AQA A Level Biology

Revision Notes

4.1.2 Eukaryotic DNA

Chromosome Structure

  • Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells)
    • 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

Chromatin and histone proteins, downloadable AS & A Level Biology revision notes

DNA is coiled around histone proteins to make chromatin

  • During interphase (S phase) the DNA replicates to create two identical strands of DNA called chromatids, joined together by a narrow region called the centromere
  • The two chromatids that make up the double structure of a chromosome are known as ‘sister chromatids’
  • It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as one chromatid goes into one daughter cell and one goes into the other daughter cell during mitosis, ensuring the daughter cells are genetically identical
  • Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes
  • The ends of the chromatids in chromosomes are ‘sealed’ with protective structures called telomeres

_Structure of chromosome, downloadable AS & A Level Biology revision notes

Simplified diagram of the structure of a chromosome

Chromosome structure, downloadable AS & A Level Biology revision notes

Diagram illustrating the structure of a chromosome before and after the S phase

Exam Tip

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

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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