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Learn4BI1Paper 2B · Paper 2 (all content)Reproduction and inheritanceChromosomes and cell division
Reproduction and inheritance · Subtopic 2 of 5

Chromosomes, Genes and Cell Division

The nucleus contains chromosomes, long strands of DNA. A gene is a length of DNA that codes for a specific protein; different versions of a gene are alleles. The base sequence is copied into mRNA, which carries it to ribosomes where amino acids are joined in the matching order. Human body cells are diploid (46 chromosomes in 23 pairs); gametes are haploid (23). Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction. Meiosis produces four genetically different haploid gametes.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define chromosome, gene and allele.
  • Outline how a gene codes for a protein.
  • Compare mitosis and meiosis.
  • Explain how sex is inherited.
Pause and think

Every cell in your body (except gametes) has the same DNA. So how can a muscle cell differ from a nerve cell?

Section 2 of 14

Core idea

Start hereInstructions for a body

A fertilised egg contains all the instructions needed to build you, written in a four-letter code repeated three billion times.

The big idea

Genes on chromosomes code for proteins. Mitosis copies cells exactly for growth; meiosis halves the chromosome number and creates variety in gametes.

1. From gene to protein

Protein synthesis

The base sequence decides the amino acid sequence.

2. Mitosis vs meiosis

Worked exampleCounting chromosomes

A dog body cell has 78 chromosomes.

Goal
Chromosomes in a skin cell after mitosis and in a sperm cell.
Strategy
Mitosis keeps number; meiosis halves it.
  1. mitosis:78\text{mitosis}: 78
    Identical diploid cells.
  2. meiosis:39\text{meiosis}: 39
    Haploid gametes.
Answer78 and 39.
✓ Check39 + 39 = 78 at fertilisation ✓.
Key takeawaySex determination Eggs always carry X; sperm carry X or Y. XX = female, XY = male, each with probability 12\frac12.
Pause and think

Why would gametes made by mitosis be a problem?

Summary

  • Gene → mRNA → protein.
  • Mitosis: growth, identical.
  • Meiosis: gametes, haploid, varied.
Key words
Diploid
Two sets of chromosomes.
Haploid
One set of chromosomes.
Allele
Alternative version of a gene.
HistoryHuman Genome Project. The Human Genome Project (1990–2003) read all 3 billion base pairs of human DNA and found we have only about 20,000 protein-coding genes, far fewer than expected.
Fun factTwo metres of DNA. Unwound, the DNA in one human cell is about 2 m long, packed into a nucleus just 0.006 mm across.
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