What is the difference between homologous chromosomes and sister chromatids?

1. Different timing of separation: Sister chromatids are the chromatids located on two different homologous chromosomes during meiosis and appear during prophase I of meiosis. Homologous chromosomes refer to a pair of chromosomes that are similar in size and shape. 2. Structural differences: Segments of sister chromatids may undergo reciprocal exchange (crossing over), which constitutes one mechanism of genetic recombination; the number of homologous chromosomes equals the number of centromeres.