CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.7.8 Structure & Properties of DNA

Significance of Hydrogen Bonding in DNA

  • Deoxyribo nucleic acid (DNA) is made up of nucleotides
  • Each nucleotide is made up of a phosphate group, a sugar molecule and an amino acid base
  • Four types of bases can be found in the DNA structure: Adenine (A), Guanine (G), Thymine (T) and Cytosine (C)
  • Two strands of DNA makes up the double helix structure
  • The bases from one strand forms hydrogen bonds with a complimentary base of the opposite strand of DNA
    • Adenine (A) forms two hydrogen bonds with Thymine (T)
    • Guanine (G) forms three hydrogen bonds with Cytosine (C)
  • In addition to hydrogen bonds between two bases, there are Van der waals forces between one pair and another
  • DNA replication is a vital process whereby two identical DNA molecules are produced by replicating one
  • In order for this to occur, the DNA double helix must first separate
  • This separation of the two strands is through the breaking of the intermolecular forces and the hydrogen bonds that hold the base pairs together
  • Once separated, each of the bases on the strands are matched with their complementary base
  • So forming two new pairs of strands, that are identical to the original DNA molecule

Primary, Secondary & Tertiary Structure of DNA

Primary structure of proteins

  • Amino acid monomers polymerise through condensation polymerisation to make polymer chains containing peptide links (referred to as amide links in chemistry)
  • Dipeptides are formed when two amino acids polymerise
  • Proteins are essentially polypeptides as many amino acid molecules join together to form proteins
    • Once polymerisation occurs, the monomers are not referred to as amino acids
    • They become known as amino acid residues – when two amino acids combine, a water molecule is lost


  • The primary structure consists of a chain of amino acids covalently bonded together through peptide bonds
  • Each amino acid residue has a name
    • For example glycine has the abbreviation ‘Gly’
  • Primary protein structure is mostly limited to considering just the covalent bond between amino acids and another type of covalent bonding known as a sulfur bridge
  • A sulfur bridge is formed if two cysteine molecules are next to each other when the protein folds
  • The sulphur atoms form their own covalent bonds

Secondary structure of proteins

  • The secondary structure takes into account interactions between strands of proteins
  • There are two types: α-helices and β-sheet
  • α-helix is formed as a protein strand is folded into a coil like that of a screw
    • Hydrogen bonds form between the oxygen atom of the peptide C=O and nitrogen atom of the peptide N-H
    • These hydrogen bonds stabilise the α-helix structure of the protein
  • β-sheet is formed when chains of proteins lie alongside each other as they fold
  • Hydrogen bonds form between the oxygen atom of C=O groups on the chain and the hydrogen atoms of N-H groups on another part of the protein chain

Tertiary structure of proteins

  • The tertiary structure arises through more complex folding of secondary protein structures
  • They are stabilised through hydrogen bonds, van der waals forces, covalent bonds (sulphur bridges) and ionic interactions between amino acid R groups
  • Some amino acid bases contain extra amine groups (-NH2) and some contain extra acid groups (-COOH)
  • When these groups are ionised, a species called a Zwitterion forms
  • A proton is transferred from the acid group (forming -COO) to an amine group forming -NH3+)
  • This ionic interaction can be seen with lysine and aspartic acid
  • Some of the amino acid residues are nonpolar where there is an abundance of van der waals interactions
  • All of these interactions work in unison to form a highly stabilised tertiary protein structure.

Exam Tip

  • The biggest difference between the 3 levels of protein folding is the degree of interactions
  • So pay close attention to your explanations of hydrogen bonds, sulphur bridges, ionic interactions in exam questions

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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