DNA (deoxyribonucleic acid) and other naturally occurring polymers — AQA GCSE Chemistry

DNA is a naturally occurring polymer with a very large molecular size, which is why it is described as a large molecule or macromolecule. It is essential for life because it stores the information that living organisms need to grow, develop and function, and it is passed from parents to offspring. In chemistry terms, DNA is a polymer made from repeating monomer units called nucleotides. Its large size allows it to carry a very long sequence of coded information in a stable form inside cells. For example, the instructions for making proteins are encoded in the order of nucleotides along a DNA chain. You should be able to state that DNA is a large molecule, recognise it as a natural polymer and explain its importance to living organisms.

DNA acts as a coded set of instructions. The sequence of nucleotides along a DNA chain encodes the information that controls how a living organism or virus develops and how it functions. Development includes growth and the formation of different cell types, while functioning includes the everyday chemical reactions and processes that keep an organism alive. In viruses, DNA can also carry the instructions needed for the virus to replicate inside a host cell. The code works because the order of the four nucleotide bases is read in groups, and this order determines the sequence of amino acids in proteins. For example, a particular stretch of DNA may encode a protein enzyme needed for a specific reaction. You should be able to explain that DNA stores instructions and that the sequence of nucleotides carries the code.

Most DNA molecules consist of two polymer chains that wind around each other to form a double helix. Each chain is a polymer built from repeating monomer units called nucleotides. There are four different nucleotides in DNA, and their order along a chain carries the genetic code. The two chains are held together by bonds between the bases of the nucleotides, and the pairing of bases means the chains are complementary. The double helix shape is like a twisted ladder, with the two chains forming the sides and the paired bases forming the rungs. For example, the four nucleotides differ in their bases, usually represented by the letters A, T, C and G, and the sequence of these bases along one chain encodes information. You should be able to state the number of chains, the number of different monomers and the double helix form.

Naturally occurring polymers are large molecules built from repeating smaller units joined by covalent bonds. Proteins are polymers of amino acids; the sequence of different amino acids gives each protein its specific shape and function, such as enzymes or structural collagen. Starch and cellulose are both polymers of glucose, but they differ in how the glucose units are joined and arranged. Starch is a storage polysaccharide in plants, while cellulose forms strong plant cell walls. Recognising that these materials are polymers, and identifying the type of monomer each is built from, links biological molecules to the polymer chemistry you study elsewhere. For example, a protein chain can be represented as a sequence of amino acid units, whereas cellulose can be represented as a chain of glucose units.

A monomer is a small molecule that can join with many identical or similar molecules to form a polymer. For the naturally occurring polymers important for life, you need to name the monomer type for each. Proteins are made from amino acids; different amino acids can be joined in a chain, and the order of amino acids determines the protein. Starch and cellulose are both made from glucose, so glucose is the monomer for both. DNA is made from nucleotides, which are the monomers of nucleic acids. When naming monomers, be precise: 'amino acid' not 'protein', and 'glucose' not 'sugar' in general. A useful check is to ask whether the named monomer could join repeatedly to build the polymer; if it cannot, it is probably not the monomer.