6.2.5 DNA

• Deoxyribonucleic acid (DNA) is a large molecule which is essential to all life
• It contains genetic information which it encodes as instructions which organisms need to develop and function correctly
• DNA consists of four different monomers called nucleotides which contain small molecules called bases and which are abbreviated to A, T, C, and G which are bound together by polymerisation
• The nucleotides form two strands that intertwine, giving the famous double helix shape of DNA
• The bases on either polymer chain pair up in specific sequences forming cross links that hold the strands together, giving rise to the double helix shape
• It is a complex molecule that contains genetic information which is stored in the order in which the bases organise themselves, which is a code for the organisms gene

Diagram showing the complex double helix structure of DNA

Other Naturally Occurring Polymers

• Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula C_x(H₂O)_y
• There are simple carbohydrates and complex carbohydrates
• Simple carbohydrates are called monosaccharides and are sugars such as fructose and glucose
• Complex carbohydrates are called polysaccharides such as starch and cellulose
• The monomers from which starch and cellulose are made are both sugars
• Starch is used to store energy and cellulose is a stiff polymer used in plant cell walls to provide support
• Complex carbohydrates are condensation polymers formed from simple sugar monomers and, unlike proteins, are usually made up of the same monomers
• An H₂O molecule is eliminated when simple sugars polymerise
  • The linkage formed is an -O- linkage and is called a glycosidic linkage

Diagram of the starch amylose showing glycosidic linkages (-O-) which bind the monomers together, Amylose makes up approximately 20-30% of starch

Natural Polymers with Biological Functions

• Proteins are also important natural polymers with specific biological functions
• Some examples of proteins and their functions include:
  • Haemoglobin which transports oxygen in the blood
  • Antibodies in the immune system help protect the body from viruses and bacteria
  • Enzymes which are biological catalysts