DNA (OCR Gateway GCSE Chemistry)

• 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

Amino Acids 

• Amino acids are small molecules containing the amino, NH₂, and carboxylic acid, COOH, functional groups
• The NH₂ group is basic and behaves in a similar way to ammonia
• The COOH group is acidic and is called a carboxyl group
• There are twenty naturally occurring amino acids and they all have the same general structure
• Different combinations of amino acids (monomers) join together to form very long protein molecules 

The structure of naturally occurring amino acids have an amino group on the second carbon along from the carboxyl group. The R represents a varying side group.