Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Self and the Beginning of Life 25

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

Table 3.3. The Standard Genetic Code

The code is composed of three-letter words (codons) on the

messenger RNA, each codon corresponding to one amino

acid. Each letter has four possibilities: U (uracil), C (cytosine),

A (adenine), G (guanine). There are 64 codons, but only 20

amino-acid building blocks available for making proteins,

resulting in some amino acids being assigned to more than

one codon. For example, methionine and tryptophan are each

assigned only one codon; leucine, arginine, and serine are each

assigned six codons; while others have an intermediate number

of codons. Some codons (UAA, UAG, UGA) do not code for

amino acids but signify the end of a protein (stop codon).

Amino Acids Codons

Alanine GCA GCC GCG GCU

Arginine AGA AGG CGA CGC CGG CGU

Asparagine AAC AAU

Aspartic Acid GAC GAU

Cysteine UGC UGU

Glutamic Acid GAA GAG

Glutamine CAA CAG

Glycine GGA GGC GGG GGU

Histidine CAC CAU

Isoleucine AUA AUC AUU

Leucine UUA UUG CUA CUC CUG CUU

Lysine AAA AAG

Methionine AUG

Phenylalanine UUC UUU

Proline CCA CCC CCG CCU

Serine AGC AGU UCA UCC UCG UCU

Threonine ACA ACC ACG ACU

Tryptophan UGG

Tyrosine UAC UAU

Valine GUA GUC GUG GUU

Fig. 3.5. The Central Dogma of molecular biology as proposed by Francis Crick. The
flow of information is from DNA to RNA and then to protein. The flow of information
from DNA to DNA happens only during cell replication.