Structure of DNA 

• Nucleotides (deoxyribose sugar, phosphate and base), sugar–phosphate backbone, base pairing (adenine - thymine and guanine - cytosine), by hydrogen bonds and double stranded antiparallel structure, with deoxyribose and phosphate at 3' and 5' ends of each strand respectively, forming a double helix.

  • The base sequence of DNA forms the genetic code.

Organisation of DNA 

• Prokaryotes have a single, circular chromosome and smaller circular plasmids.

• Eukaryotes all have linear chromosomes, in the nucleus, which are tightly coiled and packaged with associated proteins. 

  • The associated proteins are called histones.

• They also contain circular chromosomes in their mitochondria and chloroplasts. 

• Yeast is a special example of a eukaryote as it also has plasmids.