134
UNIT ONE
Messenger
RNA
Transfer
RNA
Next amino acid
Anticodon
Codons
Growing
polypeptide
chain
1
1
2
2
3
3
4
4
5
5
6
6
7
C
UG
G
Messenger
RNA
Transfer
RNA
Next
amino acid
Ribosome
1
1
2
2
3
3
7
4
4
5
5
6
7
C
C
C
G
U
C
U
G
C
G
U
Messenger
RNA
Transfer
RNA
Next amino acid
Anticodon
Codons
Growing
polypeptide
chain
1
1
2
2
3
3
4
4
5
5
6
6
7
Peptide bond
C
U
G
C
G
U
CC
G
C
G
U
6
Messenger
RNA
Transfer
RNA
Next
amino acid
1
1
2
2
3
3
4
4
5
5
6
7
6
7
U
C
GG
AA
AA
A
A
GGG
G
GG
GG
CC
CC
C
C
C
UU
U
C
GG
AA
AA
A
A
GGG
G
GG
GG
CC
CC
C
C
C
UU
U
C
GG
AA
AA
A
A
GGG
G
GG
GG
CC
CC
C
C
C
UU
U
C
GG
AA
AA
A
A
GGG
G
GG
GG
CC
CC
C
C
C
UU
The transfer RNA molecule
for the last amino acid added
holds the growing polypeptide
chain and is attached to its
complementary codon on mRNA.
A second tRNA binds
complementarily to the
next codon, and in doing
so brings the next amino
acid into position on the ribosome.
A peptide bond forms, linking
the new amino acid to the
growing polypeptide chain.
The tRNA molecule that
brought the last amino acid
to the ribosome is released
to the cytoplasm, and will be
used again. The ribosome
moves to a new position at
the next codon on mRNA.
A new tRNA complementary to
the next codon on mRNA brings
the next amino acid to be added
to the growing polypeptide chain.
2
1
3
4
FIGURE 4.24
A closer look at protein synthesis. Molecules of transfer RNA (tRNA) attach to and carry speciF
c amino acids, aligning them in the
sequence determined by the codons of mRNA. These amino acids, connected by peptide bonds, form a polypeptide chain of a protein molecule.
Protein synthesis occurs on ribosomes.
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