A chiasma is a physical point of contact between two paired chromosomes, which are thread-like structures that carry genetic information. This crossover point is analogous to twisting two separate pieces of thread together until they cross. These structures form during meiosis, a type of cell division that produces reproductive cells like sperm and eggs. The formation of a chiasma is an observable result of crossing over, where genetic material is exchanged between chromosomes.
The Formation Process During Meiosis
Chiasmata (the plural of chiasma) are formed during Prophase I of meiosis. The process begins when homologous chromosomes—one inherited from each parent—find and align with each other in a pairing called synapsis. During synapsis, the paired chromosomes are held together by a protein-and-RNA structure called the synaptonemal complex, which acts as a scaffold.
Along this synaptonemal complex, protein assemblies called recombination nodules mark the locations where genetic material will be exchanged. At these sites, enzymes cleave the DNA of two non-sister chromatids—one from each homologous chromosome. The broken ends are then rejoined to the other chromatid, an event known as crossing over that results in the physical exchange of chromosome segments.
After the exchange is complete, the synaptonemal complex disassembles. The homologous chromosomes start to separate but remain physically linked at the sites where crossing over occurred. These cross-shaped connection points are the chiasmata. They serve to hold the homologous chromosomes together as a pair, called a bivalent, until they are pulled apart later in meiosis. The number of chiasmata can vary.
The Role in Genetic Recombination
The formation of a chiasma is the physical manifestation of genetic recombination, which creates new combinations of genetic traits. Each chromosome contains genes, and different versions of these genes are called alleles. During crossing over, the exchange of segments between homologous chromosomes shuffles these alleles, creating chromatids that are a mosaic of genetic information from both parents. The resulting chromosomes in the gametes will carry a mix of genes different from either parent’s original chromosomes.
This genetic shuffling is a primary driver of diversity in sexually reproducing organisms. It can be compared to shuffling two related decks of playing cards to create new hands. These new combinations of alleles provide the raw material for evolution. Genetic variation within a population means some individuals may have traits that make them better suited to survive environmental changes or resist diseases.
This enhanced variation increases a population’s capacity to adapt over time. Natural selection can then act on this wider pool of traits, favoring combinations that confer a survival or reproductive advantage. Without the exchanges marked by chiasmata, the genes on a chromosome would be inherited as a single block, limiting the novelty that fuels evolution.
Consequences of Formation Errors
A chiasma also has a structural role, physically linking homologous chromosomes to ensure they align correctly at the cell’s equator before being pulled apart. This connection is necessary for the cell’s machinery to properly segregate the chromosomes. A failure to form at least one chiasma per homologous pair can have significant consequences.
If a pair of homologous chromosomes is not linked by a chiasma, it may fail to align properly on the meiotic spindle. This can lead to an error called nondisjunction, where the chromosomes are not segregated correctly. In such cases, both homologous chromosomes might be pulled to the same daughter cell, while the other receives none.
This error results in gametes—sperm or egg cells—with an abnormal number of chromosomes, a condition known as aneuploidy. Should such a gamete participate in fertilization, the resulting embryo will have an incorrect chromosome count. A well-known example of this is Trisomy 21, or Down syndrome, which often results from nondisjunction of chromosome 21. Studies show that reduced chiasma formation on chromosome 21 is associated with an increased risk of this event.