describe the principle of the universal genetic code in which different triplets of DNA bases either code for specific amino acids or correspond to start and stop codons
The universal genetic code is the set of rules by which the sequence of nucleotides in DNA (and the transcribed mRNA) determines the sequence of amino acids in a protein. Each codon consists of three consecutive nucleotides (a triplet) and specifies either a particular amino‑acid, a start signal for translation, or a stop signal that terminates translation.
Key Principles
Each codon is made up of three nucleotides: \$3\$ bases per codon.
There are 64 possible codons (\$4^3\$), but only 20 standard amino acids, so the code is degenerate – several codons can code for the same amino acid.
All organisms use essentially the same code, which is why it is called “universal”.
One codon (ATG in DNA, AUG in mRNA) functions as the start codon, signalling the beginning of translation.
Three codons act as stop signals (TAA, TAG, TGA in DNA; UAA, UAG, UGA in mRNA) and do not code for any amino acid.
Steps in Translating the Genetic Code
DNA is transcribed to messenger RNA (mRNA) by RNA polymerase.
The ribosome binds to the mRNA at the start codon (AUG).
Transfer RNA (tRNA) molecules bring the appropriate amino‑acyl‑tRNA matching each codon.
Peptide bonds form between successive amino acids, extending the polypeptide chain.
When a stop codon enters the ribosomal A‑site, release factors trigger termination and the newly synthesized protein is released.
DNA Codon Table (Partial)
DNA Triplet (Codon)
Amino Acid (Three‑letter code)
Notes
ATG
Met
Start codon
TTT
Phe
TTC
Phe
GCT
Ala
GCC
Ala
GCA
Ala
GCG
Ala
TAA
—
Stop codon
TAG
—
Stop codon
TGA
—
Stop codon
Suggested diagram: Flow of genetic information from DNA → mRNA → protein, highlighting the start codon (ATG/AUG) and the three stop codons (TAA/UAA, TAG/UAG, TGA/UGA).