Chromosomes (WJEC GCSE Biology: Combined Science)

• In eukaryotic cells, the nucleus contains linear, thread-like structures called chromosomes
  • Note that the ‘X’ shaped chromosomes that are often shown in pictures occur when the linear chromosomes are copied prior to cell division
• Chromosomes are made from highly coiled strands of DNA
• Chromosomes contain sections of DNA known as genes
• In the body cells of organisms, chromosomes are normally found in pairs; the chromosomes of each pair contain the same genes
  • One chromosome from each pair is inherited from the mother and the other from the father
  • Different species of organisms have different numbers of chromosome pairs, e.g.
    • Humans have 46 chromosomes in 23 pairs
      • Red blood cells are an exception; they lose their nucleus and therefore have no chromosomes
    • Horses have 64 chromosomes in 32 pairs
• As chromosomes come in pairs with the same genes, it can also be said that genes come in pairs

Chromosome pairs diagram

Cells contain pairs of chromosomes, and therefore pairs of genes; human cells contain 23 pairs of chromosomes, as shown above