5.1Heredity
Meiosis — AP BIOLOGY Revision Notes & Topical Questions
5 min read 11 parts Lesson 1 of 5 2 quizzes
Lesson summary
5.1 Meiosis — Notes — AP Biology notes: Explain how meiosis passes chromosomes from one generation to the next.Meiosis = nuclear division that produces haploid gametes (or spores) from a diploid cell, with one DNA replication and two divisions.Chromosome number — daughter cells h…
- Homologous chromosomes
- Crossing over
- Reduction division
- Nondisjunction
- Chromosome number — daughter cells have half the chromosomes of the parent cell (humans 2n = 46, n = 23)
- Counting — chromosomes are counted by the number of centromeres
Section 1 of 11What meiosis does
1
Meiosis = nuclear division that produces haploid gametes (or spores) from a diploid cell, with one DNA replication and two divisions.
2
Chromosome number — daughter cells have half the chromosomes of the parent cell (humans 2n = 46, n = 23).
3
Fertilisation — two haploid gametes fuse to make a diploid zygote, so the chromosome number is kept constant between generations.
4
Homologous chromosomes — pairs with the same length, centromere position and genes (loci), one from each parent, which may carry different alleles.
Section 2 of 11Chromosomes and chromatids
Chromosomes and chromatids
Counting — chromosomes are counted by the number of centromeres.
After replication — 46 chromosomes stay 46 (still 46 centromeres) but are now made of 92 chromatids.
Chromatid = one strand of a replicated chromosome.
Section 3 of 11Meiosis I
1
Prophase I — the nuclear envelope breaks down, chromosomes condense (each of two sister chromatids) and homologues pair (synapsis) into bivalents (tetrads).
- Crossing over of non-sister chromatids occurs at chiasmata.
- Spindle fibres form.
2
Metaphase I — homologous pairs line up on the equator in random orientation (independent assortment).
3
Anaphase I — homologues are pulled to opposite poles while the centromeres do not divide, so sister chromatids stay together.
4
Telophase I and cytokinesis — nuclear envelopes form around the two groups (and in animal cells a cleavage furrow forms),
- giving two haploid cells with each chromosome still of two chromatids.
5
Reduction division — the chromosome number is halved in meiosis I.
Section 4 of 11Meiosis II
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No replication — there is no interphase between meiosis I and II, so DNA is not replicated.
2
Similarity — the second division is almost identical to mitosis.
3
Prophase II and metaphase II — chromosomes condense and line up in single file at the equator,
- and the kinetochore of each chromatid attaches to a microtubule from the poles.
4
Anaphase II — proteins at the centromeres break down and sister chromatids are pulled to opposite poles.
5
Telophase II and cytokinesis — nuclear membranes form and four haploid cells result.
Section 5 of 11Stages of meiosis summarised
| Stage | Key events |
|---|---|
| Prophase I | Chromosomes condense; homologues pair into bivalents; crossing over between non-sister chromatids |
| Metaphase I | Bivalents line up at the equator in random orientation |
| Anaphase I | Homologues separate to opposite poles; centromeres stay intact |
| Telophase I and cytokinesis | Two haploid cells, each chromosome still two chromatids |
| Meiosis II | Sister chromatids separate, giving four haploid cells |
Section 6 of 11Meiosis I and meiosis II compared
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Pairing | Homologues pair (tetrads) | None |
| Crossing over | Yes | No |
| Metaphase | Pairs at the plate | Single chromosomes at the plate |
| Anaphase | Homologues separate | Sister chromatids separate (like mitosis) |
| Chromosome number | 2n to n | n to n |
| Result | 2 haploid cells | 4 haploid cells |
Section 7 of 11Mitosis and meiosis compared
| Feature | Mitosis | Meiosis |
|---|---|---|
| Rounds of division | One | Two |
| Daughter cells | Two | Four |
| Ploidy of daughters | Diploid | Haploid |
| Genetically | Identical | Different |
| Occurs in | All organisms | Animals, plants and fungi |
| Purpose | Growth and repair (somatic cells) | Gametes for sexual reproduction |
| Recombination | No | Yes |
| Chromosome assortment | None | Independent |
- Shared features — both start from a diploid cell, have prophase, metaphase, anaphase and telophase, line chromosomes up in metaphase,
- and end with cytokinesis.
Section 8 of 11Errors in meiosis
Errors in meiosis
Nondisjunction — failure of homologues (anaphase I) or sister chromatids (anaphase II) to separate.
Result — gametes with an extra or missing chromosome (aneuploidy), e.g. trisomy 21.
Precise terms — homologues pair in meiosis I, sister chromatids separate in meiosis II, crossing over is between non-sister chromatids,
- and the cells made are haploid and genetically different.
Section 9 of 11More from the specification
Variation
- Differences between individuals of the same species.
Genetic diversity
- The range of alleles within a population.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Meiosis | two divisions giving four haploid cells |
| Homologous chromosomes | matching pair, one from each parent |
| Synapsis | pairing of homologues in prophase I |
| Bivalent | a pair of homologues joined in prophase I |
| Chromatid | one strand of a replicated chromosome |
| Crossing over | exchange of segments between non-sister chromatids |
| Reduction division | meiosis I halving the chromosome number |
| Gamete | haploid sex cell |
| Nondisjunction | failure of chromosomes to separate |
Section 11 of 11Exam technique
Avoid these mistakes
Saying chromatids separate in anaphase I — homologues separate in anaphase I and chromatids in anaphase II.
Saying DNA is replicated between meiosis I and II — there is no replication.
Counting chromatids as chromosomes — count centromeres.
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