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

Features of genetic code, Genetics - wikidoc

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