In-Depth Notes on Gene Expression and Codon Deciphering

Gene Expression - Cracking the Code

Overview

  • Cracking the genetic code refers to understanding how genes are translated into proteins.

Key Discoveries

  • 1961: Key experimenters included Marshall Nirenberg and Heinrich Matthaei.
  • They used polymononucleotides to identify that the codon UUU encodes the amino acid phenylalanine.
  • Additional contributions by Khorana using polydinucleotides, polytrinucleotides, and polytetranucleotides helped to deduce the amino acid sequences and their corresponding codons.
  • There remained some ambiguities in the codon assignments.

Resolution of Ambiguities

  • Nirenberg and Philip Leder resolved previous ambiguities using:
    • Trinucleotide mRNAs of known sequences.
    • tRNAs charged with radioactive amino acids.
    • Ribosomes for the synthesis of proteins.

Nonsense Codons

  • Defined as codons that terminate the translation of a polypeptide chain.
  • Examples of nonsense (stop) codons include:
    • UAA (ochre)
    • UAG (amber)
    • UGA (opal)

mRNA Structure and Function

  • mRNA is read in the 5’ to 3’ direction, which corresponds to the N-terminal to C-terminal direction in the polypeptide.
  • DNA acts as a template strand during transcription, while the other strand acts similarly to RNA (RNA-like strand).

Universal Code and Exceptions

  • The universal genetic code is almost universally applicable across living organisms:
    • All living organisms utilize a similar genetic code allowing mRNA from one organism to be translated into proteins in another.
  • Exceptions include:
    • Some mitochondrial genomes.
    • Certain ciliates, which may exhibit alternative coding rules.