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The Genetic code translates a set of 3 RNA bases into amino acid.
Each codon, a unit of 3 bases represent some amino acid sequence.
The genetic code is degenerative. Separate codons can code for the same amino acid.
The genetic code is unambiguous. One codon does not code for more than one amino acid.
All species share the universal genetic code.
Proteins consist of 20 different amino acids in nature.
The genetic code has 64 different codons.
Many of the 3 letter codons code for same amino acids. Redundancy or degenerate genetic code.
Important codons:
Start codon is methionine, AUG.
Stop codons UAA, UAG, UGA
Mnemonic for the stop codons = You Are Annoying. You Go Away and You Are Gone.
In all organisms (plants, animals and microorganisms) the genetic material is the DNA which is made up of repeating units of nucleotides. The sequence in which these nucleotides are arranged on the DNA determines the structure and function of the gene. In protein synthesis, the information contained in the gene is transcribed into mRNA by using a complimentary base pair. It should be noted however, that thymine T does not occur in RNA but is replaced with uracil U.
For instance, a gene with the following nucleotide sequence CTTAGCTTC after transcription will give a mRNA of the following sequence GAAUCGAAG. After transcription, the mRNA is translated into proteins whose amino acid sequence is determined by the nucleotide sequence on the mRNA. This process therefore involves the translation of the 4-letter language of DNA into 20 letter language (amino acids) of the protein. It has however, been known that it is only likely that at least 3 nucleotides of the DNA were required to code for each amino acid.
With this combination therefore, 64 different code words are therefore possible. Series of experiments were then carried out to determine the 3 letter code words for the different amino acids. One of such experiments was carried out by Nirenberg in 1961 by using synthesized nucleotides.

The steps involved
a. Synthesis of poly ‘U’ nucleotides.
b. Addition of 1 amino acid per test tube to make 20 tubes of labelled amino acids.
c. Addition of required enzymes to each of the tubes to form protein from the polynucleotide.
From this experiment, it was found that only one of the tubes that protein was synthesized and in that tube all the amino acids of the protein are phenyl alanine. Hence, indicating that UUU codes for phenylalanine.
Other experiments were then carried out by using different combinations that makes the 64 code words. This has lead to the elucidation of the genetic code for all the amino acids and this codons are represented on the table below.

FEATURES OF THE GENETIC CODE
The genetic code has the following characteristics:
a. There is no punctuation in the reading frame of the triplet codon of a gene.
b. The aa code words are identical in all species that have been tested. However, of recent the protein synthesis using mitochondria ribosomes, tRNA and mRNA show a few differences in code words, e. g. AUA code for methionine rather than isoleucine. Similarly, UGA code for tryptophan rather than termination code.
c. A given amino acid may have more than one codon. Only methionine (AUG) and tryptophan (UGG) have single codons.
Genetic code can therefore, be said to be a set of triplet code words in DNA coding for the amino acids of protein.