60
Chapter 3
Protein chain
Tryptophan tRNA
Ribosome
Amino acid
Valine tRNA
Val
Ser
Site 2
Site 1
Trp
Ala
Large ribosome
subunit
Small ribosome
subunit
Direction of synthesis
Anticodon
mRNA
U
A
G
G
U
G
C
C
C
A
C
A
A
U
U
U
C
C
G
G
A
C
G
G
Figure 3–21
Sequence of events during protein synthesis by a ribosome.
mRNA, making room for the binding of the next amino acid–
tRNA molecule. This process is repeated over and over as amino
acids are added to the growing peptide chain, at an average rate
of two to three per second. When the ribosome reaches a termi-
nation sequence in mRNA specifying the end of the protein, the
link between the polypeptide chain and the last tRNA is broken,
and the completed protein is released from the ribosome.
Messenger RNA molecules are not destroyed during
protein synthesis, so they may be used to synthesize many
more protein molecules. In fact, while one ribosome is mov-
ing along a particular strand of mRNA, a second ribosome
may become attached to the start site on that same mRNA
and begin the synthesis of a second identical protein molecule.
Thus, a number of ribosomes, as many as 70, may be moving
along a single strand of mRNA, each at a different stage of the
translation process (
Figure 3–22
).
Molecules of mRNA do not, however, remain in the cyto-
plasm indefi nitely. Eventually cytoplasmic enzymes break them
down into nucleotides. Therefore, if a gene corresponding to
a particular protein ceases to be transcribed into mRNA, the
protein will no longer be formed after its cytoplasmic mRNA
molecules have broken down.
Once a polypeptide chain has been assembled, it may
undergo posttranslational modifi cations to its amino acid
sequence. For example, the amino acid methionine that is used
to identify the start site of the assembly process is cleaved from
the end of most proteins. In some cases, other specifi c peptide
bonds within the polypeptide chain are broken, producing a
number of smaller peptides, each of which may perform a dif-
ferent function. For example, as illustrated in
Figure 3–23
,
fi ve different proteins can be derived from the same mRNA as
a result of posttranslational cleavage. The same initial polypep-
mRNA
Growing
polypeptide
chains
Free
ribosome
subunits
Completed
protein
Ribosome
Figure 3–22
Several ribosomes can simultaneously move along a strand of
mRNA, producing the same protein in different stages of assembly.
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