Protein Synthesis

• DNA has a triplet code, a sequence if 3 nucleotide bases
• Types of RNA involved in protein synthesis:

  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA) - 20 types
  • Ribosomal RNA (rRNA) - made in nucleolus, makes up the ribosome
• mRNA:

  • made in nucleus and moves to ribosomes
  • codes for sequence of amino acids
  • a codon is a set of 3 bases on mRNA
  • mRNA sequences are a series of codons
  • they dictate which amino acids will be added to the polypeptide chain
• Start codon - AUG, starts translation

  Stop codon - UAA, UAG, UGA, stops translation and production of polypeptide chain
• tRNA:

  • made in nucleus
  • found in cytoplasm and ribosomes
  • clover leaf shape
  • carries a specific amino acid to ribosomes
  • an anticodon is a specific exposed 3 bases on one loop which forms complementary bases pairing with codon mRNA at ribosome
  • tRNA holds amino acids in place for peptide bond formation at the ribosome
  • can be reused
• rRNA:

  • made in nucleolus
  • make up ribosomes
• Ribosomes:

  • site of protein synthesis aka translation
• Things needed for protein synthesis:

  • ATP
  • Helicase - to break H bonds to separate 2 DNA strands
  • RNA polymerase - to synthesised a new strand of RNA (5' to 3') and catalyse the formation of phosphodiester bonds
  • Peptidyl transferase - to catalyse the formation of peptide bonds
• Transcription:

  • DNA is copied to mRNA
  • takes place in nucleus
• Stage one: Transcription

  1. DNA double helix unwinds
  2. Helicase breaks H bonds - only 1 strand is used as template
  3. Free, activated RNA nucleotides form H bonds with bases on DNA template strand
  4. RNA polymerase attach to template to catalyse the formation of mRNA, RNA polymerase joins activated RNA nucleotides together, catalysing phosphodiester bonds formation
• Stage two: RNA processing

  • the pre-RNA has exons and introns
  • introns are non-coding sequences that are removed via RNA splicing
  • exons are coding sequences that are joined together to form mature mRNA, that leaves the nucleus via the nuclear pore
• Translation:

  • mRNA is translated into polypeptide chain
  • takes place at ribosomes at RER/cytoplasm
• Stage three: Translation

  1. mRNA binds to ribosome
  2. tRNA carries specific amino acid and binds to large subunit of ribosome, anticodon of tRNA forms complementary base pair to codon on mRNA by forming H bonds
  3. Second tRNA binds with next codon, the two tRNAs hold amino acids side by side to form peptide bond
  4. Peptidyl transferase in ribosome catalyse the formation of the peptide bond
  5. Ribosome moves along one more codon on mRNA, process continues until stop codon is reached