• The nucleic acid DNA is a?polynucleotide?– it is made up of?many nucleotides?bonded together in a?long chain

A DNA nucleotide

• DNA molecules are made up of?two polynucleotide strands?lying side by side, running in opposite directions – the strands are said to be?antiparallel
• Each DNA polynucleotide strand is made up of?alternating deoxyribose sugars and phosphate groups bonded together?to form the?sugar-phosphate backbone.?These bonds are?covalent?bonds?known as?phosphodiester bonds
  • The phosphodiester bonds link the?5-carbon of one deoxyribose sugar?molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the?3-carbon of the deoxyribose sugar molecule of the next nucleotide?in the strand
  • Each DNA polynucleotide strand is said to have a?3’ end and a 5’ end?(these numbers relate to which carbon on the pentose sugar could be bonded with another nucleotide)
  • As the strands run in opposite directions (they are?antiparallel), one is known as the?5’ to 3’ strand?and the other is known as the?3’ to 5’ strand
• The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule

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