Question

In what stage of meiosis is crossing over most likely to occur?
Answer
Prophase I
Metaphase I
Anaphase II
Prophase II

Answer 1

I think it's Prophase I

Answer 2

Crossing over does indeed usually occur during Prophase I; specifically during the Pachytene ("thick threads") substage, once homologous chromosomes form synaptonemal complexes. Homologous chromosomes are bound together and exchange some DNA with non-sister chromatids over regions of homology, within small structures called chiasma. Chromosomal mutations can also occur during this stage if the transfer of DNA is nonreciprocating or chiasma are malformed, e.g., if chiasma form between nonhomologous regions of DNA. These chiasma remain bound as the synaptonemal complex disperses and the diplotene substage of Prophase I begins.