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Translation is the actual
synthesis of a protein under the direction of mRNA.
During this process the
nucleotide sequence
of an mRNA (messenger RNA) is translated into the
amino acid sequence
of a protein. The protein synthesis requires a
technical machinery of high complexity. As compared
to information transfer between nucleic acid
molecules, where direct copying occurs on the basis
of base complementarity, the translation process
involves a greater number of chemical reactions and
the participation of additional nucleic acid and
protein components. One of these components, the
ribosome, provides the
basic machinery for the translation process. The
major role of the ribosome is to catalyse coupling of
amino acids into protein according to the sequence
specified by the mRNA. The amino acids are brought to
the ribosome by tRNA
(transfer RNA) molecules.
The
nucleotide sequence
of the mRNA is composed of four different nucleotides
whereas a protein is built up from 20 amino acids. To
allow the four nucleotides to specify 20 different
amino acids, the
nucleotide sequence
is interpreted in codons, groups of three
nucleotides. These codons have their corresponding
anticodon in the tRNA. Furthermore each anticodon is
linked to one particular amino acid. Thus, each codon
specifies one amino acid. This is referred to as the
genetic code. In
addition to the main components of the translational
system listed above, the translation process also
involves a large number of protein factors that
facilitate binding of mRNA and tRNA to the ribosome.
Protein synthesis consumes a large part of the energy
produced in the cell.
Translation may be divided into three distinct steps.
The first, initiation,
results in the formation of an initiation complex in
which the ribosome is bound to the specific
initiation (start) site on the mRNA while the
initiator tRNA is annealed to the initiator codon and
bound to the ribosome. The second stage, elongation, consists of joining
amino acids to the growing
polypeptide chain
according to the sequence specified by the message.
Incorporation of each amino acid occurs by the same
mechanism. Thus, the same steps are repeated over and
over again until the termination codon is reached in
the message. The termination codon gives the signal
for the third and last stage of protein synthesis,
the termination, in
which the ready-made protein is released from the
ribosome.
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