Protein Synthesis

• Griffith experiment


  2 strains of pneumococcus: S-strain with smooth, shiny colonies due to slime capsule, R-strain with rough appearance due to absence of slime capsule
• Griffith experiment


  Mice injected with: 1) Live S-strain → mice died, 2) Live R-strain → mice survived, 3) Heat-killed S-strain → mice survived, 4) Heat-killed S-strain + Live R-strain → mice died, live S-strain bacteria found in blood samples
• Hershey-Chase experiment


  Studied T2 bacteriophage, used radioactive labelling to show viral DNA enters bacterial cells while protein coat remains outside
• Genetic code is the sequence of bases in DNA and mRNA that determines the sequence of amino acids in a polypeptide
• Characteristics of the genetic code

  • Triplet code, non-overlapping, degenerate, universal, unpunctuated, polarity, different frequencies in different organisms
• Only a small fraction (less than 2%) of DNA codes for proteins, the rest is non-coding DNA
• Central dogma of protein synthesis

  Information on DNA is transcribed onto RNA and then translated into a chain of amino acids forming a protein
• Transcription


  Initiation: RNA polymerase unwinds DNA and binds to promoter sequence, 2) Elongation: RNA polymerase adds free nucleotides to form mRNA complementary to template strand, 3) Termination: Transcription stops when RNA polymerase meets termination sequence
• Post-transcriptional processing in eukaryotes

  1° mRNA with introns and exons is processed by removing introns and splicing exons, adding 5' cap and 3' poly-A tail
• Amino acid activation


  Activating enzyme binds with specific amino acid and ATP, 2) ATP loses 2 phosphates to form AMP, 3) Amino acid-AMP complex forms, 4) Specific tRNA binds to active site, 5) Enzyme catalyses reaction to attach amino acid to tRNA, 6) Activating enzyme releases tRNA-amino acid complex
• Translation


  Initiation: Start codon AUG binds to initiator tRNA, small ribosomal subunit joins, 2) Elongation: Ribosome moves along mRNA, adding amino acids to polypeptide chain, 3) Termination: Stop codon enters A-site, release factor causes polypeptide dissociation and ribosome dissociation
• Post-translational processing


  Newly synthesised polypeptide is folded into 3D shape by chaperone proteins, may be modified by attachment to lipids or oligosaccharides
• Lac operon


  Group of genes that function as a single transcription unit, controlling production of enzymes for lactose metabolism in E. coli
• Control of lac operon

  When lactose is absent, repressor protein binds to operator preventing RNA polymerase binding. When lactose is present, it binds to repressor, allowing RNA polymerase to transcribe lac operon genes
• Mutations


  Changes in the hereditary material, can be chromosomal or gene mutations
• Types of point mutations

  • Addition, deletion, base pair substitution, inversion
• Effects of mutations


  Silent, missense, nonsense, frameshift
• Causes of mutations


  • Spontaneous errors, induced by mutagens like radiation or chemicals
• Mutations give rise to new alleles and genetic variation, allowing populations to evolve