Mitosis
Build the cycle, and watch the chromosome number.
Put the whole cell cycle in order — interphase first, then the four phases of karyokinesis, then cytokinesis. The badge shows the chromosome number at every step; follow it all the way down.
Tap the card that comes next
Interphase is NOT a resting phase — it is where the DNA is actually duplicated. Follow the badge: the DNA doubles in S, but the chromosome NUMBER stays at 4 right through prophase and metaphase. It rises to 8 only for the instant of anaphase, and only because each chromatid gains its own centromere — then each daughter cell ends with 4 again. This is why Selina says mitosis keeps the same normal chromosome number: it is describing the daughter cells, not that instant.
Free play — put the stages in order yourself, then play the whole thing through as one continuous process. Switch to when you're ready to test yourself.
What this lesson covers
What you do
- Play it through and watch the badge. Students commonly think the number doubles at prophase or halves at anaphase — the sequence shows it does neither.
Challenges to clear
- Build the whole cycle in the correct order.
- Lay the S phase in its correct place — DNA is copied before division begins, not during it.
The task
Put the whole cell cycle in order — interphase first, then the four phases of karyokinesis, then cytokinesis. The badge shows the chromosome number at every step; follow it all the way down.
What this lesson covers
- G₁ — First Growth Phase — RNA and proteins are synthesised and the volume of cytoplasm increases. The cell is preparing, not resting.
- S — Synthesis Phase — DNA is synthesised and the chromosomes are duplicated. Each chromosome now consists of two chromatids — but the NUMBER of chromosomes has not changed.
- G₂ — Second Growth Phase — A shorter phase in which RNA and the proteins needed for division are made.
- Prophase — Chromosomes become short and thick and are visible as two chromatids joined at the centromere. The nuclear membrane and nucleolus disappear and the spindle forms.
- Metaphase — Chromosomes line up in one plane at the equator, each attached to the spindle by its centromere.
- Anaphase — The centromere holding the two chromatids splits, and the sister chromatids are drawn to opposite poles by the shortening spindle fibres. Strictly, the chromosome count doubles at this instant: a chromosome is defined by having its own centromere, so the moment each chromatid gains one it counts as a chromosome in its own right. No new DNA is made here — the DNA was already copied back in S phase. Each pole receives 4, so every daughter cell still ends up with the diploid number, 4.
- Telophase — The chromatids reach the poles and thin back into chromatin. The nuclear membrane and nucleoli reappear — almost the reverse of prophase.
- Cytokinesis — The cytoplasm divides — by a cleavage furrow in an animal cell, by a cell plate growing outward in a plant cell. Two daughter cells, each with the full diploid number.
Hint
Interphase is NOT a resting phase — it is where the DNA is actually duplicated. Follow the badge: the DNA doubles in S, but the chromosome NUMBER stays at 4 right through prophase and metaphase. It rises to 8 only for the instant of anaphase, and only because each chromatid gains its own centromere — then each daughter cell ends with 4 again. This is why Selina says mitosis keeps the same normal chromosome number: it is describing the daughter cells, not that instant.