ch 14

Cards (57)

  • Translation
    Biological polymerization of amino acids into polypeptide chains
  • Requirements for translation
    • Amino acids
    • Messenger RNA (mRNA)
    • Ribosomes
    • Transfer RNA (tRNA)
  • tRNA
    Adaptor molecule that adapts genetic information present as specific triplet codons in mRNA to corresponding amino acid
  • tRNA
    • Small in size and very stable
    • 75-90 nucleotides
    • Transcribed as larger precursors and cleaved into mature tRNA molecules
    • Several regions of complementary nucleotide stretches in tRNA primary sequence yield 'stem loops' and determine structure
  • Modified bases in tRNA
    Inosine, dihydrouridine, and pseudouridine are common modified bases
  • All tRNA have ACC at the 3' end and G at the 5' end
  • tRNA anticodon
    • Complementarily base-pairs with codon in mRNA
    • Corresponding amino acid is covalently linked to CCA sequence at 3' end of all tRNAs
  • Aminoacylation: tRNA charging

    1. Amino acid is activated by reaction with ATP to create an aminoacyl adenylic acid (5' phosphate and carboxyl group of amino acid)
    2. Aminoacyl tRNA synthetase enzyme catalyzes this process
  • Aminoacyl tRNA synthetase

    • Enzyme that catalyzes aminoacylation
    • ~31 tRNAs and 20 different tRNA Aminoacyl tRNA synthetases
    • Highly specific; recognize only one amino acid
    • Mischarged tRNAs are "edited" by removing the incorrect amino acid
  • Ribosome
    • Have an essential role in expression of genetic information
    • Consist of ribosomal proteins and ribosomal RNAs (rRNAs)
    • Consists of large and small subunits
    • Prokaryote ribosomes are 70S, Eukaryote ribosomes are 80S
    • rRNAs provide for important catalytic functions associated with translation
    • Bacterial cell contains approximately 10,000, many more in eukaryotic cells
    • rRNA genes highly redundant
  • Translation of mRNA can be divided into three steps
    1. Initiation
    2. Elongation
    3. Termination
  • Initiation of translation
    1. Requires small and large ribosomal subunits, mRNA molecule, GTP, charged initiator tRNA, Mg2+, Initiation factors
    2. Translation starts with AUG (fmet)
  • Shine-Dalgarno sequence
    Ribosomal binding site in prokaryotes, precedes by 8 bases upstream of AUG start codon in mRNA of bacteria, base-pairs with region on 16S rRNA of 30S small subunit, facilitating initiation
  • A and P sites in ribosomes
    • Charged tRNAs enter the A site, peptidyl transferase catalyzes peptide bond formation between the amino acid on the tRNA at the A site and the growing peptide chain bound to the tRNA in the P site
    • The uncharged tRNA moves to the E (exit) site
    • The tRNA bound to the peptide chain moves to the P site
  • mRNA is read in the 5'-3' direction, the polypeptide chain is synthesized in the N-C direction
  • 23S rRNA
    Is the peptidyl transferase catalyst, catalyzes peptide bond formation between amino acid on tRNA at A site and growing peptide chain bound to tRNA in P site
  • Termination
    Signaled by stop codons (UAG, UAA, UGA) in A site, GTP-dependent release factors stimulate hydrolysis of polypeptide from peptidyl tRNA—released from translation complex
  • Polyribosomes (polysomes) are mRNAs with several ribosomes translating at once
  • Translation in eukaryotes
    • Ribosomes are larger than in bacteria
    • Transcription and translation are spatially and temporally separated
    • Presence of 7-methylguanosine essential, 3' poly-A required
    • No 'Shine-Delgarno like sequence', small ribosomal subunit associates with m7G cap (cap-dependent translation)
    • Requires more factors for initiation, elongation, and termination
    • Does not require formylmetionine, uses a unique tRNA tRNAimet
    • Poly-A binding proteins bind eIF4G, which binds eIF4E (cap binding protein) – 'closed loop translation'
    • Ribosomes can be associated with the endoplasmic reticulum
  • Kozak sequence
    Eukaryotic mRNAs contain a purine (A or G) three bases upstream from the AUG initiator codon, followed by G, considered to increase efficiency of translation by interacting with initiator tRNA
  • Closed loop translation in eukaryotes: The mRNA forms a loop that is closed where the cap and tail are brought together
  • Errors in metabolism like alkaptonuria and phenylketonuria result from mutations that lead to metabolic blocks, hundreds of medical conditions caused by errors in metabolism due to mutant genes
  • Sir Archibald Garrod's work on inborn errors of metabolism set the foundation for the "one gene-one enzyme" hypothesis
  • Individuals with alkaptonuria secrete homogentisic acid (2,5-dihydrophenylacetic acid) in urine, which turns black when exposed to air and also accumulates in cartilaginous tissues
  • Inborn Errors of Metabolism
    Errors in metabolism that result from mutations leading to metabolic blocks
  • Inborn Errors of Metabolism
    • Alkaptonuria
    • Phenylketonuria
  • Alkaptonuria
    Individual cannot metabolize alkapton
  • Phenylketonuria
    Individual is unable to convert phenylalanine to tyrosine
  • Hundreds of medical conditions caused by errors in metabolism due to mutant genes
  • Sir Archibald Garrod studied inborn errors of metabolism early in the 20th century
  • Garrod's work set the foundation for the "one gene-one enzyme" hypothesis
  • Homogentisic acid
    Also called alkapton (or alcapton), turns black when urine is exposed to air, also accumulates in cartilaginous tissues
  • Garrod also studied pentosuria, albinism, cystinuria and other diseases
  • Alkaptonuria
    • Reflects the absence of homogentisic acid oxidase activity
    • Inherited as a single gene recessive trait
  • Garrod was working at the same time as Mendel's work was being rediscovered
  • Garrod didn't fully understand Mendelian genetics, but understood the significance of multiple alkaptonurics in families and many parents of alkaptonurics being first cousins
  • Phenylketonuria (PKU)

    Phenylalanine hydroxylase is inactive, so phenylalanine is not converted to tyrosine, resulting in mental retardation
  • Beadle and Tatum demonstrated a link between gene and enzyme well in advance of the discovery of DNA structure, transcription or translation
  • One-Gene:One-Enzyme Hypothesis
    Genes act by regulating definite chemical events