Mathematical Logic: Since there are 4 different nitrogenous bases (A, C, G, U) and 20 amino acids, a singlet code () or doublet code () would be insufficient. A triplet code provides possible combinations, which is more than enough to cover all 20 amino acids.
Non-overlapping Reading: The code is read in discrete sets of three bases without any overlap. This means each base in a sequence is part of exactly one codon, ensuring that a single mutation typically only affects one amino acid unless it causes a shift in the entire reading frame.
Commaless Structure: There are no 'commas' or spacers between codons in a gene. The sequence is read continuously from a fixed starting point, meaning the deletion or insertion of a single base can disrupt every subsequent codon in the sequence.
The Start Codon: The codon AUG serves as the universal initiation signal for translation. In addition to signaling the start of a polypeptide, it also codes for the amino acid Methionine, which is typically the first amino acid in a newly synthesized protein chain.
Stop Codons: There are three specific codons—UAA, UAG, and UGA—that do not code for any amino acid. Instead, they act as 'punctuation marks' that signal the ribosome to terminate translation and release the completed polypeptide chain.
Reading Frames: Because the code is read in triplets, any sequence has three potential reading frames. The presence of a specific start codon ensures that the cellular machinery begins reading at the correct position to produce the intended protein.
| Feature | Codon | Anticodon |
|---|---|---|
| Location | Found on the mRNA molecule | Found on the tRNA molecule |
| Function | Specifies the amino acid to be added | Matches with the mRNA to deliver the amino acid |
| Complementarity | Complementary to the DNA template strand | Complementary to the mRNA codon |
Redundancy vs. Ambiguity: Redundancy means multiple codons can lead to the same amino acid (e.g., Codon A and Codon B both result in Amino Acid 1). Ambiguity would mean one codon could result in multiple different amino acids, which does not occur in the standard genetic code.
Sense vs. Antisense Strands: The antisense (template) strand of DNA is transcribed into mRNA. The resulting mRNA sequence is identical to the sense (coding) strand of DNA, except that Uracil (U) replaces Thymine (T).
Directionality Matters: Always remember that mRNA is synthesized and read in the 5' to 3' direction. When solving problems involving DNA to mRNA conversion, ensure you account for the antiparallel nature of the strands.
Identify the Start: When given a long sequence of bases, always look for the first AUG to establish the correct reading frame. Ignore any bases that appear before the start codon, as they are part of the non-translated leader sequence.
Check for Stop Codons: If a mutation is described, check if it creates a premature stop codon (nonsense mutation). This is a common exam scenario that results in a truncated, usually non-functional, protein.
Verify Redundancy: If asked about the effect of a point mutation, use a codon table to see if the new codon codes for the same amino acid. If it does, it is a 'silent mutation' and will have no effect on the protein's primary structure.
