OCR Gateway GCSE Chemistry

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6.2.5 DNA

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

The DNA helix is made from two strands of DNA held together by hydrogen bonds, IGCSE & GCSE Biology revision notesDiagram showing the complex double helix structure of DNA

Other Naturally Occurring Polymers

  • Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula Cx(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

Amylose, IGCSE & GCSE Chemistry revision notes

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 

Amino Acid - General Formula, IGCSE & GCSE Biology & Chemistry revision notes

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.

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