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

This guide follows the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. It covers specification statements 3.9 to 3.16 of Topic 3, Cell Structure, Reproduction and Development, including Core Practical 6. Topics 1 to 4 belong to the International AS (IAS), so this unit is IAS content that also supports the International A2 (IA2) topics. There are no tiers.

Use it with the revision notes and the practice questions. Course pages: Edexcel A Level Biology hub, printable checklist, free diagnostics and exam preparation.

What this unit covers

Statement	What you must be able to do
3.9 (i), (ii)	Know that a locus is the location of genes on a chromosome; understand linkage of genes on a chromosome
3.10	Explain how meiosis gives variation: independent assortment in metaphase I, crossing over in prophase I. Names of the stages of prophase are not required
3.11	Explain how mammalian gametes are specialised, including the acrosome and the zona pellucida
3.12	Know mammalian fertilisation: acrosome reaction, cortical reaction, fusion of nuclei
3.13	Know fertilisation in flowering plants, from pollen tube growth to fusion of nuclei (plus the recommended practical on pollen tube growth)
3.14	Explain the role of mitosis and the cell cycle in making genetically identical cells for growth and asexual reproduction
3.15	Core Practical 6: prepare and stain a root tip squash to observe the stages of mitosis
3.16	Calculate mitotic indices

3.9 Loci and linkage

A gene is a length of DNA coding for a polypeptide. A locus (plural loci) is the fixed position of a gene on a chromosome. In a homologous pair, both chromosomes carry the same genes at the same loci, but they may carry different alleles.

Genes whose loci are on the same chromosome are linked. They tend to be inherited together because the chromosome is passed on as one unit. Linked genes do not assort independently, so a dihybrid cross gives ratios different from the 9:3:3:1 (or 1:1:1:1 for a test cross) expected for genes on different chromosomes.

Crossing over can still separate linked alleles. The nearer two loci are, the less likely a chiasma forms between them, so fewer recombinant offspring appear.

Worked example: linkage. A plant heterozygous at two linked loci (alleles A, a and B, b) is crossed with a plant homozygous recessive for both (aabb). The heterozygote’s two chromosomes carry AB and ab. Of 1000 offspring:

Phenotype	Number
AB (parental)	442
ab (parental)	438
Ab (recombinant)	62
aB (recombinant)	58

If the loci were on different chromosomes you would expect 250 of each. The two parental types are far commoner, so the genes are linked. Recombinants = 62 + 58 = 120. Recombinant frequency = 120 / 1000 = 0.120, or 12.0%. The recombinants arose from crossing over between the two loci in meiosis in the heterozygous parent.

3.10 Meiosis and genetic variation

Meiosis is a reduction division. One diploid cell (2n) divides twice, giving four haploid cells (n) that are genetically different.

	Meiosis I	Meiosis II
What separates	Homologous chromosomes	Sister chromatids
Chromosome number	Halved: 2n to n	Stays n
Variation arises from	Crossing over (prophase I); independent assortment (metaphase I)	None new

Prophase I. Chromosomes condense and homologous chromosomes pair up. Chromatids of homologous chromosomes cross over at chiasmata, breaking and rejoining so that equivalent sections, and the alleles on them, are exchanged. Each recombinant chromatid carries a new combination of alleles. You do not need the names of the stages of prophase.

Metaphase I. Homologous pairs line up on the equator. The orientation of each pair towards either pole is random and independent of every other pair. This is independent assortment of chromosomes.

Anaphase I and telophase I. Whole chromosomes move to the poles; two haploid cells form, each chromosome still made of two chromatids.

Meiosis II. Chromosomes line up singly, the centromeres divide and chromatids move apart, giving four haploid cells.

Worked example: counting combinations. An organism has 2n = 12, so n = 6 and there are 6 homologous pairs. Independent assortment alone gives 2^6 = 64 different chromosome combinations in gametes. Random fusion of two such gametes gives 64 × 64 = 4096 combinations. In humans (n = 23) a gamete has 2^23 = 8 388 608 possible combinations before crossing over is counted. Crossing over increases the variety far beyond these figures, because each chromosome is itself a new mixture.

3.11 Mammalian gametes

Feature	Sperm	Egg cell (secondary oocyte)
Nucleus	Haploid, tightly packed in the head	Haploid, large
Acrosome	Membrane-bound vesicle at the head tip containing hydrolytic enzymes	Absent
Mitochondria	Many, in the mid-piece, to supply ATP for swimming	Many, spread through the cytoplasm
Movement	Flagellum (tail)	Non-motile; moved along the oviduct by cilia
Outer layers	Cell surface membrane	Zona pellucida (a glycoprotein layer) outside the membrane
Cytoplasm	Very little	Large volume with food stores; cortical granules just under the membrane

The acrosome lets the sperm digest a path through the zona pellucida. The zona pellucida contains the molecules the sperm recognises and binds to, and after fertilisation it hardens. Cortical granules hold the enzymes that cause that change.

3.12 Fertilisation in mammals

1. Sperm reach the egg and bind to the zona pellucida.
2. Acrosome reaction: the acrosome membrane fuses with the sperm membrane and releases its enzymes by exocytosis. The enzymes digest the zona pellucida.
3. The sperm head passes through the digested zona and its membrane fuses with the egg’s surface membrane.
4. Cortical reaction: cortical granules release their contents by exocytosis. The zona pellucida changes so that it cannot bind further sperm, which prevents polyspermy (more than one sperm entering).
5. The sperm nucleus enters the egg. The egg completes meiosis II.
6. The haploid sperm nucleus and haploid egg nucleus fuse, restoring the diploid number in the zygote.

3.13 Fertilisation in flowering plants

1. A pollen grain lands on a compatible stigma and absorbs sugars and water. It germinates, growing a pollen tube.
2. The pollen tube grows down the style, digesting tissue as it goes and guided by chemicals from the ovule (chemotropism). The tube nucleus controls growth.
3. The generative nucleus divides by mitosis to form two male nuclei.
4. The tube enters the ovule through the micropyle and releases the male nuclei into the embryo sac.
5. One male nucleus fuses with the egg nucleus, giving the diploid zygote.
6. The other male nucleus fuses with the two polar nuclei, giving the triploid endosperm nucleus.

Recommended practical: pollen tube growth. Pollen is grown on a slide or in a drop of solution containing sucrose, and the tubes are measured with a graticule after a fixed time. Sucrose concentration, temperature and pH are variables.

Worked example: rate. After 60 minutes, mean tube lengths were: 0% sucrose, 48 µm; 5%, 310 µm; 10%, 720 µm; 15%, 540 µm; 20%, 150 µm. Rate = length / time. At 10%: 720 / 60 = 12.0 µm per minute. At 0%: 48 / 60 = 0.8 µm per minute. At 20%: 150 / 60 = 2.5 µm per minute. Growth peaks near 10% and falls at 20%. A reasonable explanation is that a very concentrated solution has a lower water potential, so water leaves the pollen grain and the tube grows poorly. State the trend before explaining it, and say “suggests”.

3.14 Mitosis and the cell cycle

The cell cycle is interphase followed by mitosis and cytokinesis. Interphase has G1 (growth, protein synthesis, organelle increase), S (DNA replication) and G2 (further growth, checks, preparation).

Mitosis is nuclear division giving two nuclei genetically identical to the parent nucleus. Because DNA replicated in S phase and each chromosome (two sister chromatids) is shared out exactly, both daughter cells have the same genotype.

Stage	What you see
Prophase	Chromosomes condense and become visible as two chromatids; nuclear envelope breaks down; spindle forms
Metaphase	Chromosomes lined up on the equator, attached to spindle fibres at their centromeres
Anaphase	Centromeres divide; chromatids pulled to opposite poles, centromere first
Telophase	Chromatids (now chromosomes) reach poles and decondense; nuclear envelopes reform

Cytokinesis then divides the cytoplasm. Mitosis produces genetically identical cells for growth, replacing damaged cells and asexual reproduction.

3.15 Core Practical 6: root tip squash

Typical method (follow your centre’s version):

1. Cut about 1 cm from the tip of a growing root, where cells divide.
2. Warm it in hydrochloric acid for a few minutes to soften the tissue and separate the cells.
3. Rinse, place on a slide and add a stain that binds DNA, such as toluidine blue or acetic orcein.
4. Lower a cover slip and squash it gently, so cells form a single layer through which light passes.
5. Observe with a light microscope, starting at low power. Identify cells in each stage and make labelled drawings.

Safety: take care with acid and stain. Staining is needed because chromosomes are almost colourless in living cells.

3.16 Mitotic index

Mitotic index = number of cells in mitosis / total number of cells observed. It is a ratio, or a percentage if multiplied by 100.

Worked example. Across several fields of view you count 412 cells: 29 in prophase, 11 in metaphase, 7 in anaphase, 10 in telophase, and the rest in interphase. Cells in mitosis = 29 + 11 + 7 + 10 = 57. Mitotic index = 57 / 412 = 0.138, or 13.8%.

If the whole cell cycle takes 18 hours, the time spent in mitosis is about 0.138 × 18 = 2.5 hours. The time in prophase is 29 / 412 × 18 = 1.3 hours. This assumes cells are spread evenly through the cycle.

Comparison. A treated root tip has 21 cells in mitosis out of 390. Mitotic index = 21 / 390 = 5.4%. Percentage decrease = (13.8 − 5.4) / 13.8 × 100 = about 61% (using unrounded values, 61.1%). A substance that cuts the mitotic index may slow or stop cells entering mitosis.

Common errors

• Writing that the genes are “on the locus”. The locus is the position.
• Saying homologous chromosomes separate in meiosis II. They separate in meiosis I.
• Describing crossing over as happening between sister chromatids. It involves non-sister chromatids of a homologous pair.
• Confusing the acrosome reaction (digests the zona) with the cortical reaction (hardens it).
• Forgetting that two male nuclei are produced in the pollen tube, and what each fuses with.
• Counting only prophase to metaphase cells as dividing. Count all four stages.
• Giving a mitotic index without checking the total counted is all cells, not just dividing ones.

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.