HighSchoolQ

🧬 Biology · Genetics

Mitosis and the Cell Cycle: notes and common mistakes

Study notes

Key ideas

  • The cell cycle is interphase (G1, S, G2) plus M phase (mitosis and cytokinesis). Interphase is not a rest, and in a typical dividing cell it takes up much more of the cycle than M phase.
  • DNA is copied in S phase, not in prophase. After S, each chromosome is two sister chromatids joined at a centromere, but it is still one chromosome.
  • Mitosis is PMAT. Prophase: chromosomes coil up and the nuclear envelope breaks down. Metaphase: chromosomes line up in the middle. Anaphase: sister chromatids separate and move to opposite poles. Telophase: two nuclei re-form.
  • Cytokinesis divides the cytoplasm: a cleavage furrow in animal cells, a cell plate in plant cells. In the strict model it is separate from mitosis, and it overlaps late mitosis.
  • Mitosis makes two genetically identical daughter cells with the same chromosome number as the parent cell: 46 in a human body cell, not 23.
  • Count chromosomes by counting centromeres. Count chromatids only while the sister chromatids are still joined.

Steps

  1. Ask first: has the cell copied its DNA yet? If not (G1), there are no sister chromatids, and the chromosome number is the starting number (46 in a human body cell).
  2. Ask next: have the sister chromatids separated yet? They separate at the start of anaphase.
  3. If the copying is done but the sisters have not separated (end of S phase through metaphase), count centromeres to get the chromosomes (46). Then double that to get the chromatids (92).
  4. If the sisters have separated (anaphase and telophase), count each chromatid as a chromosome: 92 in the whole cell, 46 at each pole, until cytokinesis is complete.
  5. Once cytokinesis is complete, count per cell: each daughter cell has 46 chromosomes.

Formulas

  • Chromatids while sisters are joined: chromatids = 2 × chromosomes (end of S phase through metaphase)
  • Chromosomes in the cell in anaphase: 2 × the number in G1 (for example 46 → 92)
  • Chromosomes in each daughter cell: the same as the parent cell had in G1 (for example 46 → 46)

Watch out

  • Mitosis is not the whole cell cycle. Interphase takes up much more of it.
  • Chromosomes are copied in S phase, not in prophase.
  • The daughter cells from mitosis do not have half the chromosomes. They have the same number as the parent. Halving the number is what meiosis does.
  • Cytokinesis is not one of the four PMAT stages. It overlaps late anaphase and telophase.
  • An X-shaped chromosome is one chromosome, not two. It becomes two chromosomes only after the sister chromatids separate in anaphase.

Why it works

  • Why copy the DNA before dividing? If a cell split its DNA without copying it, each daughter cell would get only part of the instructions. Copying first gives each cell a full set.
  • Why does mitosis matter? It lets an organism grow, heal and replace worn-out cells while keeping the same DNA in each cell, and it lets some organisms reproduce without a partner.
  • Why learn to count carefully? The counts are a quick test of whether you understand what each stage does. If your numbers are wrong, the model in your head is wrong.

Common mistakes

✗ MistakeMitosis is the whole cell cycle.
✓ InsteadThe cell cycle is interphase (G1, S, G2) plus M phase (mitosis and cytokinesis). In a typical dividing cell, interphase takes up much more of the cycle than M phase.
✗ MistakeThe chromosomes are copied in prophase.
✓ InsteadThe DNA is copied in S phase of interphase. By prophase each chromosome already has two sister chromatids, so a human cell in prophase has 46 chromosomes and 92 chromatids.
✗ MistakeThe daughter cells from mitosis each get half of the chromosomes.
✓ InsteadEach daughter cell gets one complete copy of every chromosome, so it has the same number as the parent. Halving the number is what meiosis does.
✗ MistakeCytokinesis is the fifth stage of mitosis.
✓ InsteadIn the strict textbook model, mitosis means division of the nucleus, and cytokinesis is a separate process that divides the cytoplasm and overlaps late mitosis. Some books count it as a last step of M phase, so check which model your book uses.
✗ MistakeInterphase is a resting stage between divisions.
✓ InsteadThe cell is busy in interphase. It grows, does its normal job, and copies its DNA in S phase.
✗ MistakeA cell with 46 chromosomes has 92 chromosomes after S phase.
✓ InsteadAfter S phase there are still 46 chromosomes. Each now has two sister chromatids, so there are 92 chromatids. The cell holds 92 chromosomes only from the start of anaphase until cytokinesis is complete.
✗ MistakeThe two sister chromatids of an X-shaped chromosome are two chromosomes.
✓ InsteadWhile the sister chromatids are joined at a centromere, they are one chromosome. Each becomes its own chromosome after it separates in anaphase.

Teach it back

Explain to a friend (or a pet) how one human skin cell with 46 chromosomes becomes two cells that each have 46 chromosomes. Say when the DNA is copied, what happens in each of the four stages, how the cytoplasm divides, and how many chromosomes and chromatids you would count at metaphase and at anaphase.

Explaining it out loud shows what you really know. Did you…

  • I said that the DNA is copied in S phase of interphase, before mitosis starts, so each chromosome has two sister chromatids.
  • I named prophase, metaphase, anaphase and telophase in order and gave one key event for each.
  • I said that the sister chromatids separate in anaphase and that each is then counted as a chromosome.
  • I gave the counts: 46 chromosomes and 92 chromatids at metaphase, 92 chromosomes in the cell at anaphase, and 46 in each daughter cell.
  • I said that cytokinesis divides the cytoplasm (a furrow in animal cells, a cell plate in plant cells) and that it overlaps late mitosis.

Your 3-week memory plan

  1. TodayRead the lesson, then try the flashcards.
  2. TomorrowRecall it from memory first, then check.
  3. In 3 daysTake the true/false and practice questions.
  4. In 1 weekMix it with older topics and teach it back.
  5. In 3 weeksA last quick review before it fades.

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