Chromosomes (AQA GCSE Biology): Revision Note

Chromosomes: Basics

• In eukaryotic cells, the nucleus contains thread-like structures called chromosomes

• Chromosomes are made from highly coiled strands of relatively long DNA. Each chromosome is made from one DNA molecule

• In the body cells of diploid organisms, chromosomes are normally found in pairs

• One chromosome from each pair is inherited from the mother, the other from the father

• Different species of organisms have different numbers of chromosomes in their nuclei

  • Humans have 46 chromosomes in the nucleus of all their body cells, found in 23 pairs

  • Horses have 64 chromosomes in 32 pairs

  • Red blood cells are an exception – they lose their nuclei and therefore have no chromosomes

• Before a cell can divide, its genetic material needs to be doubled. This results in the characteristic ‘X’ shaped chromosomes seen in micrographs of cells preparing to divide

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