What is crossing over of homologous chromosomes? When does it happen? What does it lead to?

This is a mechanism that works while the cell is dividing, by which homologous chromosomes are changed by strips of each of the chromosomes, which are called chromatids. (At the end of the first prophase of meiosis, contact is maintained between the various at a number of points called chiasmas. As a result, there is an exchange of chromatid regions. This leads to a change in the distribution of genes in chromosomes, and hence to genetic variants of the created gemetes. This is a significant factor in evolution).