protein synthesis

• DNA contains information for the synthesis of proteins
• Genome


  The total set of all genes
• Gene


  The region of DNA which codes for a polypeptide, a specific sequence of DNA which codes for a polypeptide
• Genes code for individual proteins


  Determines the exact sequence of amino acids, primary structure, secondary structure, and function
• Transcription


  DNA is copied to mRNA, takes place in the nucleus
• RNA processing


  RNA goes through processing and modification to be ready for translation
• Translation


  mRNA is translated into a polypeptide chain, a protein
• Exons and Introns


  Exons are coding sequences, Introns are non-coding sequences in a gene
• Splicing


  Introns are spliced out, exons are joined together to form mature mRNA
• Triplet code


  Three nucleotide bases in DNA and RNA, three bases code for one amino acid
• The triplet code is degenerate, meaning multiple triplet codes can code for the same amino acid
• Codon


  Three bases in RNA, code for one amino acid in the protein
• Types of RNA involved in protein synthesis

  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
• Messenger RNA (mRNA)


  • Single-stranded, contains bases A, U, C, and G
• Transfer RNA (tRNA)


  • Transfers amino acids to the ribosome, 20 types for 20 types of amino acids
• Ribosomal RNA (rRNA)


  • Makes up the ribosome, involved in protein synthesis
• mrna


  Single-stranded RNA with bases A, U, C, and G (Uracil, Cytosine, Guanine) that is a copy of a gene coding for a polypeptide, made in the nucleus and moves to ribosome to code for a sequence of amino acids
• mrna Codon


  Three bases on the mrna that code for one amino acid
• mrna Start Codon


  Aug, codes for Methionine, the first amino acid in a polypeptide chain
• mrna Stop Codons


  UAA, UAG, UGA, code for a stop codon with no amino acid
• trna


  Transfer RNA shaped like a cloverleaf with three loops, carries specific amino acids to the ribosomes, has an anticodon that binds to the complementary codon at the ribosome
• trna Anticodon


  Forms complementary base pairs with the codon at the ribosome
• trna holds the amino acids in place, brings them to the ribosome, and can detach from an amino acid to be reused
• rrna


  Ribosomal RNA, makes up the ribosome, single-stranded, assembles into ribosomes with proteins, has one large and one small subunit, site of protein synthesis (translation)
• Ribosomes have two subunits, small and large, with specific binding sites for mrna and trna carrying amino acids
• Protein synthesis


  1. Two binding sites for tRNA carrying amino acids

  2. Large subunits contain peptidyltransferase which catalyzes the formation of peptide bonds to form a polypeptide

  3. Requires enzymes, ATP, occurs in the G1 geothermal phase 3 interface

  4. Enzymes needed include helicase and RNA polymerase

  5. RNA polymerase synthesizes the new strand of RNA in the 5' to 3' direction and catalyzes the formation of phosphodiester bonds

  6. Peptidyltransferase is needed to catalyze the formation of peptide bonds at the ribosome for the synthesis of a polypeptide chain
• Enzymes needed for protein synthesis

  • Helicase
  • RNA polymerase
  • Peptidyltransferase
• Transcription


  1. Copying of DNA gene to RNA

  2. Double helix unwinds, helicase breaks hydrogen bonds, only part of DNA unwinds

  3. RNA nucleotides complementary base pair with the DNA strand, RNA polymerase catalyzes the formation of mRNA in the 5' to 3' direction

  4. Primary transcript or pre-mRNA is formed

  5. Pre-mRNA is processed to remove introns and join exons to form mature mRNA
• Translation


  1. MRNA binds to ribosome, ribosome assembles around mRNA starting at the start codon AUG

  2. TRNA carrying methionine binds to the start codon

  3. Ribosome moves along the mRNA, reading the codons and bringing in the corresponding amino acids to form a polypeptide chain
• Translation


  Process of converting mRNA into a sequence of amino acids to form a protein
• Start codon


  • Always starts the translation process, typically AUG
• Stop codon


  • Ends the translation process, signals the ribosome to stop protein synthesis
• One tRNA molecule carries a specific amino acid to the ribosome by binding to the mRNA codon through complementary base pairing
• Peptide bond formation


  Occurs between amino acids held by two tRNA molecules in the ribosome, catalyzed by peptidyl transferase
• Multiple ribosomes can travel down one mRNA strand simultaneously, producing many polypeptides at the same time
• mRNA is short-lived to control gene expression and protein production efficiently
• Terminology in protein synthesis

  • DNA base = superfood
  • mRNA base = codon
  • tRNA carries anticodon to ensure correct amino acid is added to the polypeptide chain