OCR AS Biology

Revision Notes

2.3.3 DNA: Structure

DNA Structure

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

DNA polynucleotide strand, downloadable AS & A Level Biology revision notes, downloadable AS & A Level Biology revision notes

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

DNA polynucleotide strand (2), downloadable AS & A Level Biology revision notes

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 complementary base pairs

DNA molecule with hydrogen bonding, downloadable AS & A Level Biology revision notes

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 formed by the twisting of the DNA molecule

DNA double helix formation, downloadable AS & A Level Biology revision notes

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

Exam Tip

Make sure you can name the different components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, phosphodiester bonds, hydrogen bonds) and make sure you are able to locate these on a diagram.

You must know how many hydrogen bonds occur between the different base pairs.

Remember that the bases are complementary so the number of A = T and C = G, as you could be asked to determine how many bases are present in a DNA molecule if given the number of one of the bases.

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