CIE AS Biology (9700) exams from 2022

Revision Notes

5.1.1 Chromosome Structure

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
  • 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

Exam Tip

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

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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