Chiasmata

Chiasmata (singular: chiasma) are the physical X-shaped junctions where homologous chromosomes have exchanged genetic material during crossing over. They become visible in prophase I of meiosis and represent the sites where chromatids from homologous chromosomes are still attached after swapping segments.

• Chiasmata hold homologous chromosomes together until anaphase I. This helps ensure proper segregation - each homologous pair must have at least one chiasma for meiosis I to proceed correctly.
• Under a microscope, chiasmata look like X-shaped overlaps between homologous chromosome pairs. These are literally the crossover points.
• Terminology tip: use "chiasma" for a single crossover point and "chiasmata" for multiple. (Don't confuse chiasma with the similarly named "optic chiasm," which is unrelated and refers to crossing of optic nerves.)

• An exam might show an image of paired chromosomes in meiosis I and point to the X-shaped connection asking "What is this structure?" The answer is a chiasma (the site of crossing over).
• Genetics problems on meiosis sometimes mention that homologous chromosomes remain connected at chiasmata. If a question asks how the homologous chromosomes stay together until Anaphase I, citing chiasmata (from crossing over) is the key.