• 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 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.