• The nucleic acid DNA is a?polynucleotide?– it is made up of?many nucleotides?bonded together in a?long chain
• A?nucleotide?is a building block consisting of a?phosphate group,?2-deoxyribose?and a?nitrogenous base?such as?adenine,?guanine,?cytosine?and?thymine

The components that make up nucleotide in DNA

A DNA nucleotide

A single DNA polynucleotide strand showing the positioning of the ester bonds

Hydrogen bonding

• The two antiparallel DNA polynucleotide strands that make up the DNA molecule are?held together by hydrogen bonds?between the nitrogenous bases
• These hydrogen bonds always occur between the?same pairs of bases:
  • The purine?adenine?(A) always pairs with the pyrimidine?thymine?(T) – two hydrogen bonds are formed between these bases
  • The purine?guanine?(G) always pairs with the pyrimidine?cytosine?(C) – three hydrogen bonds are formed between these bases
  • This is known as?complementary base pairing
  • These pairs are known as?DNA base pairs

   

A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds

Double helix

• DNA is not two-dimensional as seen in the diagram above
• DNA is described as a?double helix
• This refers to the?three-dimensional shape?that DNA molecules form

DNA molecules form a three-dimensional structure known as a DNA double helix

• The weaker hydrogen bonds between the base pairs are more easily broken than the covalent bonds in the sugar-phosphate strands
• This is what allows the strands to break apart during cell division
• New nucleotides from the cells move in to pair up with the exposed bases, which then link by sugar-phosphate bonding and the helix molecules are replicated