AQA AS Biology

Revision Notes

1.5.3 The Structure of DNA

The Structure of DNA

  • 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 DNA 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 that DNA molecules form

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.

Author: Amelia

While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.
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