A chiasma is a point of contact between sister chromatids that forms during meiosis, a process of cell division and replication used by a wide variety of organisms. At the chiasma, the chromatids can exchange genetic information, resulting in new combinations of genetic material. When the chromatids separate and become sister chromosomes, they will contain a different mix of genetic material than their parent chromosomes. This allows organisms to evolve genetically, creating new traits and passing them down to descendants.
The chromatids are joined at the centromere, a point roughly in the middle of each chromatid. The formation of a chiasma occurs during meiosis I, the first phase in meiosis, when the paired chromosomes exchange genetic material before splitting into two cells, each containing half the genetic material of the original parent cell. Chiasmata play an important role in the process of replicating genetic material.
By essentially shuffling the genome by exchanging information at the chiasma, organisms are capable of generating theoretically endless combinations of new traits. Some of these combinations do not work out, for a variety of reasons, and they are not passed down. Others prove to be successful, and will start to disseminate through the population as a result. Over time, organisms can undergo major evolutionary shifts as their genomes change and some individuals thrive as a result of their inherited characteristics.
At times, errors during division can involve the chiasma. If two daughter chromatids fail to separate during meiosis, the product of that particular round of cell division will have an odd number of chromosomes. The exchange of genetic material at this point can also become garbled or confused, resulting in passing down deleterious traits or ending with some cells with an excess of genetic material, while others may be missing pieces of genetic material. In some cases, this can be harmful, as the missing or excess material might be important and could express itself in the form of a genetic defect.
People can see the chiasma with the assistance of microscopy during the process of meiosis, when the chromatids are joined together in a distinctive X shape. This shape is also replicated on charts and graphs depicting meiosis. The term “chiasma,” which comes from the Greek, refers generally to an intersection or crossing. This term is also used in anatomy to refer to bundles of nerves at the point of intersection, like the optic chiasm, where the optic nerves cross each other.