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Translation process
The translation process is
divided into three steps:
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Initiation: When a small
subunit of a ribosome charged with a tRNA+the amino
acid methionine encounters an mRNA, it attaches and
starts to scan for a start signal. When it finds the
start sequence AUG, the codon (triplet) for the amino
acid methionine, the large subunit joins the small
one to form a complete ribosome and the protein
synthesis is initiated.
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Elongation: A new tRNA+amino acid enters the ribosome,
at the next codon downstream of the AUG codon. If its
anticodon matches the mRNA codon it basepairs and the
ribosome can link the two aminoacids together.(If a
tRNA with the wrong anticodon and therefore the wrong
amino acid enters the ribosome, it can not basepair
with the mRNA and is rejected.) The ribosome then moves
one triplet forward and a new tRNA+amino acid can enter
the ribosome and the procedure is repeated. |
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Termination: When the ribosome
reaches one of three stop codons, for example UGA,
there are no corresponding tRNAs to that sequence.
Instead termination proteins bind to the ribosome and
stimulate the release of the polypeptide chain (the
protein), and the ribosome dissociates from the mRNA.
When the ribosome is released from the mRNA, its
large and small subunit dissociate. The small subunit
can now be loaded with a new tRNA+methionine and
start translation once again. Some cells need large
quantities of a particular protein. To meet this
requirement they make many mRNA copies of the
corresponding gene and have many ribosomes working on
each mRNA. After translation the protein will usually
undergo some further modifications before it becomes
fully active.
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