Which of the Following Does Not Occur During Mitosis?

The answer

The events that do not occur during mitosis are the ones unique to meiosis or to interphase. On most exams the correct choice is one of these:

• Synapsis and crossing over of homologous chromosomes — these happen only in meiosis (prophase I), never in mitosis.
• DNA replication — this happens during the S phase of interphase, before mitosis begins, not during mitosis itself.

Mitosis is the part of the cell cycle that physically separates chromosomes that were already copied into two genetically identical daughter nuclei. It does not copy DNA, and it does not shuffle genes between homologous chromosomes.

Why DNA replication is the classic trap answer

Students often assume DNA is copied during mitosis because mitosis produces two cells, each with a full set of DNA. But the copying is done ahead of time. The cell cycle runs interphase (G1 → S → G2) → mitosis (M) → cytokinesis. DNA is duplicated in S phase, giving each chromosome two identical sister chromatids joined at a centromere. Mitosis then simply pulls those sister chromatids apart. So if a question lists "DNA replication" as an option for what happens in mitosis, that option is wrong — replication belongs to interphase.

Why crossing over and synapsis are wrong for mitosis

Synapsis (homologous chromosomes pairing up to form tetrads) and crossing over (exchanging segments between those homologs) are the mechanisms that generate genetic variation in meiosis. Mitosis keeps homologous chromosomes separate and unpaired, which is exactly why it produces two cells that are genetically identical to the parent. If crossing over occurred in mitosis, the daughter cells would no longer be identical — defeating mitosis's whole purpose of faithful copying for growth and repair.

What DOES occur during mitosis

To rule options in and out confidently, know the four phases and their real events:

• Prophase — chromosomes condense; the nuclear envelope breaks down; the spindle forms.
• Metaphase — chromosomes line up single-file along the metaphase (equatorial) plate.
• Anaphase — sister chromatids are pulled apart to opposite poles.
• Telophase — two new nuclear envelopes form; chromosomes decondense.

Cytokinesis then divides the cytoplasm into two cells. Any answer describing these events does occur in mitosis; anything about DNA copying, homolog pairing, gene swapping, or halving the chromosome number does not.

The bigger picture: mitosis vs. meiosis

Mitosis makes two diploid, identical cells for growth and repair, with no crossing over and no change in chromosome number. Meiosis makes four haploid, genetically varied cells (gametes), with synapsis, crossing over, and two divisions that halve the chromosome number. Keeping these two straight is the fastest way to answer any "which does not occur in mitosis" question: if the event creates variation or reduces chromosome number, it belongs to meiosis, not mitosis.