The genetic code is the rulebook for translating DNA into proteins. It's a triplet code where three nucleotides (a codon) specify one amino acid. This system is non-overlapping, unambiguous, and nearly universal across all life forms.

There are 64 possible codons, with 61 coding for amino acids and 3 serving as stop signals. The code is degenerate, meaning multiple codons can specify the same amino acid. This redundancy provides a buffer against mutations and influences codon usage in different organisms.

The Genetic Code

Features of genetic code

• Triplet code consists of three nucleotides (codon) that specify one amino acid
• Non-overlapping means codons are read sequentially without any overlap between them
• Comma-less indicates there is no punctuation or breaks between codons
• Unambiguous signifies that each codon specifies only one amino acid without ambiguity
• Degenerate means multiple codons can code for the same amino acid (synonymous codons)
• Near-universal denotes that the genetic code is conserved across most organisms (bacteria, plants, animals)
• Exceptions exist in mitochondrial and certain protozoal genetic codes that slightly deviate from the standard code

Codons and amino acids

• 64 possible codons exist based on the four nucleotide bases (A, U, C, G)
  • 61 codons specify amino acids and are called sense codons
  • 3 codons are stop codons that terminate protein synthesis
• 20 amino acids are coded by the genetic code
  • Multiple codons can code for the same amino acid due to degeneracy
• Codon-amino acid correspondence is determined by the nucleotide sequence
  • First nucleotide of the codon has the strongest influence on amino acid specificity
  • Second nucleotide has a moderate influence on amino acid choice
  • Third nucleotide (wobble position) has the least influence on amino acid selection

Start and stop codons

• Start codon AUG codes for methionine and initiates protein synthesis
  • Methionine is the first amino acid incorporated into the growing polypeptide chain
• Stop codons UAA (Ochre), UAG (Amber), and UGA (Opal) terminate protein synthesis
  • Release factors (RF1, RF2) recognize stop codons and release the polypeptide chain from the ribosome
  • Premature stop codons can result in truncated proteins

Degeneracy in genetic code

• Degeneracy refers to multiple codons coding for the same amino acid
  • 61 codons code for 20 amino acids, resulting in redundancy
  • Amino acids can be coded by 1-6 different codons (Met, Trp have one codon; Leu, Ser, Arg have six)
• Wobble hypothesis explains the flexibility in base pairing at the third position of the codon
  • Allows tRNAs to recognize multiple codons through non-Watson-Crick base pairing (G-U wobble)
  • Reduces the number of tRNAs required to decode the genetic code
• Implications of degeneracy include redundancy and protection against mutations
  • Silent mutations change the codon but not the amino acid, minimizing impact on protein function
  • Missense mutations change the amino acid but may have minimal effect if the new amino acid is similar
• Codon usage bias refers to the preferential use of certain codons for an amino acid
  • Varies among species and genes based on tRNA abundance and translational efficiency
  • Highly expressed genes tend to use optimal codons that match abundant tRNAs