AP Biology · Unit 4 · Topic 4.5
Follow one cell around the cycle with two counters running: how many chromosomes it has, and how much DNA. Watch which one moves at each stage.
Learning objective 4.5.A Describe the events that occur in the cell cycle.
Two counters run the whole time: how many chromosomes the cell has, and how much DNA. Step forward one stage at a time and watch which counter moves. S phase moves only the DNA, anaphase moves only the chromosomes, and cytokinesis halves both.
This is the single most common mistake on this topic. Replication produces a second chromatid, joined to the first at the centromere. One centromere means one chromosome, so a cell that has just replicated all of its DNA still has exactly the number of chromosomes it started with — each one now made of two identical halves.
Step from G1 to S and look at the two counters. The DNA counter doubles. The chromosome counter does not move.
Nothing is copied in anaphase. The chromosome count doubles because the centromeres split, and every chromatid that was half of a chromosome becomes a whole chromosome of its own. The DNA was already there — it was made hours earlier, in S.
Same eight stages along the bottom of both graphs. The step in the top graph and the step in the bottom graph are at different stages — that is the whole idea, and it is much easier to see than to remember.
A flat section of the DNA graph does not mean nothing is happening. G1, G2 and all of mitosis are flat, and they are the stages in which the cell grows, builds the spindle, condenses the chromosomes, lines them up and pulls them apart. The graph is only tracking one quantity.
If a question shows you a DNA-content graph and asks where S phase is, look for the only rising section. If it asks where cell division happened, look for the drop.
The graph plots DNA per cell. When one cell becomes two, the DNA in each of them is half what the single cell held — not because anything was destroyed, but because it was shared out. Plot the total DNA in the dish instead and the line never falls.
Anaphase does not appear on the DNA graph at all. It is inside the flat plateau. That is why a question about chromosome number cannot be answered from a DNA-content graph alone, and why the second graph is drawn here.
You cannot watch one root-tip cell for a day. But you can look at hundreds of cells at one instant, and each of them was caught at a random moment in its cycle. So the fraction of cells you find in a phase is the fraction of the cycle that phase takes up.
| Phase | Cells | Fraction | Duration |
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The arithmetic is one line: duration = (cells in that phase ÷ total cells) × cycle length. Everything else is careful counting.
Switch between the two fields above. The second has twice as many cells in every phase and gives identical durations, because the method uses proportions rather than totals. Counting more cells makes the estimate less noisy; it does not shift it.
It is worth knowing which way the error goes. Anaphase lasts about twenty minutes out of twenty-four hours, so in a field of 200 cells you would expect roughly three of them — and finding two or four instead moves your estimate a long way. Rare phases are the ones a small count gets worst.
Under a microscope this is obvious and slightly disappointing: most of what you see is interphase nuclei with no visible chromosomes. Mitosis is the part of the cycle that looks like something, and it is the small part.
Answer these in your own words before you open the answers.
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| S phase | Anaphase |
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If you can fill this in from memory, the chromosome-counting questions stop being guesswork. Almost every one of them turns on knowing which of these two events has already happened.