Answer: Sister chromatids from two homologous chromosomes exchange DNA
Explanation: Before meiosis occurs, chromosomes within a cell are duplicated. Humans naturally carry two copies of each chromosome. When duplication occurs, each chromosome and its duplicated copy is naturally bound to each other via telomeres. These bound chromosomes are known as sister chromatids. Meanwhile, each pair of chromosomes and its duplicated copy are known as homologous pairs.
Crossing over occurs in the step between prophase 1 and metaphase 1. After the chromosomes have been duplicated, homologous chromosomes pairs line up and swap parts of their chromosomes. By doing so, the gametes of meiosis have increased genetic diversity, thus making them more likely to survive environmental changes.