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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

238 Biopolymers

TABLE III The Genetic Code
First base Second base of codon Third base
of codon of codon
↓ U C A G ↓

UUU UAU UGU U
Phe UCU Tyr Cys

UUC UCC  UAC UGC C
U Ser
UUA UAA UGA Stop A
Leu UCA Stop

UUG UCG  UAG UGG Trp G
  
CAU U
CUU


 CCU His CGU
CUC  CCC  CAC CGC  C
C Leu Pro Arg
CAA A
CUA CCA CGA
  Gln 
CUG  CCG  CAG CGG  G
AAU AGU U
 
ACU
AUU
 Asn Ser
AUC Ile ACC  AAC AGC C
A Thr
AUA  AAA AGA A
ACA
 Lys Arg
AUG Met, Start ACG  AAG AGG G
GAU U
  
GUU


 GCU Asp GGU
GUC  GCC  GAC GGC  C
G Val Ala Gly
GAA A
GUA GCA GGA
  Glu 
GUG  GCG  GAG GGG  G
message on the m-RNA (Fig. 21). (GUG or UUG is used RNA can bind (Fig. 22). In this example, the second codon
more rarely as an initiating codon.) The larger ribosome codes for serine. A peptide bond is formed between the
subunit then binds to the whole complex. carboxyl group of the methionine and the amino group
In a simplified picture of protein synthesis, we can con- of the second acid serine, and the new dipeptide remains
sider that an intact ribosome has three binding sites, called bound to the second t-RNA, at the A-site. The 23s RNA of
the P-, A-, and E-sites, for t-RNAs. At this stage, the initi- the larger ribosomal subunit plays an important role in the
ating t-RNA is bound at the P-site. A second t-RNA, with catalysis of peptide bond formation. The ribosome now
its amino acid, becomes bound at the A-site. The codon on moves with respect to the t-RNA and m-RNA, so that the
the m-RNA next to AUG determines which amino-acyl-t- dipeptidyl-t-RNA is located at the P-site (Fig. 22), and the

FIGURE 21 Initial stages of protein biosynthesis.