1.3 4

• DNA


  Controls protein synthesis and so DNA instructions control how the cell is built and how it works
• Proteins


  Made up of amino acids, joined together in countless combinations to make an almost infinite variety of proteins
• Genetic code


  Based on genes, a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain, that affect a characteristic in the phenotype of the organism
• Triplet code


  A sequence of three bases along a strand of DNA codes for a particular amino acid, or signals the beginning or end of an amino acid sequence
• Codon


  A sequence of three bases on the DNA or RNA
• mRNA


  Formed as a complementary strand to the DNA, a reverse image of the original base sequence
• The genetic code is identical throughout the living world
• Large parts of the DNA do not code for proteins
• Non-coding DNA sequences are very important, involved in regulating the protein-coding sequences
• The genetic code is non-overlapping and degenerate
• M.W. Nirenberg (1927-2010) and H. Matthaei (1929-): 'Prepared artificial mRNA where all the bases were uracil, and found chains of a single type of amino acid, phenylalanine. UUU appeared to be the mRNA codon for phenylalanine.'
• The minimum length of artificial mRNA that would bind to a ribosome was three bases long - a single codon</b>
• Non-overlapping code


  Triplets of bases follow each other along the strand like beads on a necklace, and do not overlap
• Overlapping code


  Triplets of bases overlap, so a single sequence could code for multiple amino acids
• Non-overlapping code


  • Relatively short lengths of DNA could carry instructions for many different proteins
  • The amino acids that could be coded for side by side would be limited
• Overlapping code


  • Very economical
  • Very limiting in the amino acids that could be coded for side by side
• Experimental observations suggest the genetic code is non-overlapping
• A single nucleotide change in a non-overlapping code only affects one amino acid
• Degenerate code


  The genetic code contains more information than is needed, where often only the first two nucleotides in a codon determine the amino acid
• Degenerate code


  • Protects living organisms from the effects of mutations, as a change in the final base may not affect the amino acid
• Translation


  The process by which proteins are produced, via RNA, using the genetic code found in the DNA. It takes place on the ribosomes.
• Ribosomes


  The site of protein synthesis in the cell
• Triplet code


  The code of three bases, which is the basis of the genetic information in the DNA
• Gene


  A sequence of bases on a DNA molecule that contains coding for a sequence of amino acids in a polypeptide chain that affect a characteristic in the phenotype of the organism
• Codon


  A sequence of three bases in DNA or mRNA
• Complementary strand


  The strand of RNA formed that complements the DNA acting as the coding strand