Question

1. what is the defining event in prophase?

sister chromatids break apart and move to the opposite ends of the cell.
chromosomes line up in the middle of the cell.
nuclear membrane forms around the two sets of chromatids at the far ends of the cell.
dna condenses to chromosomes. the nuclear membrane breaks down.

Explanation:

1. Analyze the first option: Sister chromatids breaking apart and moving to opposite ends occurs in anaphase, not prophase. So this option is incorrect.
2. Analyze the second option: Chromosomes lining up in the middle of the cell is the event of metaphase, not prophase. So this option is incorrect.
3. Analyze the third option: The formation of the nuclear membrane around chromatids at the ends of the cell happens in telophase, not prophase. So this option is incorrect.
4. Analyze the fourth option: During prophase, the chromatin (DNA) condenses into visible chromosomes, and the nuclear membrane breaks down, which are defining events of prophase.

Answer:

The correct option is: DNA condenses to Chromosomes. The nuclear membrane breaks down.