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
T C A G
F
i
r
s
t

P
o
s
i
t
i
o
n
T
Phenylalanine
Phenylalanine
Leucine
Leucine
Serine
Serine
Serine
Serine
Tyrosine
Tyrosine
stop
stop
Cysteine
Cysteine
stop
Tryptophan
T
C
A
G
T
h
i
r
d

P
o
s
i
t
i
o
n
C
Leucine
Leucine
Leucine
Leucine
Proline
Proline
Proline
Proline
Histidine
Histidine
Glutamine
Glutamine
Arginine
Arginine
Arginine
Arginine
T
C
A
G
A
Isoleucine
Isoleucine
Isoleucine
Methionine
Threonine
Threonine
Threonine
Threonine
Asparagine
Asparagine
Lysine
Lysine
Serine
Serine
Arginine
Arginine
T
C
A
G
G
Valine
Valine
Valine
Valine
Alanine
Alanine
Alanine
Alanine
Aspartic acid
Aspartic acid
Glutamic acid
Glutamic acid
Glycine
Glycine
Glycine
Glycine
T
C
A
G


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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