Protein synthesis

• Nucleotide bases


  • A
  • G
  • C
  • T

  
• Protein synthesis


  Transcription & translation
• Gene


  Sequence of DNA nucleotides that contains the information to make a polypeptide using the genetic code

  
• Codon


  Triplet of bases that codes for a specific amino acid
• Genetic code


  Each triplet of bases codes for a specific amino acid
• DNA contains the information to make proteins, but DNA stays in the nucleus and proteins are synthesised by ribosomes in the cytoplasm - so the genetic code needs to be carried by a messenger to the ribosomes

  
• Amino acids


  • thr
  • glu
  • met
• Universal


  Same triplet (codon) code used by all organisms
• Degenerate


  Most amino acids are coded for by more than one codon, e.g. glycine = GGA, GGC, GGG and GGT
• DNA 'un-zips'


  Enzyme RNA polymerase joins RNA nucleotides together using phosphodiester bonds to make mRNA

  
• Uracil replaces Thymine in RNA
• Messenger RNA (mRNA)


  Copy of the 'sense' strand of the DNA - it contains the genetic code to make the polypeptide

  
• Transcription


  Making an mRNA copy of the DNA nucleotide sequence in a gene

  
• Transfer RNA (tRNA)


  Family of RNA molecules that each carry an amino acid, identity of the amino acid carried depends on sequence of the anti-codon

  
• Ribosomes (ribosomal RNA)


  Made of RNA and proteins, synthesise polypeptides

  
• Anti-codon


  Triplet of exposed bases that bind to codon of mRNA

  
• Translation


  Ribosomes synthesise the polypeptide by 'reading' the mRNA, tRNA molecule binds to each codon on mRNA, amino acid carried by tRNA is added to the polypeptide chain

  
• Part of the human insulin mRNA sequence

  • aug ggc cug ugg aug cgc cuc cug ccc cug cug gcg cug cug gcc
• The sequence of the 'sense' DNA strand from this part of the gene is: atg ggc ctg tgg atg cgc ctc ctg ccc ctg ctg gcg ctg ctg gcc
• The sequence of the DNA strand this mRNA was transcribed from is: tac ccg gac acc tac gcg gag gac ggg gac gac cgc gac gac cgg

  
• The amino acid sequence this mRNA codes for is: met gly leu trp met arg leu leu pro leu leu ala leu leu ala
• Transcription in eukaryotes


  • Requires promoter, transcription factors, and RNA polymerase

  
• Transcription


  1. Promoter - region where transcription factors and RNA polymerase bind

  2. Transcription factors - proteins that bind to DNA and help RNA polymerase bind to promoter

  3. RNA polymerase - synthesises mRNA, catalyses formation of phosphodiester bonds between RNA nucleotides

  
• Gene


  DNA sequence that codes for the synthesis of a single polypeptide chain

  
• Eukaryotic genes


  • Consist of promoter, exons (coding sequence), and introns (non-coding sequence)

  
• Introns


  Non-coding sequence, some evidence for role in regulation of gene expression, but mostly old inactive sequence

  
• Intron (non-coding) sequences must not be present in the mRNA at translation

  
• Splicing


  1. Process of removing introns from the newly synthesised mRNA

  2. Splicesome (complex of snRNPs - small nuclear ribonucleoproteins) cuts intron at 5' end and joins to branch site, then cuts intron at 3' end and joins together exons

  
• Alternative splicing


  Regulated process in eukaryotes that allows a single gene to code for several proteins, by either including or excluding particular exons from the final mRNA

  
• Alternative splicing greatly increases the biodiversity of proteins that can be encoded by the genome; in humans, ~95% of multi-exonic genes are alternatively spliced

  
• Mature mRNA molecules


  • Have 5' cap of methylated guanines and 3' sequence of ~200 adenines (polyA tail) to prevent degradation by enzymes

  
• Genetic code


  Each triplet of bases (codon) codes for a specific amino acid

  
• Encoding information to make a polypeptide


  Within the sequence of DNA nucleotides of a gene

  
• Nucleotide bases


  • A
  • G
  • C
  • T
• Genetic code


  • Universal - same triplet (codon) code used by all organisms
  • Degenerate - most amino acids are coded for by more than one codon, e.g. glycine = GGA, GGC, GGG and GGT

  
• Mutation


  A change to the sequence of bases in the DNA of a gene

  
• Main types of mutation

  • Substitution - replacement of one nucleotide with a different nucleotide
  • Insertion - addition of one or more nucleotides
  • Deletion - loss of one or more nucleotides
• Point mutations


  • Mutations that change the identity of one amino acid, so protein of reduced/altered function is made - usually harmful to the organism
  • Caused by substitution of nucleotides (1/2 or 3) giving a codon that codes for a different amino acid

  
• Neutral mutations


  • Mutations that do not change the identity of any amino acids, so a fully-functional protein is made so they have no effect on the organism
  • Caused by substitution of nucleotides (1/2 or 3) giving a codon that codes for the same amino acid

  
• Neutral mutations


  • Bacterial resistance to antibiotics