Biol 3130 Lecture 9

Flow of Genetic Information

Translation

• tRNA charging (attaching amino acids to appropriate tRNA)

• Incorporation of the initiator tRNA with small ribosomal subunit

• Locating the translational start site on the mRNA

• Assembly of full ribosome

• Synthesis of the first peptide bond

• Translocation and elongation

• Termination and release

Transfer RNAs (tRNAs)

• Adaptor molecules

• Transfer RNAs convert the genetic code found on mRNA codons to amino acids

• read the codon sequence through their anticodon

  • anticodon sequence corresponds to the amino acid attached to the tRNA

  • anticodon: Triplet nucleotide sequence on tRNA molecules that base-pairs with a corresponding mRNA codon within the ribosome during translation

• amino acid (technically, an aminoacyl when attached) is attached to the 3′ end of the tRNA, at a CCA-3′ sequence

  • 2′ or 3′ OH of the 3′ A residue covalently attaches to the amino acid group

Anticodon-Codon Base-pairing

• When the anticodon and the codon base pair, just like all types of base-pairing, strands are antiparallel.

• In the example shown for a Phe tRNA:

  • The codon sequence is: 5′-UUC-3′

  • The anticodon sequence is: 5′-GAA-3′

tRNA Charging

• refers to the attachment of an amino acid to the appropriate tRNA

• correct amino acid needs to be attached to the tRNA with the corresponding anticodon

• tRNA charging is a two-step process that is catalyzed by the enzyme aminoacyl-tRNA synthetase

tRNA Charging Steps

Step 1

• activation of amino acid

• an amino acid is activated by displacing the diphosphate on ATP

  • powered by energy released by breaking a phosphoanyhdride bond

• Aminoacyl-AMP is formed and pyrophosphate is released

Step 2

• transfer to the tRNA

• the aminoacyl group is transferred to the tRNA

E. coli Ribosome

• Ribosomes are composed of a large subunit + a small subunit

• subunit components are named according to their sedimentation coefficient: (how far they travel through a solution when spun in an ultracentrifuge)

  • Sedimentation coefficients depend on mass, density, and shape of the particles

• E. coli ribosome (70S) is composed of a 50S large subunit, and a 30S small subunit

  • Each subunit is composed of rRNAs, and many proteins