Pearson Edexcel International A Level Biology: Meiosis, Gametes, Fertilisation and Mitosis (YBI11) – Revision Notes

These notes condense the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, statements 3.9 to 3.16 and Core Practical 6. This is Topic 3, which belongs to the International AS (IAS). For full explanations and worked examples use the study guide, then test yourself with the practice questions.

Also useful: the Edexcel A Level Biology hub, the printable checklist and the free diagnostics.

Key definitions

Term	Meaning
Locus	The position of a gene on a chromosome
Linked genes	Genes with loci on the same chromosome, inherited together unless crossing over separates them
Homologous chromosomes	Pair with the same genes at the same loci, one from each parent; alleles may differ
Chiasma	Point where non-sister chromatids have crossed over
Independent assortment	Random orientation of each homologous pair at metaphase I
Acrosome reaction	Release of enzymes from the sperm acrosome, digesting the zona pellucida
Cortical reaction	Release of cortical granule contents from the egg, altering the zona so no more sperm bind
Polyspermy	Entry of more than one sperm into an egg
Mitotic index	Cells in mitosis / total cells counted

3.9 Linkage in brief

• Same chromosome, so same inheritance unit.
• Test cross of a double heterozygote: linked genes give two common parental types and two rare recombinant types, not 1:1:1:1.
• Recombinant frequency = recombinant offspring / total offspring.
• Smaller recombinant frequency means loci are closer together.

3.10 Meiosis: method in steps

1. Prophase I: homologous chromosomes pair; crossing over between non-sister chromatids at chiasmata; alleles are exchanged.
2. Metaphase I: homologous pairs on the equator; each pair faces either pole at random.
3. Anaphase I: homologous chromosomes pulled apart (chromosome number halved).
4. Telophase I and cytokinesis: two haploid cells.
5. Meiosis II: chromosomes line up singly; centromeres divide; chromatids separate.
6. Result: four haploid cells, genetically different.

Names of the stages of prophase are not required.

Source of variation	Stage	Effect
Crossing over	Prophase I	New allele combinations on each chromatid
Independent assortment	Metaphase I	2^n chromosome combinations (n = haploid number)
Random fusion of gametes	Fertilisation	Further new combinations

Reminder: n = 4 gives 2^4 = 16 gamete types; two such parents give 16 × 16 = 256 zygote combinations before crossing over.

3.11 and 3.12 Gametes and mammalian fertilisation

Gamete part	Role
Acrosome (sperm)	Enzymes to digest the zona pellucida
Mid-piece mitochondria	ATP for the flagellum
Flagellum	Swimming
Zona pellucida (egg)	Binds sperm; barrier after cortical reaction
Cortical granules (egg)	Enzymes released at cortical reaction
Egg cytoplasm	Food stores for early development

Order to remember: bind, acrosome reaction, digest zona, membranes fuse, cortical reaction, nucleus enters, nuclei fuse.

• The cortical reaction is the block to polyspermy.
• The zygote is diploid. Each gamete is haploid.

3.13 Fertilisation in flowering plants

1. Pollen grain germinates on the stigma; pollen tube grows.
2. Tube grows down the style towards the ovule.
3. Generative nucleus divides by mitosis: two male nuclei.
4. Tube enters through the micropyle.
5. Male nucleus 1 + egg nucleus gives the diploid zygote.
6. Male nucleus 2 + two polar nuclei gives the triploid endosperm nucleus.

Recommended practical: pollen tube growth. Vary one factor (for example sucrose concentration or temperature), measure tube length at a fixed time, calculate rate = length / time.

3.14 and 3.15 Mitosis and Core Practical 6

Stage	Key feature
Prophase	Chromosomes condense; envelope breaks down
Metaphase	Chromosomes on the equator
Anaphase	Chromatids pulled apart to poles
Telophase	Nuclear envelopes reform; chromosomes decondense

Cell cycle: G1, S (DNA replication), G2, mitosis, cytokinesis. Mitosis gives genetically identical cells for growth, repair and asexual reproduction.

Root tip squash in steps (typical method; follow your centre’s version):

1. Cut the tip (actively dividing region).
2. Warm acid treatment softens and separates cells.
3. Stain the DNA.
4. Squash under a cover slip to one layer.
5. View, identify stages, draw and label.

Practical points: use only the tip, because cells further back are not dividing; keep acid and stain off skin; view at low power first, then increase magnification; draw only what you see, with clear lines and labels, and note the stage of each cell.

3.16 Mitotic index: method

1. Count all cells in the field(s), including interphase cells.
2. Count cells showing any stage of mitosis.
3. Divide mitotic cells by total cells (multiply by 100 for a percentage).
4. To estimate time in a stage: (cells in stage / total cells) × length of the cycle.

Worked reminder: 36 cells in mitosis out of 240 gives 36 / 240 = 0.15, or 15%. If the cycle lasts 20 hours, mitosis lasts about 0.15 × 20 = 3 hours.

Must-know distinctions

	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Making gametes
Divisions	One	Two
Daughter cells	Two, diploid, identical	Four, haploid, different
Homologues pair up	No	Yes, in prophase I
Crossing over	No	Yes
What separates	Sister chromatids	Homologues (I), then chromatids (II)

	Mammals	Flowering plants
Male gamete	Sperm, swims	Male nucleus, carried by pollen tube
Entry route	Through the zona pellucida	Through the micropyle
Nuclear fusion	One sperm nucleus + egg nucleus	Male nucleus 1 + egg nucleus; male nucleus 2 + two polar nuclei
Products	Diploid zygote	Diploid zygote and triploid endosperm nucleus
Block to extra male gametes	Cortical reaction	Not named in these statements

Worked reminders

Linkage. A test cross of a double heterozygote gives 380 AB, 372 ab, 25 Ab and 23 aB (800 in total). Expected for unlinked genes: 200 of each. Recombinants = 25 + 23 = 48. Recombinant frequency = 48 / 800 = 0.060, or 6.0%. The parental classes dominate, so the loci are linked and close together.

Variation count. Independent assortment alone gives 2^n gamete types. For n = 3 that is 8. Two parents give 8 × 8 = 64 zygote combinations. Crossing over is extra, so the true number is larger.

Rate. Pollen tubes reach 450 µm in 30 minutes. Rate = 450 / 30 = 15 µm per minute. Always state the unit and the time.

Mitotic index with two slides. Slide 1: 27 of 180 cells in mitosis, so 15.0%. Slide 2: 9 of 150, so 6.0%. A real investigation should count several fields on each slide, and the same number of cells each time where possible, so the comparison is fair.

How to write answers

• For “explain how meiosis causes variation”, name the stage, name the event, then state the effect on alleles or chromosomes.
• For fertilisation, write in time order and name both the structure and the change it causes.
• For practical questions, give the step and its purpose in the same sentence: “warm acid, to separate the cells”.
• For calculations, write the formula, the substitution, the answer and the unit.
• For data, quote figures from the table, compare them, then offer an explanation using careful language such as “suggests”.

Quick self-test

1. What is a locus?
2. A test cross gives 460 parental and 40 recombinant offspring out of 500. What is the recombinant frequency?
3. Which stage separates homologous chromosomes?
4. An organism has n = 5. How many chromosome combinations can independent assortment give in its gametes?
5. Name the structure in the sperm that contains digestive enzymes.
6. What does the cortical reaction prevent?
7. How many nuclei does the generative nucleus form in the pollen tube, and by which type of division?
8. What does one male nucleus fuse with to make endosperm?
9. In which phase of the cell cycle is DNA replicated?
10. In a root tip, 18 of 150 cells are in mitosis. Calculate the mitotic index as a percentage.
11. Why is the root tip squashed under a cover slip?

Answers

1. The position of a gene on a chromosome.
2. 40 / 500 = 0.080, or 8.0%.
3. Anaphase I (after pairing in prophase I and alignment in metaphase I).
4. 2^5 = 32.
5. The acrosome.
6. Polyspermy (more than one sperm entering the egg).
7. Two male nuclei, by mitosis.
8. The two polar nuclei, giving a triploid nucleus.
9. S phase.
10. 18 / 150 × 100 = 12%.
11. To spread the cells into a single thin layer so light passes through and chromosomes can be seen.

Where marks are usually lost

• Writing “gene” for “locus”, or “allele” for “gene”.
• Saying chromatids separate in meiosis I.
• Describing crossing over as an exchange between sister chromatids, or between different chromosomes (not homologues).
• Not linking each source of variation to its stage: crossing over to prophase I and independent assortment to metaphase I.
• Mixing up the acrosome reaction and the cortical reaction, or giving one without its purpose.
• Writing that the pollen tube grows “towards the ovule” without a cause, or leaving out the micropyle.
• Dividing mitotic cells by interphase cells instead of by all cells.
• Forgetting units or the percentage sign in rate and index answers.
• Describing a squash without explaining why each step is done.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology specification, Issue 2, February 2021 (Pearson Education Limited), statements 3.9 to 3.16 and Core Practical 6.