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Biology AS AQA

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  1. 1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    9 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    14 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    4 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-1-dna-genes-and-chromosomes
    10 主题
  20. 4-2-dna-and-protein-synthesis
    3 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
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Exam code:7401

Chromosome structure

  • In the nucleus of eukaryotic cells, DNA molecules are very long, linear

  • Together, a DNA molecule and its associated proteins form a chromosome

    • The main proteins present are the large, positively charged globular proteins called histones. Their role is to organise and condense the DNA tightly so that it fits into the nucleus

    • The other proteins are enzymes used in copying and repairing the DNA

  • The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of

  • Chromatids are organised into sections, which is a series of genes

  • The ends of the chromatids in chromosomes are ‘sealed’ with protective structures called telomeres

Diagram depicting chromosome structure, showing chromatids, chromatin, and DNA wrapped around histone proteins, emphasising genetic organisation.
DNA is coiled around histone proteins to make chromatin
Diagram of a chromosome showing chromatids, centromere, and telomeres, linked to DNA structure with highlighted genes 1 and 2.
Simplified diagram of the structure of a chromosome

Examiner Tips and Tricks

It is important to distinguish when the terms chromatin, chromatids and chromosomes are used.

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