What you need to know
- 3.9–3.10Loci, linkage and meiotic variation.
- 3.11–3.13gameteA haploid sex cell, such as a sperm or an egg cell. and fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote. in mammals and plants.
- 3.14–3.16Cell cycle, mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent., CP6 and mitotic indices.
By the end, you should be able to…
- Choose the correct chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. process.
- Support explanations with mechanism and evidence.
- Check calculations and biological units.
Build your understanding
Plan a linked explanation
Identify the process, stage and structure before describing a consequence. For genetic identity, link replication to equal distribution of sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere.. For variation, link crossover or independent orientation to different alleleA different version of the same gene, found at the same locus. combinations.
Use homologous chromosomesA pair of chromosomes, one from each parent, with the same genes at the same loci but possibly different alleles. for separation in meiosisTwo nuclear divisions that produce four genetically different haploid nuclei, used to make gametes. I and sister chromatids for mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. or meiosis II. State whether a number describes the whole cell, each nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. or each final daughter cell.
Read questions as evidence problems
A microscope description must contain observable support for a stage. A percentage needs a denominator. A treatment result requires controls and replication before a causal explanation is secure.
For fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote., distinguish movement to a target, membrane events and nuclear fusion. For plants, name both fusion partners and both products.
Make your reasoning visible
Write a mechanism chain
Feature → process → consequence makes a stronger explanation than repeating the question.
Check scope
Link each answer to the stated system; do not transfer mammalian acrosome reactions to pollen tubes.
Avoid this misconception
The claim: Every division halves chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. number.
The correction: mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. preserves it; meiosisTwo nuclear divisions that produce four genetically different haploid nuclei, used to make gametes. I reduces ploidy; meiosis II separates chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere. without halving the chromosome number per nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. again.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
A diploid cell with 2n = 14 undergoes normal mitosis. Describe how each daughter nucleus receives 14 chromosomes.
Show suggested answer
DNA replicates before mitosis, making sister chromatids. Chromosomes align individually and sister chromatids separate to opposite poles; each new nucleus receives one copy of every chromosome.
Practice question
Retrieval and explanation · suggested answer
A nucleus is haploid but its chromosomes each have two chromatids. Which stage boundary could this represent? Explain.
Show suggested answer
After meiosis I and before meiosis II: homologues have separated, leaving one chromosome of each pair, but sister chromatids remain joined.
Practice question
Retrieval and explanation · suggested answer
Explain two mechanisms during meiosis that produce different allele combinations.
Show suggested answer
In prophase I crossing over exchanges corresponding segments between non-sister chromatids of homologues. In metaphase I independent orientations of bivalents lead to different combinations of maternal and paternal homologues in the products.
Practice question
Retrieval and explanation · suggested answer
A root sample contains 225 interphase, 30 prophase, 18 metaphase, 12 anaphase and 15 telophase cells. Calculate the mitotic index.
Show suggested answer
Mitotic cells = 75; total = 300. Index = 75/300 = 0.25, or 25%.
Practice question
Retrieval and explanation · suggested answer
A mammalian sperm reaches an egg but cannot release its acrosomal enzymes. Explain a likely effect and distinguish the cortical reaction.
Show suggested answer
Penetration of surrounding layers may be impaired. The cortical reaction is release of egg cortical granules after sperm entry, helping prevent additional sperm entry; it is a different event.
Practice question
Retrieval and explanation · suggested answer
A plant egg has 11 chromosomes. Give the chromosome numbers of its zygote and primary endosperm nucleus, explaining the fusions.
Show suggested answer
Zygote: 22 from egg 11 + male 11. Endosperm nucleus: 33 from two polar nuclei, each 11, + male 11.
Practice question
Retrieval and explanation · suggested answer
Why should a root-tip investigation sample several fields and several roots?
Show suggested answer
Several fields reduce dependence on an unrepresentative view; independent roots capture biological variation. Fields from one root alone are not independent biological replicates.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A diploid nucleus has 12 chromosomes and 24 DNA molecules. Can you distinguish mitotic metaphase from metaphase I from these numbers alone?
Show a hint
The same replicated starting content can be arranged differently.
Explore the reasoning
No. Both can contain 12 replicated chromosomes. Inspect whether chromosomes align individually or homologues pair as bivalents; chromosome arrangement supplies the missing evidence.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does the idea of a locus explain why alleles of one gene pair up in meiosis?
Locus and linkage
Practice question
Retrieval and explanation · suggested answer
How does the idea of a locus explain why alleles of one gene pair up in meiosis?
Lesson: 3B 3: Sexual Reproduction and Meiosis
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Why “shuffling” needs a mechanism
3B · Exam PracticeNHGRI describes crossing over as exchange between homologous chromosomes. In explanations, a metaphor such as shuffling is useful only when the physical event is also identified. In prophase…Specification 3.10Read & use the scienceClose article
Why this matters
NHGRI describes crossing over as exchange between homologous chromosomes. In explanations, a metaphor such as shuffling is useful only when the physical event is also identified. In prophase I, non-sister chromatids of homologous chromosomes exchange corresponding sections. In metaphase I, homologous pairs align independently. Those are different sources of variation and should not be merged into the word shuffle. Geneticists use recombinant offspring to estimate the distance between linked loci, but the exam answer starts with the chromosome event. A drawing can help: mark two loci on each homologous chromosome before and after a crossover. The labels explain the change better than an unqualified metaphor.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.10 understand the role of meiosis in ensuring genetic variation through the production of non-identical gametes as a consequence of independent assortment of chromosomes in metaphase I and crossing over of alleles between chromatids in prophase I
Use the science
Why would “the genes shuffle” be an incomplete exam answer?
Explore a possible response
It does not identify homologues, non-sister chromatids, exchange of corresponding segments or the stage; nor does it distinguish crossing over from assortment.
Explore the source
- NHGRI: Crossing OverNational Human Genome Research Institute
Breeding begins with chromosome behaviour
3B · Sexual Reproduction and MeiosisThe John Innes Centre studies meiotic recombination in Arabidopsis and wheat. Understanding where chromosome exchanges occur can inform plant breeding. Crossing over creates new combinations between homologous chromatids;…Specification 3.10Read & use the scienceClose article
Why this matters
The John Innes Centre studies meiotic recombination in Arabidopsis and wheat. Understanding where chromosome exchanges occur can inform plant breeding. Crossing over creates new combinations between homologous chromatids; independent assortment changes which whole homologues enter each gamete. Plant breeders cannot choose the exact crossover in every meiosis, but mapping recombination frequencies can show whether useful alleles are strongly linked. The John Innes work is therefore a research connection, not an exam requirement to know a crop variety. In an explanation, identify the chromosome event and the resulting allele combination. Saying that plants simply mix their traits skips the mechanism the specification assesses.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.10 understand the role of meiosis in ensuring genetic variation through the production of non-identical gametes as a consequence of independent assortment of chromosomes in metaphase I and crossing over of alleles between chromatids in prophase I
Use the science
Why might tightly linked alleles make combining desirable traits harder?
Explore a possible response
If desired alleles occur on opposite homologues, a crossover between their loci may be needed to bring them together; close linkage makes this less likely.
Explore the source
- John Innes Centre: meiotic recombination researchJohn Innes Centre
From cells to a research question
3B · MitosisScientists studying cell-cycle regulation need to distinguish entry into division from successful completion. A count of dividing cells is one observation, not a complete explanation. A mitotic index…Specification 3.16Read & use the scienceClose article
Why this matters
Scientists studying cell-cycle regulation need to distinguish entry into division from successful completion. A count of dividing cells is one observation, not a complete explanation. A mitotic index is the number of cells visibly in mitosis divided by the total cells counted. It is a snapshot of a population, so its interpretation depends on how long cells spend in each stage as well as how many enter division. Researchers may also track labelled cells through time. This distinction matters when testing a drug: more cells with condensed chromosomes could mean division is frequent, or that the cells are stuck before completion. A root-tip sample and a tumour sample are different biological settings, but the sampling caution is shared.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.16 be able to calculate mitotic indices
Use the science
What extra observation could distinguish faster cycling from arrest?
Explore a possible response
Follow cells over time and count completed divisions, using suitable controls alongside the snapshot counts.
Explore the source
- Nobel Prize: the 2001 Physiology or Medicine awardNobel Prize Outreach