Chromosome structure

• Chromosome structure:

  • When they first become visible at the start of cell division, chromosomes appear as two threads, joined at a single point
  • Each thread is called a chromatid because DNA has already replicated to give two identical DNA molecules

  
• Chromosome structure:

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

  • 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
• Chromosome structure:

  • The tightly coiled combination of DNA and proteins is called a 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 structure called telomeres

  
• Every chromosome consists of a long DNA molecule that codes for several different proteins

  • A length of DNA that codes for a single polypeptide or proteins is called a gene