A chiasma (plural: chiasmata) is the physical point of contact between paired homologous chromosomes, appearing as a cross-shaped structure. These temporary structures form during meiosis, a specialized type of cell division. A chiasma represents a physical link that connects chromosomes from different parents.
This connection ensures the chromosomes remain correctly associated before they are distributed into new cells. These structures are visible under a microscope and were first described in 1909 by Frans Alfons Janssens. He identified their X-shaped appearance and proposed their connection to the exchange of genetic material.
Formation During Meiosis
Chiasmata form during Prophase I, the first stage of meiosis. Meiosis is the process that halves the number of chromosomes to produce sex cells, ensuring the correct chromosome count in offspring. During Prophase I, homologous chromosomes—one from each parent—pair up along their entire length. This intricate pairing process is called synapsis.
Once homologous chromosomes are paired, an event called crossing over occurs. Enzymes create breaks in the DNA of the paired chromosomes, which are then repaired in a way that exchanges segments of genetic material between them. A chiasma is the visible, physical point where the two chromosomes are held together after this exchange.
The synaptonemal complex, a protein framework, facilitates this pairing and exchange. This complex acts as a scaffold, holding the chromosomes together for the DNA exchange machinery. After the exchange is complete, the synaptonemal complex disassembles, but the chromosomes remain physically linked at the crossover locations, which are the chiasmata.
Critical Functions in Genetics
One of the primary outcomes of chiasma formation is the generation of genetic diversity. The crossing over that each chiasma represents shuffles genetic material between the maternal and paternal chromosomes. This event creates new combinations of alleles, which are different versions of the same gene. As a result, the chromosomes passed on to offspring are not identical to those of the parents.
This genetic recombination is a primary reason why siblings, apart from identical twins, exhibit a wide range of traits despite having the same parents. The number and position of chiasmata can vary, which further increases the potential for genetic variation. This shuffling of genes provides the raw material for natural selection, allowing populations to adapt to changing environments.
Chiasmata also have a structural role in the accurate segregation of chromosomes. They act as physical tethers, holding homologous chromosomes together as a pair until Anaphase I. This connection generates tension as the cell’s machinery pulls the chromosomes toward opposite poles.
This tension signals to the cell that chromosomes are correctly attached to the spindle fibers—the ropes that pull them apart. Without at least one chiasma per homologous pair, chromosomes might not align properly during Metaphase I. This link ensures one complete chromosome from the pair is pulled to each new daughter cell, preventing errors in chromosome number.
Impact of Formation Errors
Proper chiasma formation is required for the correct distribution of chromosomes into sex cells. If chiasmata fail to form, or do so in insufficient numbers, the homologous chromosomes may not remain linked. This absence of a physical connection can lead to nondisjunction, an error where chromosomes fail to separate correctly during cell division.
Nondisjunction during meiosis produces gametes (sperm or eggs) with an incorrect number of chromosomes, a condition called aneuploidy. A gamete might have an extra chromosome or be missing one. If this gamete is involved in fertilization, the resulting embryo will have an abnormal chromosome count in all its cells.
A well-known consequence of meiotic nondisjunction is Trisomy 21, commonly known as Down syndrome. This condition occurs when an individual has three copies of chromosome 21 instead of the usual two. This is often the result of a nondisjunction event in the mother’s developing egg cell, where the two copies of chromosome 21 failed to separate. Research has linked this failure to problems with chiasma formation, particularly the absence of a chiasma on chromosome 21 during the mother’s meiosis.