes as the original parent cell. Synapsis occurs exclusively in Prophase I, the first stage of Meiosis I.
The formation of the bivalent structure through synapsis is a prerequisite for crossing over, which shuffles genes and generates genetic diversity. This exchange of DNA segments creates recombinant chromosomes that are a unique mix of maternal and paternal genes. The synaptonemal complex physically supports the maturation of programmed DNA breaks into these crossovers.
The physical attachments formed by crossing over, known as chiasmata, hold the homologous chromosomes together until Metaphase I. This physical link ensures the correct separation of homologous pairs during Meiosis I, a reduction division where the chromosome number is halved. By contrast, in mitosis, the lack of synapsis and crossing over means the cell division is purely equational, resulting in genetically identical cells.