Himachal Pradesh PSC Botany Notes: Genetics: Translation

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Translation: Synthesis of a linear polymer of amino acids from a linear polymer of nucleotides

Where does it occur?

  • On the ribosome, a rRNA-protein complex that provides:
  • a scaffold for mRNA
  • sites for the docking of tRNA charged with a specific amino acid
  • an enzyme for peptide bond synthesis between amino acids
  • an enzyme for translocation of the mRNA through the ribosome

What is the function of tRNA?

Carrier of a specific amino acid during translation

What is the structure of tRNA?

  • secondary structure has some base-pairing → cloverleaf
  • information transfer at the anti-codon loop, complementary to the codon
  • note the importance of H-bonds in the genetic code
  • tertiary structure is L-shaped which places the amino acid far from the codon-anticodon site
  • degeneracy of the code produces wobble

What is the genetic code?

  • A sequence of 3 nucleotides forms a codon
  • unambiguous, each codon specifies an amino acid, or start, or stop
  • degenerate, some amino acids have multiple codons
  • 2-letters often sufficient, specifiy hydrophobic and hydrophillic amino acids

What is the enzyme that charges tRNA with an amino acid?

  • An aminoacyl-tRNA synthetase
  • it has proof reading capabilities through the precise fit of amino acid and tRNA
  • energy provided by ATP: Energy for the formation of aminoacyl-tRNA and for proof reading
  • there are at least 20 synthetases, isoaccepting for the tRNA's coding for a single amino acid

What is the mechanism of translation?

  • mRNA forms a large complex with the ribosome and protein factors
  • together they guide in the correct aminoacyl-tRNA
  • correct amino acid specified by codon-anticodon base pairing (H-bonds)
  • protein factors have proof reading capability--energy provided by GTP
  • an enzyme catalyzes polymerization of two amino acids, peptide (amide) bond formation between two amino acids
  • an enzyme catalyzes movement of mRNA through the polymerization site: Energy provided by GTP
  • mRNA translated from 5 ‘→ 3’ same direction as it is synthesized
  • Reprise: Flow of information: Central dogma
  • DNA → RNA → linear amino acid sequence → 3D-conformation of protein
  • But some viruses have only RNA as their genome: No DNA.

How do they carry out information transfer? How do they get around the unidirectional flow of information in the central dogma?

  • Use an enzyme called reverse transcriptase to transcribe RNA into DNA.
  • Example: HIV, a retrovirus
  • Then, use central dogma.
  • For HIV:
  • RNA → DNA → mRNA → linear amino acid sequence → 3D-conformation of protein.