16.1 Nuclear Division

Cambridge O Level Biology 5090

2026 📋 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Understand that chromosomes contain DNA, which carries genetic information in the form of genes
  2. Describe a haploid nucleus as a nucleus containing a single set of chromosomes
  3. Describe a diploid nucleus as a nucleus containing two sets of chromosomes
  4. State that in a diploid cell there is a pair of each type of chromosome, and in a human diploid cell there are 23 pairs
  5. Describe mitosis as nuclear division giving rise to genetically identical cells in which the chromosome number is maintained
  6. Outline the role of mitosis in growth, repair of damaged tissues, replacement of dying cells, and asexual reproduction
  7. Describe stem cells as unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions
  8. State that meiosis is involved in the production of gametes
  9. Describe meiosis as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
  10. Understand that cancers form as a result of uncontrolled cell division

📖 Main Content

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1. Chromosomes, DNA, and Genes

To understand nuclear division, you first need to understand what is inside the nucleus of a cell.

  • Inside the nucleus (the control centre of the cell) there are thread-like structures called chromosomes.
  • Each chromosome is made of a long molecule called DNA (deoxyribonucleic acid). Think of DNA as a very long instruction manual written in a chemical code.
  • Along each DNA molecule, there are sections called genes. Each gene is one specific instruction — for example, a gene might carry the instruction for your eye colour, your blood group, or how a certain protein is made.
  • In other words: chromosomes → contain DNA → which is organised into genes → which carry genetic information.

💡 Simple analogy: Think of a chromosome as a bookshelf, DNA as the books on that shelf, and genes as the individual chapters inside each book. Each chapter gives a specific instruction.

2. Haploid and Diploid Nuclei

Cells can contain different numbers of chromosome sets. This is important because it affects how cells divide and what role they play in the body.

🔹 Diploid Nucleus

  • A diploid nucleus is a nucleus that contains two sets of chromosomes.
  • This means the chromosomes come in matching pairs — every chromosome has a partner that carries genes for the same features.
  • These matching pairs are called homologous pairs (homologous means "corresponding" or "matching").
  • Most body cells (e.g. skin cells, muscle cells, liver cells) have a diploid nucleus.
  • In humans, a diploid cell contains 23 pairs of chromosomes, which is 46 chromosomes in total.

🔹 Haploid Nucleus

  • A haploid nucleus is a nucleus that contains only one set of chromosomes — meaning just one chromosome from each pair.
  • There are no matching partners — each chromosome is alone.
  • In humans, a haploid cell contains 23 chromosomes (not pairs — just 23 individual chromosomes).
  • Sex cells (gametes) — such as sperm cells and egg cells — are haploid.
FeatureDiploidHaploid
Number of chromosome setsTwo setsOne set
Chromosomes in humans46 (23 pairs)23
Example cellsBody/somatic cellsGametes (sperm & egg)
Chromosomes in pairs?YesNo

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