Biology • Year 12 • Module 5 • Lesson 8
Meiosis — Reduction Division and Continuity Across Generations
Lock in the core vocabulary of homologous pairs, diploid/haploid, meiosis I vs II, crossing over and independent assortment — and the role of reduction division in keeping chromosome number stable across generations.
1. Label the meiosis-and-fertilisation cycle
The diagram below tracks one diploid parent cell through meiosis I, meiosis II and fertilisation across one generation. Write the missing labels into boxes A–H. Each label is drawn from the lesson's Key Terms or from Cards 1–4. 8 marks
- A — ploidy of the starting parent cell (___oid) _______________________
- B — name of the division that separates homologous pairs (meiosis ___) _______________________
- C — name of the division that separates sister chromatids (meiosis ___) _______________________
- D — number of daughter cells produced at the end of meiosis _______________________
- E — ploidy of each gamete (___oid) _______________________
- F — variation event: exchange of segments between homologous chromosomes _______________________
- G — variation event: random orientation of homologous pairs before separation _______________________
- H — ploidy of the zygote after fertilisation (___oid) _______________________
| Box | Your label |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F | |
| G | |
| H |
2. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: meiosis, homologous chromosomes, diploid, haploid, crossing over, independent assortment, sister chromatids, gamete, zygote, reduction division. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | A matching pair of chromosomes carrying the same genes in the same positions, one inherited from each parent. | |
| 2.2 | Having one set of chromosomes (e.g. a sperm or egg cell). | |
| 2.3 | Cell division that halves chromosome number to produce four genetically varied sex cells. | |
| 2.4 | A haploid sex cell that fuses at fertilisation. | |
| 2.5 | Exchange of corresponding segments between non-sister chromatids of homologous chromosomes during meiosis I. | |
| 2.6 | Having two sets of chromosomes, one from each parent. | |
| 2.7 | The diploid cell formed when two haploid gametes fuse at fertilisation. | |
| 2.8 | The two identical copies of one chromosome joined at a centromere after DNA replication. | |
| 2.9 | Random orientation of each homologous pair before separation, producing different chromosome combinations in gametes. | |
| 2.10 | An alternative name for meiosis I, because it halves the chromosome number. |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 10 marks (1 for T/F, 1 for the correction where needed)
3.1 Meiosis produces four genetically identical daughter cells. T / F
3.2 Meiosis I is the reduction division because homologous chromosomes are separated into different cells. T / F
3.3 Crossing over creates brand-new alleles by changing the DNA base sequence directly. T / F
3.4 If gametes were produced by mitosis instead of meiosis, the chromosome number of the species would double every generation. T / F
3.5 Independent assortment occurs in meiosis II, when sister chromatids are pulled apart at random. T / F
4. Cloze — chromosome arithmetic across one generation
Fill each blank using a term from the word bank. Each word is used once. 8 marks
Word bank: diploid · haploid · meiosis · fertilisation · homologous · four · halves · reduction
In sexually reproducing organisms, somatic cells are usually (4.1) _____________, meaning they contain two sets of chromosomes arranged as (4.2) _____________ pairs.
Gametes, however, must be (4.3) _____________ so that the chromosome number is not doubled each generation. The cell division that produces them, (4.4) _____________, (4.5) _____________ chromosome number and produces (4.6) _____________ daughter cells from one parent cell. Meiosis I is therefore also called the (4.7) _____________ division. At (4.8) _____________, two gametes fuse to restore the diploid chromosome number in the zygote.
5. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
5.1 What is the function of meiosis at the level of the species (not the individual cell)?
5.2 What is the function of meiosis I specifically (compared to meiosis II)?
5.3 What is the function of crossing over in meiosis?
5.4 What is the function of fertilisation in maintaining a species' chromosome number?
Q1 — Labelled diagram
A: diploid (2n). B: meiosis I. C: meiosis II. D: four (4). E: haploid (n). F: crossing over. G: independent assortment. H: diploid (2n).
Q2 — Term–definition matches
2.1 homologous chromosomes • 2.2 haploid • 2.3 meiosis • 2.4 gamete • 2.5 crossing over • 2.6 diploid • 2.7 zygote • 2.8 sister chromatids • 2.9 independent assortment • 2.10 reduction division.
Q3 — True / false with correction
3.1 False. Correction: meiosis produces four haploid daughter cells that are genetically unique from each other and from the parent cell. Crossing over and independent assortment ensure each receives a different combination of alleles.
3.2 True.
3.3 False. Correction: crossing over creates new combinations of existing alleles by exchanging segments between homologous chromosomes. New alleles arise through mutation, not crossing over.
3.4 True.
3.5 False. Correction: independent assortment occurs in meiosis I, when homologous chromosome pairs line up randomly before separation. Meiosis II separates sister chromatids.
Q4 — Cloze answers
4.1 diploid • 4.2 homologous • 4.3 haploid • 4.4 meiosis • 4.5 halves • 4.6 four • 4.7 reduction • 4.8 fertilisation.
Q5.1 — Function of meiosis (species level)
Meiosis produces haploid gametes from diploid parent cells, so that fertilisation between two gametes restores rather than doubles the species' chromosome number each generation. It also generates genetic variation among gametes through crossing over and independent assortment, supporting continuity of species under changing conditions.
Q5.2 — Function of meiosis I
Meiosis I separates the two members of each homologous chromosome pair into different daughter cells. This is the reduction division — it halves the chromosome number from diploid to haploid. (Meiosis II then separates sister chromatids in each haploid cell, but does not change chromosome number.)
Q5.3 — Function of crossing over
Crossing over exchanges corresponding segments between non-sister chromatids of homologous chromosomes during meiosis I. This produces new combinations of existing alleles on each chromosome, increasing the genetic variation among the resulting gametes. (It does not create new alleles — that comes from mutation.)
Q5.4 — Function of fertilisation in chromosome-number stability
Fertilisation fuses two haploid gametes (n + n) to form a diploid (2n) zygote, restoring the species' normal chromosome number. Together with meiosis, this keeps the chromosome number stable across generations rather than doubling each time.