DNA and chromosomes

• In prokaryotic cells, such as bacteria, the DNA molecules are short, circular and not associated with proteins

  • Therefore do not have chromosomes
• In the nucleus of eukaryotic cells, the DNA molecules are very long, linear and associated with proteins called histones to form chromosomes
• The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotic cells, is short, circular and not associated with proteins
• Histones role:

  • To organise and condense the DNA tightly so that it fits into the nucleus
• Homologous pair - Two chromosomes that carry the same genes but not necessarily the same alleles of the genes
• Homologous chromosomes - A set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilisation