Introduction The ability of all living organisms to efficiently and accurately translate genomic information into functional proteins is a remarkable process that is the result of billions of years of evolution. This flow of information from DNA to protein requires that the three polymerization reactions fundamental to life. DNA replication, transcription, and translation, proceed with optimized levels of fidelity and speed. Thus, the accuracy of the three polymerization reactions can be ranked according to their importance in maintaining the integrity of the organism. DNA replication proceeds with an impressive level of accuracy, where an incorrect nucleotide is incorporated only once in 108–1010events (Kunkel and Bebenek, 2000), whereas transcription and translation proceed with considerably lower levels of fidelity, with misincorporation rates of 1 in 104 and 1 in 103–104, respectively (Bouadloun et al., 1983; Edelmann and Gallant, 1977; Kramer and Farabaugh, 2007; Laughrea et al., 1987;Rosenberger and Foskett, 1981). Protein synthesis as a chemical reaction 1. Each amino acid is attached to a tRNA molecule specific to that amino acid by a high-energy bond derived fromATP. The process is catalyzed by a specific enzyme called a synthetase (the tRNA is said to be “charged” when the amino acid is attached): There is a separate synthetase for each amino acid. 2. The energy of the charged tRNA is converted into a peptide bond linking the amino acid to another one on the ribosome: 3. New amino acids are linked by means of a peptide bond to the growing chain: 4. This process continues until aa n (the final amino acid) is added. The whole thing works only in the presence of mRNA, ribosomes, several additional protein factors, enzymes, and inorganic ions. Ribosomes Figure 1; The addition of a single amino acid to the growing polypeptide chain in the course of translation of mRNA. Steps of protein synthesis 1. Initiation o The first step in initiation is the binding of the mRNA to the 30S subunit (The binding is stimulated by the initiation factor IF3. o The initiation factor IF2 binds to GTP and to the initiator fMet-tRNA and stimulates the binding of fMet-tRNA to the initiation complex, leading the fMet-tRNA into the P site, as shown in the middle of o A ribosomal protein splits the GTP bound to IF2, helping to drive the assembly of the two ribosomal subunits. At this stage, the factors IF2 and IF3 are released. Figure 2; Steps in the initiation of translation 2. Elongation Steps are as fallows. o Elongation factor EF-Tu mediates the entry of amino-acyl-tRNAs into the A site. To do so, EF-Tu first binds to GTP. This activated EF-Tu–GTP complex binds to the tRNA. Next, hydrolysis of the GTP of the complex to GDP helps drive the binding of the aminoacyl-tRNA to the A site, at which point the EF-Tu is released, leaving the new tRNA in the A site o Elongation factor EF-Ts mediates the release of EF-

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COMSATS Institute of Information Technology, Abbottabad
Course title and code Molecular Genetics (Bio344)
Assignment number 04
Assignment title How proteins are synthesized at molecular level
Submitted by Zohaib HUSSAIN
Registration number Sp13-bty-001
Submitted To Dr. Sabaz Ali Khan
Date of submission Wednesday, June 10, 2015

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Introduction
The ability of all living organisms to efficiently and accurately translate genomic information
into functional proteins is a remarkable process that is the result of billions of years of evolution.
This flow of information from DNA to protein requires that the three polymerization reactions
fundamental to life. DNA replication, transcription, and translation, proceed with optimized
levels of fidelity and speed. Thus, the accuracy of the three polymerization reactions can be
ranked according to their importance in maintaining the integrity of the organism. DNA
replication proceeds with an impressive level of accuracy, where an incorrect nucleotide is
incorporated only once in 108–1010events (Kunkel and Bebenek, 2000), whereas transcription
and translation proceed with considerably lower levels of fidelity, with misincorporation rates of
1 in 104 and 1 in 103–104, respectively (Bouadloun et al., 1983; Edelmann and Gallant,
1977; Kramer and Farabaugh, 2007; Laughrea et al., 1987;Rosenberger and Foskett, 1981).
Protein synthesis as a chemical reaction
1. Each amino acid is attached to a tRNA molecule specific to that amino acid by a high-
energy bond derived fromATP. The process is catalyzed by a specific enzyme called
a synthetase (the tRNA is said to be “charged” when the amino acid is attached):
There is a separate synthetase for each amino acid.
2. The energy of the charged tRNA is converted into a peptide bond linking the amino acid to
another one on the ribosome:

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3. New amino acids are linked by means of a peptide bond to the growing chain:
4. This process continues until aa n (the final amino acid) is added. The whole thing works
only in the presence of mRNA, ribosomes, several additional protein factors, enzymes, and
inorganic ions.
Ribosomes
Figure 1; The addition of a single amino acid to the growing polypeptide chain in the course
of translation of mRNA.

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Steps of protein synthesis
1. Initiation
o The first step in initiation is the binding of the mRNA to the 30S subunit (The binding is
stimulated by the initiation factor IF3.
o The initiation factor IF2 binds to GTP and to the initiator fMet-tRNA and stimulates the
binding of fMet-tRNA to the initiation complex, leading the fMet-tRNA into the P site, as
shown in the middle of
o A ribosomal protein splits the GTP bound to IF2, helping to drive the assembly of the two
ribosomal subunits. At this stage, the factors IF2 and IF3 are released.
Figure 2; Steps in the initiation of translation

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2. Elongation
Steps are as fallows.
o Elongation factor EF-Tu mediates the entry of amino-acyl-tRNAs into the A site. To do so,
EF-Tu first binds to GTP. This activated EF-Tu–GTP complex binds to the tRNA. Next,
hydrolysis of the GTP of the complex to GDP helps drive the binding of the aminoacyl-
tRNA to the A site, at which point the EF-Tu is released, leaving the new tRNA in the A site
o Elongation factor EF-Ts mediates the release of EF-Tu–GDP from the ribosome and the
regeneration of EF-Tu–GTP.
o In the translocation step, the polypeptide chain on the peptidyl-tRNA is transferred to the
aminoacyl-tRNA on the Asite in a reaction catalyzed by the enzyme peptidyltransferase.
The ribosome then translocates by moving one codon farther along the mRNA, going in the
5′ → 3′ direction. This step is mediated by the elongation factor EF-G ) and is driven by
splitting a GTP to GDP. This action releases the uncharged tRNA from the P site and
transfers the newly formed peptidyl-tRNA from the A site to the P site.

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Figure 3; Steps in elongation
3. Termination
In the earlier discussion of the genetic code, we described the three chain-termination codons
UAG, UAA, and UGA. Interestingly, these three triplets are not recognized by a tRNA, but
instead by protein factors, termed release factors, which are abbreviated RF1 and RF2. RF1

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recognizes the triplets UAA and UAG, and RF2 recognizes UAA and UGA. A third
factor, RF3, also helps to catalyze chain termination. When the peptidyl-tRNA is in the P site,
the release factors, in response to the chain terminating codons, bind to the A site.
The polypeptide is then released from the P site, and the ribosomes dissociate into two subunits
in a reaction driven by the hydrolysis of a GTP molecule
Figure 4 ; Stepsleadingtoterminationof proteinsynthesis

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References
1. Thompson RC, Karim AM. The accuracy of protein biosynthesis is limited by its speed: high
fidelity selection by ribosomes of aminoacyl-tRNA ternary complexes containing
GTP[gamma S] Proc Natl Acad Sci USA. 1982;79:4922–4926. [PMC free article] [PubMed]
2. Bouadloun F, Donner D, Kurland CG. Codon-specific missense errors in vivo.EMBO
J. 1983;2:1351–1356. [PMC free article] [PubMed]
3. Griffiths AJF, Miller JH, Suzuki DT, et al.New York An Introduction to Genetic Analysis.
7th edition.: W. H. Freeman; 2000.