What Is The Genetic Code?

It has been proved that the nucleotide sequence in DNA determines the sequence of amino acids in a protein. This is known as the co-linearity hypothesis, meaning that a certain length of DNA corresponds to (is collinear with) a sequence of amino acids. In fact, a particular sequence of nucleotides is the code for a specific amino acid. Since there are four types of nucleotides in a DNA molecule and amino acids in a protein are at least of 20 types, each amino acid is coded by a triplet of bases (nucleotides). This is known as the triplet code hypothesis and the code constitutes the genetic code. The triplet code results in 64 codons. Three of the codons, UAA, UAG and UGA do not code for any amino acid and were, therefore, once unfortunately called nonsense codons. They are called unfortunate because they are in fact, the stop signals and, hence, not nonsense. AUG is likewise the start signal but it also codes for methionine if it falls in the middle. The remaining 60 codons are for 20 amino acids. Naturally, most amino acids are specified by more than one codon. The code is, therefore, called degenerate. The code is universal since it is identical in all organisms, from micro-organisms to human.

Genetic code is a set of three nucleotides on the DNA or mRNA which specifies particular amino acid. It was hypothesized that a set of three nucleotides in any combination (out of four G, C, T and A nucleotides) would produce 64 combinations which are enough to code for 20 amino acids. Marshell Nrenberg, Philip Leader and Har Gobind Khorana tested all the 64 codons by making artificial mRNA. Later studies indicated that three codons UAA, UAG, and UGA do not code for any amino acid and hence called nonsense codons. These are present at the end of the gene and hence are called stop codon. Every mRNA (gene) starts with the initiation code AUG which encodes the amino acid methionine. Similarly UUU-UUC codes for phenylalanine etc.
At regards the organism level, genetic codes are universal i.e. each code specifies the specific amino acid in all organism including bacteria. However, genetic coding is different at organism and organelle like mitochondria level e.g. UGA is a stop codon in all organisms in nucleus, while some codon specifies amino acid tryptophan in mitochondria.
Recognition of genetic code: each kind of activating enzyme (aminoacyl-tRNAsynthetase) recognizes and binds to a specific amino acid such as tryptophan. This tRNA binds anticodon ACC on codon UGG on mRNA.