Chromosome Structure (CIE A Level Biology)

• 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

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

Simplified diagram of the structure of a chromosome

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