3. What important event occurs during prophase I of meiosis that does not happen in mitosis?
A. replication of DNA
B. crossing over between homologous chromosomes
C. the nuclear envelope disappears
D. formation of spindle fibers
4. What occurs during crossing over?
A. two complete chromatids switch places
B. chromosomes move the other side of the cell
C. portions of homologous chromosomes interchange
D. chromosome number is cut in half