Mitosis

Prophase: Chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and the mitotic spindle (made of microtubules) begins to form from centrosomes moving to opposite poles.

Metaphase: Spindle fibers attach to the centromeres of the chromosomes and align them along the metaphase plate (the cell's equator), ensuring each daughter cell will receive one copy of each chromosome.

Anaphase: The centromeres split, and the sister chromatids are pulled apart toward opposite poles by the shortening spindle fibers; once separated, they are referred to as individual chromosomes.

Telophase: Chromosomes reach the poles and begin to de-condense back into chromatin, while new nuclear envelopes reform around each set of chromosomes, resulting in two distinct nuclei.

Chromosome Counting: Always remember that the number of chromosomes is determined by the number of centromeres. During anaphase, the chromosome count temporarily doubles because sister chromatids become independent chromosomes.

Identifying Phases: Look for specific visual cues: Prophase (messy nucleus), Metaphase (middle alignment), Anaphase (moving away), and Telophase (two distinct clusters).

DNA Content vs. Chromosome Number: DNA mass doubles during the S phase, but the chromosome number remains the same until anaphase. Use the notation $2n$ to track ploidy; mitosis always maintains $2n$ in both daughter cells.

Interphase is not Mitosis: Students often mistake interphase as the first stage of mitosis. In reality, mitosis only begins after interphase is complete.

Chromatid vs. Chromosome: A common error is calling sister chromatids "chromosomes" before they separate. They are only called chromosomes once the centromere splits during anaphase.

Genetic Variation: Mitosis does NOT produce genetic variation (unlike meiosis). Any variation in mitotic daughter cells is the result of rare mutations, not the process itself.