Advanced Placement (AP) Biology Practice Exam

During which phase of meiosis does crossing over primarily occur?

1. Prophase I

2. Metaphase I

3. Telophase II

4. Interphase

The correct answer is: Prophase I

Crossing over primarily occurs during Prophase I of meiosis. This is a crucial step in the first division of meiosis, where homologous chromosomes pair up in a process called synapsis. While they are paired, segments of chromatids can exchange genetic material, leading to genetic recombination. This increases genetic diversity in gametes, which is essential for evolution and the adaptation of populations over time. In contrast, Metaphase I primarily involves the alignment of homologous pairs at the cell equator, while Telophase II marks the end of meiosis II, where the separated chromatids are packaged into new nuclei. Interphase, although critical for DNA replication and preparation for cell division, is not a phase of meiosis itself and does not include crossing over.