• 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