Which structure is the site of crossing over between homologous chromosomes during meiosis?

During meiosis, crossing over occurs at a specific region of homologous chromosomes known as the chiasma. This is where chromatids from homologous chromosomes physically exchange segments of genetic material. This process is critical for increasing genetic diversity in the gametes produced, as it creates new combinations of alleles.

The chiasma forms during prophase I of meiosis when homologous chromosomes pair up closely along their lengths, a process known as synapsis. As the chromatids break and reattach, the points at which they cross over become visible as chiasmata under a microscope.

This structure is vital for the proper distribution of genetic material during meiosis and helps ensure that each gamete contains a unique set of genes. This genetic variation is essential for evolution and adaptation in populations over time.

Other structures mentioned do not have the same role in crossing over. For example, the centromere is involved in the separation of sister chromatids during cell division but is not the site of genetic exchange. Spindle fibers help in the movement of chromosomes during cell division, and asters are involved in the organization of the spindle apparatus. However, neither actually participates in the genetic recombination of chromatid segments like the chiasma does.