Translation

• Translation of mRNA depends on ribosomes and transfer RNAs
• Translation is the biological polymerization of amino acids into polypeptide chains
• Translation requires

  • Amino acids
  • mRNA
  • Ribosomes
  • Transfer RNA tRNA
• tRNAs adapt genetic information present as specific triplet codons in mRNA to their corresponding amino acid
• tRNAs have anticodons that complement the mRNAs
• tRNAs carry the corresponding amino acid
• Ribosomes consist of ribosomal proteins and ribosomal RNAs (rRNAs)
• Ribosomes have a large subunit and a small subunit
• The rRNAs perform important catalytic functions associated with translation
• rRNAs promote the binding of the various molecules involved in translation and fine-tune the process
• The rRNA genes, called rDNA, are part of a moderately repetitive DNA fraction and are present in clusters at various chromosomal sites
• Each clustered of rDNA contains tandem repeated separated by noncoding spacer DNA
• tRNAs are small in size and very stable

  -composed of 75-90 nucleotides

  -Transcribed from DNA and contain post-transcriptionally modified bases
• Modified bases enhance H-bonding efficiency during translation
• The 2-dimensional structure of tRNAs is a clover leaf
• A tRNA has an anticodon that complementarily base-pairs with the codon in the mRNA
• The corresponding amino acid is covalently links to the CCA sequence at the 3’ end of all tRNAs
• Before translation can proceed, tRNA molecules must be chemically linked to their respective amino acids
• Activation (charging or aminoacylation) done by aminoacyl tRNA synthetase
• There are 20 different synthetases, one for each amino acid, and they are highly specific since they recognize only one amino acid
•  Translation of mRNA can be divided into 3 steps
• INITIATION, which requires

  • The small and large ribosomal subunits
  • GTP
  • Charge initiator tRNA
  • Mg2+
  • Initiation factors (Ifs)
• In bacteria, the AUG start codon is preceded by a Shine (John)-Dalgarno (Lynn) sequence (AGGAGG), which base-pairs with a region of the 16S rRNA of the 30S small subunit, facilitating initiation
• This initiation complex (small ribosomal subunit + initiation factors + mRNA at codon AUG) then combines with the large ribosomal subunit
• ELONGATION requires both ribosomal subunits assembled with the mRNA to form the P (peptidyl) site and A (aminocycle) site
• The charged tRNAs enter the A site, and 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
• The sequence of elongation and translocation is repeated over and over
• TERMINATION is signaled by a stop codon ( UAG, UAA, UGA) in the A site
• GTP-dependent release factors cleave the polypeptide chain from the tRNA and release it from the translation complex
• Polysome (or polyribosomes) are mRNAs with several ribosomes translating at once
• žIn eukaryotes,—

  • the ribosomes are larger and longer lived than in bacteria—
  • transcription occurs in the nucleus
  • The 5′ end of mRNA is capped with a 7-methylguanosine residue at maturation, which is essential for translation
  • A poly-A tail is added at the 3′ end of the mRNA.—
  • translation occurs in the cytoplasm
• žMany eukaryotic mRNAs contain a purine (A or G) three bases upstream from the AUG initiator codon, which is followed by a G
• žThis Kozak (Marilynn)sequence is considered to increase the efficiency of translation by interacting with the initiator tRNA
• žTranslation in eukaryotes generally requires more factors for initiation, elongation, and termination than translation in bacteria does.
• žMany ribosomes are not free-floating as in bacteria but instead are associated with the endoplasmic reticulum