The Cell Cycle, Mitosis and Meiosis
General Biology · Section 2.4 · 18 study cards
Chromosome vocabulary, the phases of the cell cycle, how mitosis and meiosis differ, and where variation and aneuploidy come from.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Count chromosomes by centromeres, always
Nearly every numerical question on this unit is solvable with one rule: the number of chromosomes equals the number of centromeres, and two sister chromatids sharing a centromere are one chromosome. DNA content is tracked separately, by counting chromatids. Write both numbers at every stage and the confusion disappears.
A procedure for any stage-counting question
- Write down 2n for the organism and therefore the G1 chromosome count.
- Pass through S phase: chromosome number unchanged, chromatid number doubled.
- For each stage asked about, decide whether centromeres have split yet. Before anaphase of mitosis or anaphase II they have not; after, they have, and the chromosome count doubles.
- For meiosis, remember there is no second S phase. Meiosis I halves the chromosome number, meiosis II halves the chromatid number per chromosome.
- State the answer as chromosomes per cell and chromatids per cell, and say which cell you mean, since anaphase counts refer to one cell that is about to become two.
Let the anaphases define the divisions
If you remember only one thing about meiosis, make it this: anaphase I separates homologues, anaphase II separates sister chromatids, and mitotic anaphase separates sister chromatids. That single sentence tells you which division is reductional, why meiosis II resembles mitosis, and what nondisjunction at each point produces.
Regulation is about permission, not machinery
Treat the checkpoint material as a separate question: not how does the cell divide, but is it allowed to. Each checkpoint is a specific test with a specific failure consequence, and cyclin-Cdk complexes are the switches that carry the decision. Cancer then reads as the checkpoints being ignored, which is why it belongs in this unit at all rather than in a chapter of its own.
Definitions and theorems
Worked example
An organism has a diploid number of 2n = 8. For a single cell of this organism, state the number of chromosomes and the number of chromatids present in G1, in G2, at metaphase of mitosis, at the end of anaphase of mitosis, in each daughter cell at the end of meiosis I, and in each cell at the end of meiosis II. Then explain what a karyotype of a gamete would show if nondisjunction of one pair occurred at anaphase I.
G1. Nothing has replicated. Eight chromosomes, each a single chromatid, so eight chromosomes and eight chromatids.
G2, after S phase. Every chromosome has been copied, but the copies share a centromere, so the chromosome count is unchanged. Eight chromosomes and sixteen chromatids.
Metaphase of mitosis. The chromosomes are lined up single file but no centromere has split. Still eight chromosomes and sixteen chromatids.
End of anaphase of mitosis. Centromeres have divided and each chromatid is now an independent chromosome. The one cell contains sixteen chromosomes and sixteen chromatids, eight of each heading to each pole, which is why each daughter cell ends up at eight and eight.
End of meiosis I. Homologues have been separated, so each daughter cell has one of each pair, four chromosomes. No centromere split, so each of those still has two chromatids: four chromosomes and eight chromatids per cell, and the cell is now haploid.
End of meiosis II. Sister chromatids separate with no intervening replication. Each of the four products has four chromosomes and four chromatids.
Nondisjunction at anaphase I. One pair of homologues travels to the same pole, so two of the four gametes carry five chromosomes and the other two carry three. A karyotype of an affected gamete would show one chromosome type present twice, or absent altogether, against a normal single copy of the other three types. Because the error happened in the first division, all four products are abnormal, whereas an error at anaphase II would leave two of them normal.
Common mistakes
- Counting chromatids as chromosomes. A replicated chromosome with two sister chromatids is one chromosome, because it has one centromere. Students who count forty-six chromatids as forty-six chromosomes in G2 get every subsequent number wrong, and this single rule is worth more marks than any other on the topic.
- Confusing sister chromatids with homologues. Sister chromatids are identical; homologues merely carry the same genes and often different alleles. Since meiosis I separates homologues and both mitosis and meiosis II separate sister chromatids, mixing the terms makes it impossible to say which division a described event belongs to.
- Inserting an S phase between meiosis I and meiosis II. There is none. DNA is replicated once and the cell divides twice, which is exactly how the chromosome number is halved, and assuming a second replication gives diploid gametes.
- Saying crossing over happens between sister chromatids. It happens between non-sister chromatids of homologous chromosomes. Exchange between identical sisters would produce no new combination of alleles, so it would not be a source of variation at all.
- Treating cancer as unlimited growth rather than failed control. The cells are not growing faster in any simple sense; they are dividing without the permissions that normally apply, having lost checkpoint control, density-dependent inhibition and anchorage dependence. Answers phrased as uncontrolled division earn credit only when the specific control that failed is named.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.