There are three-stages of protein
Assembly of Ribosomes on mRNAIn
prokaryotes, initiation requires the large and small ribosome subunits, the
mRNA, initiationtRNA and three Initiation Factors (IFs).
Activation of Amino Acid Amino
acids become activated by binding with amino acyltRNA synthesize enzyme in the
presence of ATP.
Amino acid (AA) + ATP AA-AMP-Enzyme
Transfer of Amino Acid to tRNAThe
AA-AMP-enzyme complex formed reacts with specific t RNA to form aminoacyltRNA
AA-AMP-Enzyme complex + tRNA AA tRNA + AMP
The cap region of mRNA binds to
the smaller subunit of ribosome.
The ribosome has two sites,
A-site and P-site.
The smaller subunit first binds
the initiator tRNA then and then binds to the larger subunit so, that
initiation codon (AUG) lies on the P-site.
The initiationtRNA, i.e.,
methionyltRNA then binds to the P-site,
Another charged aminoacyltRNA
complex binds to the A-site of the ribosome.
Peptide bond formation and
movement along the mRNA called translocation. A peptide bond is formed between
carboxyl group (?COOH) of amino acid at P-site and aminogroup (?NH) of amino
acid at A-site by the enzyme peptidyltransferase.
The ribosome slides over mRNA
from codon to codon in the 5'3'
According to the sequence of
codon, amino acids are attached to one another by peptide bonds and a
polypeptide chain is formed.
When the A-site of ribosome
reaches a termination codon which does not code for anyamino acid, no charged
tRNA binds to the A-site.
Dissociation of polypeptide from
ribosome takes place which is catalyzed by a 'release factor'.
There are three termination codons,i.e.,
UGA, UAG and UAA.
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