Gene expression and protein synthesis

• Gene expression and protein synthesis

  The process by which genetic information is used to direct the synthesis of proteins
• Mendel's Pea Plants

  1866
• Beadle and Tatum

  1940s
• Auxotrophy


  The inability of an organism to synthesize a particular compound required for its growth
• One gene-one enzyme hypothesis

  Each gene dictates production of a specific enzyme
• One gene-one polypeptide


  Each gene codes for a specific polypeptide
• Gene expression


  The process by which DNA directs protein synthesis, including transcription and translation
• Genome

  A complete set of genetic instructions for any organism, typically DNA (sometimes RNA)
• The coding system for genetic information is the same in all living organisms
• The genome is copied during the process of replication
• Transcription

  The synthesis of RNA using information in DNA
• Translation

  The synthesis of a polypeptide, using information in the mRNA
• Ribosomes

  The sites of translation
• Transcription and translation occur differently in prokaryotic and eukaryotic cells
• Genetic code


  The instructions for assembling amino acids into polypeptides encoded in DNA
• Codon


  A triplet RNA code
• Types of codons


  • Stop codons (termination codon)
  • Sense codons
• Degeneracy of the genetic code

  More than one codon may specify a particular amino acid, but no codon specifies more than one amino acid
• The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
• Reading frames


  The three possible "frames" that we can "read" the mRNA sequence
• Types of mutations


  • Base substitutions
  • Base insertions and deletions
• Missense mutation


  A mutation that changes the amino acid sequence
• Nonsense mutation


  A mutation that introduces a premature stop codon
• Silent mutation


  A mutation that does not change the amino acid sequence
• Loss-of-function mutation


  A mutation that results in reduced or abolished protein function
• Gain-of-function mutation


  A mutation that confers new or enhanced activity to a protein
• Conditional mutation


  A mutation that causes a mutant phenotype in only certain environments
• Lethal mutation


  A mutation that affects the survival of the organism
• Oncogene


  A gene that encodes a protein able to induce cancer
• Proto-oncogene


  A normal cellular gene that produces a protein that enhances cell division or inhibits normal cell death
• Transfer RNA (tRNA)


  Carries a specific amino acid on one end and has an anticodon on the other end
• Anticodon


  Base-pairs with a complementary codon on mRNA
• Aminoacyl-tRNA synthetase


  Enzyme that attaches amino acids to tRNA
• Ribosome


  Facilitates specific coupling of tRNA anticodons with mRNA codons in protein synthesis
• Translation initiation


  1. A small ribosomal subunit binds with mRNA and a special initiator tRNA

  2. Small subunit moves along the mRNA until it reaches the start codon (AUG)

  3. Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
• Elongation of the polypeptide chain

  1. Codon recognition

  2. Peptide bond formation

  3. Translocation
• Termination of translation


  1. Ribosome reaches a stop codon on mRNA

  2. Release factor promotes hydrolysis

  3. Ribosomal subunits and other components dissociate
• Post-translational modifications (PTMs)


  Modifications made to polypeptides after translation
• Protein localisation


  Targeting of proteins to specific areas of the cell
• Pure breeding


  Breeding of organisms that produce offspring with the same traits as the parents