How one cell becomes two, or four
HESI A2 topic: Cellular reproduction
Also covers HESI A2 Anatomy & Physiology (Mitosis and meiosis)
- DNA is copied in the S phase of interphase, before mitosis.
- Mitosis order: prophase, metaphase, anaphase, telophase, then cytokinesis.
- Mitosis makes 2 identical diploid cells for growth and repair.
- Meiosis makes 4 different haploid gametes; in humans, 46 becomes 23.
- Crossing over happens in prophase I; meiosis I splits homologous pairs.
Swipe the picture sideways to see all of it.
Between divisions, the cell is in interphase. Its chromosomes are long, thin threads inside the nucleus. This simplified cell has four chromosomes; a human body cell has 46.
Key facts
- Interphase has three parts: G1 (growth), S (DNA is copied, making sister chromatids), and G2 (final preparation); most of the cycle is spent in interphase.
- Mitosis phases in order: prophase, prometaphase (in some books), metaphase, anaphase, telophase; cytokinesis then splits the cytoplasm.
- In anaphase of mitosis, sister chromatids separate and move to opposite poles.
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- Cytokinesis uses a cleavage furrow in animal cells and a cell plate in plant cells.
- Human body cells are diploid (2n = 46, or 23 pairs); gametes are haploid (n = 23), and fertilization restores the diploid number.
- In meiosis I, homologous chromosomes pair, cross over in prophase I, and separate in anaphase I; in meiosis II, sister chromatids separate.
- Meiosis has one round of DNA copying and two divisions, producing four genetically different haploid cells.
Quick check1 question · 2 more at the end
During which phase of mitosis do sister chromatids separate and move to opposite ends of the cell?
Correct answer: B.
In anaphase, the sister chromatids pull apart and move to opposite poles, so each end of the cell gets a full set of chromosomes. Before that, chromosomes condense in prophase and line up at the metaphase plate in metaphase. In telophase, new nuclear envelopes form around the two sets.
A In prophase, chromosomes condense and the spindle forms; the chromatids are still joined.
B Correct: sister chromatids separate and move to opposite poles in anaphase.
C In metaphase, chromosomes line up at the metaphase plate but have not yet separated.
D In telophase, the chromatids have already reached the poles and new nuclear envelopes form.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: The cell cycle is interphase (G1, S, G2), when the cell grows and copies its DNA, then mitosis and cytokinesis. Mitosis (prophase, metaphase, anaphase, telophase) makes two genetically identical diploid cells for growth and repair. Meiosis divides twice after one DNA copy, making four genetically different haploid gametes; crossing over happens in prophase I.
1 Why do cells divide?Cells divide to grow a body, to replace and repair tissues, and to make offspring. 1 min
Cells divide to grow a body, to replace and repair tissues, and to make offspring. Multicellular organisms use mitosis for growth and for maintaining and repairing tissues, and many single-celled organisms reproduce by dividing.
Meiosis has a different job. It makes gametes (eggs and sperm) with half the usual number of chromosomes.
2 What happens in the cell cycle?A cell spends most of its life in interphase, getting ready, before the short burst of division. 1 min
A cell spends most of its life in interphase, getting ready, before the short burst of division. The full cycle:
| Phase | What happens |
|---|---|
| G1 | The cell grows, makes proteins, and stores energy |
| S | DNA is copied; each chromosome becomes two sister chromatids joined at a centromere |
| G2 | Final preparations; more proteins and some organelles are made |
| Mitosis | The nucleus divides |
| Cytokinesis | The cytoplasm divides into two cells |
Some cells leave the cycle and rest in G0. Mature heart muscle cells and nerve cells stay there permanently.
3 What happens in each phase of mitosis?Mitosis splits one set of copied chromosomes evenly into two nuclei. 1 min
Mitosis splits one set of copied chromosomes evenly into two nuclei. Watch the animation, then check yourself against this list:
- Prophase: chromosomes condense and become visible, the nuclear envelope breaks down, and spindle fibers grow between the two centrosomes. (Some books call the moment spindle fibers grab the chromosomes prometaphase.)
- Metaphase: chromosomes line up single file across the middle of the cell, at the metaphase plate.
- Anaphase: sister chromatids pull apart and move to opposite poles.
- Telophase: a nuclear envelope forms around each set, chromosomes loosen, and the spindle breaks down.
Cytokinesis then divides the cytoplasm. Animal cells pinch in two with a cleavage furrow; plant cells build a cell plate that becomes a new cell wall. The result is two daughter cells that are genetically identical to the parent.
4 What do haploid and diploid mean?Diploid cells carry two sets of chromosomes, one from each parent, and haploid cells carry one set. 1 min
Diploid cells carry two sets of chromosomes, one from each parent, and haploid cells carry one set. Human body cells are diploid (2n) with 46 chromosomes in 23 pairs. Gametes are haploid (n) with 23.
When an egg and sperm join at fertilization, the diploid number is restored.
Keep two terms apart. Homologous chromosomes are a matching pair, one from each parent, with the same genes but possibly different versions. Sister chromatids are two identical copies of one chromosome, made in the S phase.
5 How does meiosis make four different cells?Meiosis copies DNA once and then divides twice, which halves the chromosome number. 1 min
Meiosis copies DNA once and then divides twice, which halves the chromosome number.
Meiosis I separates homologous pairs:
- Prophase I: homologs pair up. Nonsister chromatids swap segments in crossing over.
- Metaphase I: pairs line up at the center, each in a random orientation.
- Anaphase I: homologs separate; sister chromatids stay joined.
Meiosis I is the reduction division: each new cell is haploid. There is no DNA copying before meiosis II.
Meiosis II separates sister chromatids, much like mitosis. Crossing over and random alignment make the four haploid cells genetically different.
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells made | 2 | 4 |
| Chromosome sets | Diploid, same as parent | Haploid, half of parent |
| Genetically | Identical to parent | All different |
| Crossing over | No | Yes, prophase I |
| Purpose | Growth, repair | Gametes |
For how alleles sort into those gametes, see our Mendelian genetics lesson.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: DNA is copied in the S phase of interphase, not during mitosis itself.
Watch out: Meiosis I separates homologous chromosomes; sister chromatids stay joined until meiosis II.
Watch out: DNA is not copied again between meiosis I and meiosis II.
Watch out: Mitosis keeps the chromosome number the same (46 to 46); meiosis halves it (46 to 23).
Watch out: Crossing over happens in prophase I of meiosis, not in mitosis.
How to remember it
Memory hook: "PMAT" (prophase, metaphase, anaphase, telophase): in Metaphase chromosomes line up in the Middle, and in Anaphase they move Away. For meiosis: "I splits pairs, II splits twins."
Self-check:
- Say in which phase DNA is copied, and what each chromosome looks like afterward.
- A cell with 2n = 8 finishes meiosis. How many chromosomes does each new cell have?
- Name the stage where crossing over happens and why it matters.