AP Biology · Unit 5 · Topic 5.1
The DNA is copied once and the cell divides twice. Follow the two counters and you can say exactly which division halves what — which is the whole of this topic.
Learning objective 5.1.A Describe the process of meiosis.
The same two counters as the cell cycle, plus a third: how many cells there are. Step forward and watch which counter moves at each stage. The chromosome number halves once, and it is worth noticing exactly where.
Step from Metaphase I to Telophase I and watch the chromosome counter. It goes from six to three, and it never halves again. By the time meiosis II starts, the cells are already haploid — meiosis II is working on cells that have finished being reduced.
The four-cell ending makes meiosis II look like the halving, and that is the trap. Count chromosomes rather than cells and the answer is unambiguous.
Compare Anaphase I and Anaphase II in the drawing. In the first, the objects moving to the poles each have two chromatids and a single centromere: whole chromosomes. In the second, the centromeres have split and single chromatids are moving. If you can say which of those is happening, you can answer almost any question about a meiosis diagram.
Look at the total-DNA counter. It rises once, at S, and then never rises again for the rest of the process — it is only ever divided up. One replication and two divisions is the entire arithmetic of halving, and it is why meiosis needs a second division at all.
Both graphs run along the same ten stages. Watch where the chromosome number comes down and stays down, and where the DNA per cell does. They happen in different divisions, and that is the whole content of this tab. (The chromosome graph also spikes at anaphase II, when centromeres split — cytokinesis undoes that one immediately.)
Two gametes fuse at fertilisation, so whatever number a gamete carries is doubled in the zygote. If gametes carried the full diploid number, the count would double every generation instead of staying constant — which is what the graph above is showing.
An exam graph of DNA per cell through meiosis has one rise and two falls. The rise is S phase. The first fall is cytokinesis at the end of meiosis I; the second is cytokinesis at the end of meiosis II.
The same graph for mitosis has one rise and one fall. If you are handed an unlabelled graph and asked which process it shows, count the falls.
Both begin with one diploid cell that has replicated its DNA once. Everything that differs after that comes from how many times the cell divides and from what each division separates.
| Mitosis | Meiosis |
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It does not. Both processes replicate once. Meiosis divides the product of one replication between four cells instead of two, and that is exactly why each of the four ends with half as much.
If meiosis replicated twice it would produce four diploid cells, and fertilisation would then double the chromosome number every generation. The single replication is not an economy; it is the mechanism.
Meiosis II has no pairing, no crossing over, and separates chromatids rather than chromosomes. It is much closer to mitosis than to meiosis I — the only difference from mitosis is that the cell it acts on is already haploid.
Meiosis produces four cells that are genetically different from the parent and from each other, where mitosis produces two that are identical to it. Two events in meiosis I are responsible — the exchange of segments between paired homologues, and the independent way each pair lines up at the plate. Topic 5.2 counts what those two events actually generate.
Answer these in your own words before you open the answers.
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| Anaphase I | Anaphase II |
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Most meiosis diagram questions are asking you to place a picture in one of these two columns. Count the chromatids on each moving object: two means anaphase I, one means anaphase II.