The Genetic Code

The Genetic Code
"The Rosetta Stone of Biology"

Introduction

The genetic code allows biologists to read the "Books of Life." Codons, which consist of three nucleotides, are the "words" of these books. Living organisms "maintain these books in volumes" called chromosomes, and the collection of chromosomes constitutes the genome.
Since there are four different nucleotides T, C, A and G, there are 4³=64 possible codons. With the exception of three particular codons, which are "stops", each codon during protein synthesis leads to one of twenty amino acids. In other words, each three-letter nucleotide sequence corresponds to a specific amino acid. This correspondence is known as the genetic code and is summarized in the following table:

Second Position of Codon

F
i
r
s
t

P
o
s
i
t
i
o
n

Phenylalanine

Leucine

Serine

Tyrosine

stop

Cysteine

stop

Tryptophan

T
h
i
r
d

P
o
s
i
t
i
o
n

Leucine

Proline

Histidine

Glutamine

Arginine

Isoleucine

Methionine

Threonine

Asparagine

Lysine

Serine

Arginine

Valine

Alanine

Aspartic acid

Glutamic acid

Glycine

How to Read the Table

The letters T, C, A and G represent respectively the four nucleotides thymine, cytosine, adenine and guanine. Of the three letters in a codon, the first appears at the left of the table, the second at the top, and the third on the right. For example, ATG codes methionine, TGG codes tryptophan, and either CAT or CAC codes histidine. The stops, which signal the end of an amino acid sequence (that is, a protein), correspond to TAA, TAG and TGA. Because the number of codons (64) is much bigger than the number of amino acids (20), the correspondence is not one-to-one: It is degenerate because several codons often correspond to the same amino acid, as is evident in the table.

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To the main report on the human genome.