The mRNA is the transcribed polynucleotide which is later translated by ribosomes using tRNA to join amino acids and make proteins. mRNA contains the genetic code which is a set of instructions that help cells to obtain the information in making proteins from the DNA. The work of Khorana and Nirenberg helped decipher the genetic code which is followed by all organisms in protein synthesis. The codes are always in a triplet (codon) form that sequence of three N-bases and they are universal. AUG triplet is the initiator codon whose presence is must at the start of the sequence to initiate the translation process. Without AUG, translation for that sequence will not occur. Also, AUG codes for the amino acid methionine.
UAA, UAG and UGA are called as terminator codon and they do not represent/code for any amino acid. They signal the end of the polypeptide chain during protein synthesis in translation. Stop codon is must to avoid formation of defective protein.