AQA GCSE Biology

Revision Notes

1.2.1 Chromosomes

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 we see in micrographs of cells preparing to divide

Chromosomes, IGCSE & GCSE Biology revision notes

Chromosomes are usually uncoiled, when a cell prepares to divide they fold up into ‘worm-like’ structures that we recognise

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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