Genetic Code and Translation

• Cells depend on machinery to translate mRNA nucleotide sequences into specific amino acids.
• An intermediate adaptor molecule is required to recognize mRNA nucleotide sequences and specific amino acids.
• Transfer RNA (tRNA) serves as this adaptor molecule.
• At least 20 codons must exist to make the genetic code, as 20 amino acids are needed for protein synthesis.
• Codons are sequences of 3 nucleotide bases on mRNA, determining the type and position of an amino acid within a protein.
• The genetic code is the collection of genetic codons.
• There are 4 different nucleotides in mRNA, and the genetic codon is a triplet, resulting in (4)³ = 64 codons.
• AUG codes for methionine.
• Nonsense (termination) codons include UAA, UAG, and UGA, and they do not code for amino acids.
• These codons terminate the translation process.

Characteristics of the Genetic Code

• Multiple codons can code for one amino acid.
• With 61 codons and only 20 amino acids, multiple codon codes are necessary for the same amino acid.
• Arginine is an example of an amino acid that has six codons.
• For certain amino acids, the first 2 bases of the codon are constant, while the 3rd base varies, which is called wobbling.
• The 3rd base does not necessarily obey the base pairing rule.
• Glycine codons GGA, GGU, GGG, and GGC correspond to tRNA molecules carry the anticodon sequence CCG.
• This tRNA can pair with GGU and GGC, meaning the 3rd base of the tRNA anticodon can pair with both U and C of the codons.
• Each codon codes for only a single amino acid.
• Some tRNA molecules, however, can use their anticodon to recognize more than one codon.
• The reading of genetic codons during protein synthesis does not involve any overlap of codons
• There is no punctuation between codons; the message is read in a continuous sequence of nucleotide triplets until a translation stop codon is reached.
• Until recently, the code was considered universal, however some mitochondrial tRNA molecules read 4 codons differently from tRNA in the cytoplasm of the same cell.
• The anticodon arm of the tRNA recognizes the mRNA codon.
• The binding of the tRNA anticodon to the codon follows the rules of complementarity and antiparallel orientation.
• The codon is read 5' → 3', while the anticodon is read 3' → 5'.
• The anticodon arm is responsible for the specificity of tRNA.

Charging of tRNA

• Charging of tRNA is the binding of the tRNA to its specific amino acid.
• Amino acyl tRNA synthetases carry out this process.
• This process occurs in two steps which are: formation of the aminoacyl-AMP-enzyme complex, and the complex recognizing a specific tRNA to which the amino acid attaches to the 3'-hydroxyl end.