the E. coli that this explanation will focus. The synthesis process is divided into
initiation, elongation, and termination stages (Fig. 2.16).
(1) Initiation: Before protein synthesis begins, the ribosome dissociates into
subunits (a). These small subunits bond near the AUG start codon on the
mRNA (see Fig. 2.15). The AUG start codon then bonds with
formylmethionyl-tRNA (fMet-tRNA), which has CAU as an anticodon, to
form an initiation complex (c). The initiation complex then associates the
larger subunit to form a complete ribosome (d). The location where the
fMet-tRNA bonds on the ribosome is called the peptidyl site (P-site).
(2) Elongation: The next codon on the mRNA (GCU in the figure) is associated
by the second aminoacyl-tRNA, here with IGC as its corresponding code unit
(e). It should be noted that the amino acid does not associates directly with the
mRNA, but uses tRNA as an adaptor. Aminoacyl-tRNA synthetase serves to
combine each amino acid with its specific tRNA. The location where the
aminoacyl-tRNA bonds is known as the aminoacyl site (A-site). The peptide
bond then forms between formylmethionine (fMet) and alanine with the enzyme peptidyl transferase (f). Under these conditions, the empty tRNA
migrates to the P-site and the fMet-tRNA from the A-site to the P-site, and the
ribosome moves the equivalent of one codon down along the mRNA chain.
A third piece of aminoacyl-tRNA (here phenylalanine-tRNA) then joins the
next codon (here UCC) (g). Steps (e) through (g) are repeated and the peptide
chain grows gradually longer (h). Termination: Once a stop codon (UAA,
UGA, or UAG) is encountered, there is no longer any tRNA with a complementary codon, and peptide chain synthesis is terminated (i). At this point,
Table 2.2 The standard genetic code
First position
(5′-end)
Second position
Third position
(3′-end)
U
C
A
G
U
UUU Phe
UUC Phe
UUA Leu
UUG Leu
UCU Ser
UCC Ser
UCA Ser
UCG Ser
UAU Tyr
UAC Tyr
UAA Stop
UAG Stop
UGU Cys
UGC Cys
UGA Stop
UGG Trp
U
C
A
G
C
CUU Leu
CUC Leu
CUA Leu
CUG Leu
CCU Pro
CCC Pro
CCA Pro
CCG Pro
CAU His
CAC His
CAA Gln
CAG Gln
CGU Arg
CGC Arg
CGA Arg
CGG Arg
U
C
A
G
A
AUU Ile
AUC Ile
AUA Ile
AUG Met
a
ACU Thr
ACC Thr
ACA Thr
ACG Thr
AAU Asn
AAC Asn
AAA Lys
AAG Lys
AGU Ser
AGC Ser
AGA Arg
AGG Arg
U
C
A
G
G
GUU Val
GUC Val
GUA Val
GUG Val
GCU Ala
GCC Ala
GCA Ala
GCG Ala
GAU Asp
GAC Asp
GAA Glu
GAG Glu
GGU Gly
GGC Gly
GGA Gly
GGG Gly
U
C
A
G
a AUG forms part of the initiation signal as well as coding internal methionine residues
2.9 What Is Genetic Information?
43
initiation, elongation, and termination stages (Fig. 2.16).
(1) Initiation: Before protein synthesis begins, the ribosome dissociates into
subunits (a). These small subunits bond near the AUG start codon on the
mRNA (see Fig. 2.15). The AUG start codon then bonds with
formylmethionyl-tRNA (fMet-tRNA), which has CAU as an anticodon, to
form an initiation complex (c). The initiation complex then associates the
larger subunit to form a complete ribosome (d). The location where the
fMet-tRNA bonds on the ribosome is called the peptidyl site (P-site).
(2) Elongation: The next codon on the mRNA (GCU in the figure) is associated
by the second aminoacyl-tRNA, here with IGC as its corresponding code unit
(e). It should be noted that the amino acid does not associates directly with the
mRNA, but uses tRNA as an adaptor. Aminoacyl-tRNA synthetase serves to
combine each amino acid with its specific tRNA. The location where the
aminoacyl-tRNA bonds is known as the aminoacyl site (A-site). The peptide
bond then forms between formylmethionine (fMet) and alanine with the enzyme peptidyl transferase (f). Under these conditions, the empty tRNA
migrates to the P-site and the fMet-tRNA from the A-site to the P-site, and the
ribosome moves the equivalent of one codon down along the mRNA chain.
A third piece of aminoacyl-tRNA (here phenylalanine-tRNA) then joins the
next codon (here UCC) (g). Steps (e) through (g) are repeated and the peptide
chain grows gradually longer (h). Termination: Once a stop codon (UAA,
UGA, or UAG) is encountered, there is no longer any tRNA with a complementary codon, and peptide chain synthesis is terminated (i). At this point,
Table 2.2 The standard genetic code
First position
(5′-end)
Second position
Third position
(3′-end)
U
C
A
G
U
UUU Phe
UUC Phe
UUA Leu
UUG Leu
UCU Ser
UCC Ser
UCA Ser
UCG Ser
UAU Tyr
UAC Tyr
UAA Stop
UAG Stop
UGU Cys
UGC Cys
UGA Stop
UGG Trp
U
C
A
G
C
CUU Leu
CUC Leu
CUA Leu
CUG Leu
CCU Pro
CCC Pro
CCA Pro
CCG Pro
CAU His
CAC His
CAA Gln
CAG Gln
CGU Arg
CGC Arg
CGA Arg
CGG Arg
U
C
A
G
A
AUU Ile
AUC Ile
AUA Ile
AUG Met
a
ACU Thr
ACC Thr
ACA Thr
ACG Thr
AAU Asn
AAC Asn
AAA Lys
AAG Lys
AGU Ser
AGC Ser
AGA Arg
AGG Arg
U
C
A
G
G
GUU Val
GUC Val
GUA Val
GUG Val
GCU Ala
GCC Ala
GCA Ala
GCG Ala
GAU Asp
GAC Asp
GAA Glu
GAG Glu
GGU Gly
GGC Gly
GGA Gly
GGG Gly
U
C
A
G
a AUG forms part of the initiation signal as well as coding internal methionine residues
2.9 What Is Genetic Information?
43