4.1+4.2

• Start codon


  TACIAUG - initiates translation at the start of a gene
• Stop codon


  3 bases at the end of every gene that do not code for an amino acid, they mark the end of the polypeptide chain
• Translation


  1. Start codon initiates

  2. Polypeptide chain is synthesised

  3. Stop codon causes ribosome to detach and translation to stop
• There are 64 possible triplet combinations in the genetic code, but only 20 amino acids
• Degenerate


  The genetic code has more triplet combinations than needed to code for the 20 amino acids
• The same triplet of bases codes for the same amino acid in all organisms
• Non-overlapping genetic code


  • Each base in a gene is only part of one triplet, read as a discrete unit
• Introns


  Sections of DNA that do not code for amino acids, removed during mRNA processing
• Exons


  Sections of DNA that do code for amino acids
• Genome is the complete set of DNA in one cell, proteome is the full range of proteins in one cell
• Genomes don't change, proteomes change depending on which proteins are currently needed
• Eukaryotic DNA

  • Linear, associated with histone proteins, coiled up into chromosomes in the nucleus
• Prokaryotic DNA


  • Circular, not associated with histones, condenses by supercoiling to fit in the cell
• Eukaryotic cells also have DNA in mitochondria and chloroplasts, similar to prokaryotic DNA
• Alleles


  Different forms of the same gene, code for slightly different versions of the same polypeptide
• Homologous chromosomes have the same genes but can have different alleles
• Exons are the sections of DNA that code for amino acids, introns are the non-coding sections
• Non-coding repeats are DNA sequences that repeat over and over but don't code for amino acids