Genetic Code
The genetic code is the way of encoding information about the structure of proteins in DNA. Each codon encodes individual amino acids, composed of 2-3 nucleotides (triple). The start codon "AUG" initiates protein synthesis, while the stop codons "UAA," "UAG," and "UGA" terminate it.
The composition of proteins includes 20 types of amino acids. Therefore, if one amino acid were encoded by only one nucleotide, there would be only 4 possible combinations.
From half of the genetic code table, it clearly emerges that there are more codons than amino acids, their number being 64, 2 to the power of 3, 3 of which do not encode any amino acids and are referred to as STOP codons.
Transcription of genetic information (DNA) takes place with messenger RNA as a matrix, derived from a single or double DNA strand. The first DNA strand, the coding strand, undergoes transcription, encoding the gene sequence and information about a given protein.
Characteristics of the Genetic Code
- Triplet: Every three consecutive nucleotides of the nucleic acid sequence signify an amino acid.
- E.g., AAG - aminokwas
- Unambiguous: One codon encodes exclusively one amino acid.
- E.g., DAG - amino acid F
- Degenerate: One amino acid can be encoded by more than one codon.
- E.g., DIG - amino acid V
- Comma-less: There are no free nucleotides.
- E.g., DPNG - CAG - CUG
- Universal: Besides a few exceptions, codons in all organisms signify the same amino acids.
- E.g., AAG - TAGG - CUG
Genetic Code Table
| First Nucleotide | Second Nucleotide | Third Nucleotide | Amino Acid |
|------------------|-------------------|------------------|------------|
| G | C | A | ARG |
| A | U | G | MET |
| G | U | A | VAL |
| U | A | A | LYS |
| C | A | A | THR |
| C | G | U | ARG |
| U | A | C | THR |
| C | C | C | PRO |
| A | U | C | THR |
| G | C | U | ALA |
| U | A | C | SER |
| U | U | C | PHE |
| A | U | G | ILE |
| A | U | U | ILE |
| A | U | G | THR |
| U | U | U | PHE |
| G | C | C | ALA |
| C | C | G | PRO |
| C | G | G | PRO |
| G | U | C | VAL |
| G | U | G | VAL |
| A | U | U | ILE |
| U | U | C | SER |
| U | U | G | SER |
| A | A | G | LYS |
| A | U | A | TYR |
| A | U | U | TYR |
| U | A | C | SER |
| U | A | U | SER |
| A | A | C | LYS |
| A | A | U | LYS |
| G | A | A | GLU |
| G | A | G | GLU |
| G | C | A | ALA |
| G | C | C | ALA |
| G | C | G | ALA |
| G | G | A | GLY |
| G | G | G | GLY |
To decipher the sequence of amino acids, we start from AUG and end at one of the three STOP codons.
For instance, the DNA sequence "CGTACATGACCATIO" translates into the following amino acids when transcribed to mRNA:
- DNA: CGTACATGACCATIO
- mRNA: GCAUG- UA CUGGUAAG
- Resulting amino acids: MET TYR TRP STOP
In conclusion, the characteristics of the genetic code dictate how the information about amino acids is encoded and eventually translated into proteins.
