Final answer:
After the nuclear envelope breaks down in mitosis, 3. the chromosomes line up at the equator of the cell during metaphase, ensuring that each daughter cell will receive an identical set of chromosomes.
Step-by-step explanation:
In mitosis, after the nuclear envelope breaks down, the chromosomes do not pair up as homologous chromosomes. This type of pairing is specific to meiosis during prophase I. In contrast, during mitotic division, each chromosome is already duplicated as sister chromatids held together at the centromere. The correct event that first happens to chromosomes after the breakdown of the nuclear envelope in mitosis is Option 3: They line up at the equator of the cell.
This stage is known as metaphase, and it is characterized by all the chromosomes aligning along the metaphase plate, an imaginary line equidistant from the two spindle pole extremities. This alignment ensures that when the chromosomes are subsequently separated into sister chromatids during anaphase, each new daughter cell will receive an identical set of chromosomes.