In Prophase I, which events occur?

In Prophase I, which events occur?

• A. Nucleus dissolves and crossing over occurs
• B. Nucleus reforms and DNA replicates
• C. Nuclear envelope reappears and spindle breaks down
• D. Chromatids split and move to opposite poles

Answer: (A)

In prophase I, chromosomes condense and homologous chromosomes pair up (synapsis) while crossing over occurs between non-sister chromatids. At the same time, the nuclear envelope breaks down and the spindle apparatus forms, so the nucleus essentially dissolves. This combination of events—nuclear envelope breakdown and crossing over—is characteristic of this stage. DNA replication happens earlier during S phase, not during prophase I, and the nuclear envelope reappears or the spindle breaks down at later stages. Chromatids splitting and moving to opposite poles happens during anaphase, not prophase I.

In prophase I, chromosomes condense and homologous chromosomes pair up (synapsis) while crossing over occurs between non-sister chromatids. At the same time, the nuclear envelope breaks down and the spindle apparatus forms, so the nucleus essentially dissolves. This combination of events—nuclear envelope breakdown and crossing over—is characteristic of this stage. DNA replication happens earlier during S phase, not during prophase I, and the nuclear envelope reappears or the spindle breaks down at later stages. Chromatids splitting and moving to opposite poles happens during anaphase, not prophase I.