The shape of the lesson
By the end of the lesson, learners can
- Eight stages, two divisions: Press Next stage and watch the two homologous pairs (red from one parent, blue from the other).
- Three sources of variation: Choose crossing over and press Run it to see the chiasma and the four chromatids AB, Ab, aB and ab.
- Spotting the stage in a photomicrograph: In exam photomicrographs and diagrams you identify the stage from where the chromosomes are and how many groups there are.
- Meiosis I separates homologous chromosomes: 2n becomes n.
- Meiosis II separates sister chromatids, as in mitosis.
- Crossing over: non-sister chromatids, prophase I, chiasmata.
- Combinations: 2n gametes; random fertilisation multiplies them.
One picture every idea returns to
A diploid (2n) cell has two complete sets of chromosomes, one set from each parent; a haploid (n) cell has one set. The two chromosomes of a homologous pair have the same genes at the same loci (but possibly different alleles), the same length and the same centromere position. Gametes must be haploid so that fertilisation restores the diploid number instead of doubling it each generation, so they are made by meiosis, a reduction division. In meiosis I homologous chromosomes pair up and are separated; in meiosis II sister chromatids are separated, giving four haploid cells. Gametes differ genetically because of crossing over between non-sister chromatids in prophase I, independent assortment (random orientation) of homologous pairs at metaphase I and of sister chromatids at metaphase II, and then random fertilisation.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Halve the chromosomes, shuffle the alleles.. Every gamete you made carries just one chromosome from each of your 23 pairs, and no two gametes carry quite the same set. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Eight stages, two divisions | Press Next stage and watch the two homologous pairs (red from one parent, blue from the other). Project tab 1 of the lab (Inheritance lab, the stages of meiosis) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Three sources of variation | Choose crossing over and press Run it to see the chiasma and the four chromatids AB, Ab, aB and ab. Project tab 2 of the lab (Inheritance lab, sources of genetic variation) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Spotting the stage in a photomicrograph | In exam photomicrographs and diagrams you identify the stage from where the chromosomes are and how many groups there are. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Spotting the stage in a photomicrograph, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not mix up the two anaphases. | In anaphase I whole homologous chromosomes separate (each still has two chromatids); in anaphase II the sister chromatids separate, just as in mitosis. The chromosome number halves in meiosis I, not meiosis II. |
| Crossing over happens between non-sister chromatids of a homologous pair, in prophase I. | Sister chromatids are identical copies, so swapping between them would change nothing. |
Meiosis and genetic variation: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Explain the meaning of the terms haploid, diploid and homologous pair of chromosomes.
Describe the behaviour of the chromosomes, the nuclear envelope and the spindle during prophase I and metaphase I of meiosis in an animal cell.
State three ways in which the appearance of a plant cell in telophase II of meiosis differs from that of an animal cell at the same stage.
The fruit fly Drosophila has 2n = 8. Calculate the number of genetically different gametes one fly can produce by independent assortment alone, and the number of different zygotes possible from two flies.
Explain how meiosis and fertilisation together make the offspring of the same two parents genetically different.
The DNA content of a cell (arbitrary units) is measured through meiosis: G1, 10; after S phase, 20; after telophase I, 10; after telophase II, 5. Explain each change.
Answer key and marking notes
- Haploid (n): a cell or nucleus with one complete set of chromosomes.
- Diploid (2n): a cell or nucleus with two complete sets of chromosomes, one from each parent.
- Homologous pair: two chromosomes with the same genes at the same loci (same length and centromere position), one maternal and one paternal; they may carry different alleles.
- Prophase I: chromosomes condense / become visible as two sister chromatids; homologous chromosomes pair up to form bivalents.
- Crossing over may occur between non-sister chromatids at chiasmata; the nuclear envelope breaks down.
- Centrioles move to opposite poles and the spindle forms.
- Metaphase I: bivalents line up on the equator, attached to spindle fibres by their centromeres, each pair oriented at random.
- The plant cell divides by a cell plate forming across the middle; the animal cell by the membrane pinching in (cleavage furrow).
- The plant cell has a cell wall around the dividing cells.
- The plant cell has no centrioles at the poles of the spindle.
- n = 4, so gametes = 24 = 16.
- Zygotes = 16 × 16 = 256 (ignoring crossing over).
- Crossing over between non-sister chromatids in prophase I produces new combinations of alleles on chromatids.
- Independent assortment / random orientation of homologous pairs at metaphase I gives each gamete a different mix of maternal and paternal chromosomes.
- Random orientation of (non-identical) sister chromatids at metaphase II adds further variation.
- Random fertilisation: any one of the many types of male gamete can fuse with any one of the many types of female gamete.
- 10 to 20: DNA replicates in S phase of interphase, so each chromosome has two chromatids.
- 20 to 10: in meiosis I homologous chromosomes separate into two cells (reduction division).
- 10 to 5: in meiosis II sister chromatids separate into four cells; each gamete has half the DNA of the original G1 cell.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement correctly describes the two chromosomes of a homologous pair in a human body cell?
- Athey carry the same genes at the same loci, but may have different alleles
- Bthey are identical copies made when DNA replicates in S phase
- Cthey carry different genes but are the same length
- Dthey both came from the same parent and have the same alleles
During which stage of meiosis do sister chromatids separate and move to opposite poles?
- Aanaphase II
- Banaphase I
- Cmetaphase II
- Dtelophase I
A diploid plant cell has 2n = 24. How many chromosomes are present in each nucleus at the end of telophase II?
- A12
- B24
- C6
- D48
In which way does meiosis in an animal cell differ from meiosis in a plant cell?
- Ain animal cells centrioles organise the spindle and the membrane pinches in to divide the cell
- Bin animal cells no spindle forms and the chromosomes move by themselves
- Cin animal cells a cell plate forms between the two new nuclei
- Din animal cells the nuclear envelope does not break down during prophase
Which process during meiosis can separate two alleles that were on the same chromatid?
- Acrossing over between non-sister chromatids in prophase I
- Bindependent assortment of homologous pairs at metaphase I
- Cseparation of sister chromatids at anaphase II
- Drandom fertilisation of the gametes
The graph shows the DNA content of a cell during meiosis. Which row correctly identifies events P, Q and R?
- AP DNA replication; Q homologous chromosomes separate into two cells; R sister chromatids separate into four cells
- BP crossing over; Q sister chromatids separate into two cells; R homologous chromosomes separate into four cells
- CP DNA replication; Q sister chromatids separate into two cells; R homologous chromosomes separate into four cells
- DP crossing over; Q homologous chromosomes separate into two cells; R DNA replication
A species has 2n = 10. Ignoring crossing over, how many genetically different zygotes could two parents produce through independent assortment and random fertilisation?
- A1024
- B32
- C100
- D64
A photomicrograph shows a single cell with chromosomes in pairs, lined up across its middle, with spindle fibres attached. Which stage is shown and why?
- Ametaphase I, because homologous chromosomes are on the equator as bivalents
- Bmetaphase II, because chromosomes are lined up on the equator
- Canaphase I, because homologous pairs are attached to the spindle
- Dprophase I, because the chromosomes are in pairs
Quiz key
Homologous chromosomes have the same genes at the same loci, the same length and centromere position, one from each parent, so their alleles can differ. Identical copies made in S phase are sister chromatids, not homologues. Homologues carry the same genes, not different ones. They come from different parents, so their alleles need not match.
Centromeres divide in anaphase II, so sister chromatids separate then. In anaphase I homologous chromosomes separate, each still with two chromatids. At metaphase II chromosomes are only lined up on the equator. Telophase I is the end of the first division, when the two haploid cells form.
Meiosis produces haploid cells with n = 12. 24 is the diploid number, kept only by mitosis. 6 would mean halving twice, but meiosis II separates chromatids and does not change the chromosome number. 48 would be a cell after replication counted as chromatids, or a polyploid.
Animal cells have centrioles at the poles and divide by a cleavage furrow. Both types of cell form a spindle. The cell plate is the plant method. The nuclear envelope breaks down in prophase in both.
Crossing over swaps sections of non-sister chromatids, so linked alleles can be recombined. Independent assortment shuffles whole chromosomes, not alleles on one chromosome. Anaphase II separates sister chromatids, moving each whole. Random fertilisation combines gametes but does not alter any chromatid.
DNA doubles at P during S phase replication. It halves at Q at the end of meiosis I, when homologues go into two cells, and halves again at R in meiosis II, when chromatids go into four cells. Crossing over exchanges DNA but does not change the amount. Chromatids separate in meiosis II, not meiosis I. R is a fall, not the rise that replication would give.
n = 5, so each parent can make 25 = 32 types of gamete, and 32 × 32 = 1024 zygote combinations. 32 is for one parent only. 100 squares 2n instead of using powers of 2. 64 adds the gametes of the two parents (32 + 32) instead of multiplying.
Bivalents on the equator of a single cell is metaphase I. At metaphase II the chromosomes are single, and two cells are present. In anaphase I the homologues are moving apart towards the poles. In prophase I the pairs have formed but are not yet lined up on the equator.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.