Mitosis and Meiosis, Grade 12: Stages, Comparison and the Slide Practical
Grade 12 material tells learners that homologous chromosomes carry the same genetic information, and that crossing over swaps identical segments. Later in the same topic it explains that gametes carry different alleles, which is the opposite.
Both cannot be true. If homologous chromosomes carried the same information, crossing over would change nothing, every gamete would be identical, and alleles would not exist. The whole of Grade 12 genetics rests on them being different.
This page gives the stages of meiosis in order, the comparison with mitosis that the exam actually asks for, the slide practical, and the one word in each place that fixes the contradiction.
Where this fits in the curriculum
| Subject | Life Sciences |
|---|---|
| Grade | 12 |
| Term | 1 |
| Topic | Topic 2, Meiosis |
| Status | No formally assessed practical. Carries a 20-mark slide activity |
| Marks | 20 for the slide activity. Our worksheet is 80 |
Meiosis I, in six stages
The first division is the one that halves the chromosome number. Everything that makes meiosis different from mitosis happens here.
| Stage | What happens |
|---|---|
| Early prophase I | Chromosomes become visible as threads, shorten and thicken. Homologous pairs lie alongside one another |
| Late prophase I | Chromosomes attach to spindle fibres by their centromeres. The nuclear membrane disappears. Crossing over happens |
| Crossing over | Chromatids touch at points called chiasmata and break there, exchanging segments |
| Metaphase I | Homologous pairs line up on the equator |
| Anaphase I | Spindle fibres pull the homologous pairs apart. One whole chromosome of each pair goes to each pole |
| Telophase I | One complete set reaches each pole. Two new cells form, each haploid |
Meiosis II, in five stages
The second division is essentially mitosis, run on both of the cells the first division made.
| Stage | What happens |
|---|---|
| Prophase II | Each chromosome is two chromatids joined at a centromere. Spindle fibres form in each cell |
| Metaphase II | Single chromosomes line up on the equator |
| Anaphase II | The centromeres split. Chromatids are pulled to opposite poles |
| Telophase II | Chromatids, now called daughter chromosomes, reach the poles. Nuclear membranes re-form |
| Cytokinesis | Four haploid cells in total from the one original parent cell |
The line that separates the two divisions in one sentence: in anaphase I whole chromosomes separate, in anaphase II chromatids separate. A learner who can say that can answer most of what this topic asks.
The contradiction at the centre of this topic
Printed in the crossing over panel: the homologous chromosomes exchange identical segments of the chromatids.
Printed in the key words: homologous pairs are chromosomes which carry the same genetic information.
Printed later in the same topic: not all the gametes carry identical genes for a characteristic, and variations of the same gene are called alleles.
| If the first two were true | What we actually observe |
|---|---|
| Crossing over would swap identical pieces, so it would change nothing | Crossing over is one of the two main sources of genetic variation |
| Every gamete from one parent would be the same | Siblings from the same parents are not identical |
| There would be no dominant or recessive versions of a gene | Alleles are the entire basis of Topic 5 |
The repair is one word in each place.
| What is printed | What is correct |
|---|---|
| Homologous chromosomes carry the same genetic information | They carry genes for the same characteristics, at the same positions. The versions of those genes, the alleles, are usually different |
| Crossing over exchanges identical segments | It exchanges corresponding segments. Same position on the chromosome, different alleles on it, which is exactly why the result is new combinations |
Ask the class this and the point lands in ten seconds: if your mother's chromosome 9 and your father's chromosome 9 carried the same information, why do you have one blood group rather than a blend of theirs?
Mitosis and meiosis, the comparison the exam asks for
| Mitosis | Meiosis | |
|---|---|---|
| Where | Somatic cells | Sex cells |
| Nuclear divisions | One | Two |
| Daughter cells | Two | Four |
| Daughter cells are | Identical to each other and to the parent | Non-identical |
| Chromosome number | Stays the same | Halved |
| Homologous pairs | Do not pair up | Pair up |
| Crossing over | Does not occur | Occurs |
| On the equator | Single chromosomes | Homologous pairs in metaphase I, single chromosomes in metaphase II |
| The centromere | Divides | Does not divide in the first division. It divides in the second |
What they share: both form daughter cells from a parent cell, both happen in the nucleus, and both run through prophase, metaphase, anaphase and telophase.
The practical: observe meiosis, 20 marks
Prepared slides and a microscope. This is the one activity in the topic and it is worth doing properly.
- Find the chromosomes at 40x first, then move up to 100x. Going straight to high power is why classes lose the cells
- Identify the stages you can actually see. A real slide does not show all of them, and it does not show them in order
- Make labelled drawings of five stages: metaphase I, anaphase I, telophase I, metaphase II and telophase II. Four marks each
Anther and pollen material is the standard choice, because a single anther contains cells caught at every stage at once. Root tip shows mitosis, not meiosis, which is the mistake most often made when ordering slides.
| What to buy | Why |
|---|---|
| Cell Division slide set of 9 | The one to buy. Nine slides covering the division stages, enough for groups to work from different cells at once |
| Mitosis and Meiosis slide set | The cheaper option, and it covers both processes this page compares |
| A biological microscope | Needs 100x. Any compound student microscope reaches it |
If you have no slides at all, the activity still runs from micrographs, models or posters, and the marks are awarded for the drawings either way.
What you should see
- Cells caught at different stages side by side in one field of view, which is the whole advantage of anther tissue
- Dark stained chromosomes against a paler cytoplasm. The stain is what makes them visible at all
- Chromosomes as distinct bodies only from late prophase onward. Before that they are a diffuse network
- Far more cells in prophase than anywhere else, because prophase takes the longest
If it does not work
| What happens | What caused it |
|---|---|
| No chromosomes visible anywhere | The slide is a root tip, not an anther. Root tips show mitosis. Check what you ordered |
| The field is empty at 100x | The cells were never centred at 40x. Find them low, then go up |
| Every cell looks the same | You are in interphase tissue. Move to a different part of the slide |
| The image is dark at high power | The condenser and the iris, not the lamp. Open the iris as magnification goes up |
| Learners cannot tell metaphase I from metaphase II | Count what is on the equator. Pairs means first division, singles means second |
Two more things the printed material gets wrong
| What is printed | What is correct |
|---|---|
| "All the cells in the body of an organism contain the same number of chromosomes" | The next sentence excludes sex cells, so the first one cannot stand. It is true of somatic cells. Mature red blood cells have no nucleus and no chromosomes at all, and gametes have half the number. Say somatic and the sentence becomes correct |
| Trisomy gives "a chromosome number of 2n + 1, except for chromosomes 13, 18, and 21" | The exception is attached to the wrong thing. Trisomy 13, 18 and 21 are all 2n + 1 like any other. What makes them exceptional is that a child can be born with them, where most other trisomies are not survivable. As printed it states something false about chromosome number |
And two statistics on the same page do not parse. "In 35 percent of cases, these abnormalities are the most common cause of pregnancy failure" mixes a frequency with a ranking: being the most common cause is not something that happens in a percentage of cases. The same sentence shape appears again for oogenesis. If a learner is asked to quote a figure from either, there is no figure in there to quote.
Abnormal meiosis, in the order it is examined
| Term | What it means |
|---|---|
| Non-disjunction | Chromosomes fail to separate during anaphase. In meiosis I a homologous pair fails to separate, in meiosis II the sister chromatids do |
| The result | Gametes with one chromosome too many or too few, so the zygote has an unequal number |
| Trisomy | Three of the same chromosome, 2n + 1. Trisomy 21 causes Down syndrome |
| Monosomy | One chromosome missing. Usually not survivable, except Turner syndrome, a single X |
| Karyotype | The number and structure of the chromosomes in a nucleus, set out in numbered pairs. The extra chromosome at pair 21 is what you are looking for |
Safety
Low hazard. This is a microscope lesson.
- Prepared slides are glass. Carry one at a time, and report breakages rather than clearing them by hand
- Rack the objective down while watching from the side, never while looking through the eyepiece, or the lens goes through the slide
- Do not use the coarse focus at 100x. Fine focus only
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 80 marks, covering both meiotic divisions, the slide drawings, the mitosis comparison and abnormal meiosis
- Marking memorandum, with the comparison table marked line by line and the five answers that look right and score nothing
Related practicals
- The nervous system and the senses, Grade 12. The two assessed practicals in this grade, reaction times and the eye dissection
- Plant and animal cells under a microscope, Grade 9. Where the microscope skill this activity assumes is actually taught, including making a wet mount
- Variation, Grade 7. Where variation is first met, five years before meiosis explains where it comes from
- Human reproduction and puberty, Grade 7. Gametes, long before gametogenesis has a name
Buy this experiment
One slide set and a microscope, and the topic is covered.
The Cell Division slide set of 9 at R325 is the one we would buy. Nine slides means groups work on different cells rather than queueing at one scope.
The Mitosis and Meiosis slide set at R190,96 is the cheaper route and covers both processes, which suits a class that only needs one pass through the topic.
Any compound microscope that reaches 100x will do this. If you are buying, the microscope range starts well below what most schools expect to pay, and the same instrument runs the Grade 9 cells practical.