Chromosome Structure (Cambridge International A-Level Biology)

Spec point — 5.1.1 · describe the structure of a chromosome, limited to: • DNA • histone proteins • sister chromatids • centromere • telomeres

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 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

  • These chromatids are 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

  • 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

Exam Hint It is important that you are able to distinguish when the terms chromatid, sister chromatids and chromosomes are used.