A LevelBiologyCambridge (CIE)Revision Notes6. Nucleic Acids & Protein SynthesisStructure of Nucleic Acids & Replication of DNAThe Structure of DNA

The Structure of DNA (Cambridge (CIE) A Level Biology): Revision Note

Exam code: 9700

Marlene

Written by: Marlene

Reviewed by: Alistair Marjot

Updated on

DNA structure

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

Diagram of a DNA nucleotide showing phosphate group, pentose sugar (deoxyribose), and a nitrogenous base (A, C, G, T) with labelled carbons.
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

    • The phosphate group is 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

Diagram of a nucleotide structure, showing phosphate groups, pentose sugars, and nitrogenous bases including thymine, guanine, and adenine.
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

Diagram showing DNA structure with nucleotide pairing: adenine-thymine (2 hydrogen bonds) and cytosine-guanine (3 hydrogen bonds), sugar-phosphate backbone.
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

Diagram of DNA structure showing base pairs, antiparallel strands, sugar-phosphate backbone, and double helix; includes key for bases.
DNA molecules form a three-dimensional structure known as a DNA double helix

Examiner Tips and Tricks

Make sure you can:

  • Name the different components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, phosphodiester bonds, hydrogen bonds)

  • 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 = the number of T and the number of C = the number of G.

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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    Author
    Reviewer
    Marlene

    Author: Marlene

    Expertise: Biology Content Creator

    Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.

    Alistair Marjot

    Reviewer: Alistair Marjot

    Expertise: Environmental Systems and Societies & Biology Content Creator

    Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.