Protein synthesis involves three main steps - initiation, elongation, and termination. In initiation, the small and large ribosomal subunits assemble along with mRNA and tRNA to form the initiation complex. In elongation, amino acids are added one by one to the growing polypeptide chain. Termination occurs when a stop codon is reached, causing the release of the completed protein. While the overall process is similar between prokaryotes and eukaryotes, there are some key differences like the number of initiation factors and whether mRNA is polycistronic or monocistronic.

1. Protein Synthesis

2. Content
• Definition
• Location in the cell
• Requirements
• Steps involved
• Initiation
• Elongation
• Termination
• Antibiotics inhibits the protein synthesis

3. • The process of protein synthesis translates the codons (nucleotide triplets) of the
messenger RNA (mRNA) into the 20-symbol code of amino acids that build the
polypeptide chain of the proteins
• mRNA translation begins from its 5′-end towards its 3′-end
• The polypeptide chain is synthesized from its amino-terminal (N-end) to its
carboxyl-terminal (C-end).
• no significant differences in the protein synthesis steps in prokaryotes and
eukaryotes, however there is one major distinction between the structure of the
mRNAs – prokaryotes often have several coding regions (polycistronic mRNA),
while the eukaryotic mRNA has only one coding region (monocistronic mRNA

4. Protein Synthesis Steps involved
• Initiation
• Elongation
• Termination

7. Protein Synthesis Initiation
• The components involved in the first step of protein synthesis are:
• The mRNA to be translated
• the two ribosomal subunits (small and large subunits)
• the aminoacyl-tRNA which is specified by the first codon in the mRNA
• guanosine triphosphate (GTP), which provides energy for the process –
eukaryotes require also adenosine triphosphate!
• initiation factors which enables the assembly of this initiation complex -
prokaryotes have 3 initiation factors are known (IF-1, IF-2, and IF-3), while
eukaryotes, there have over ten factors designated with eIF prefix.

9. Two mechanisms are involved in the
recognition sequence
• Shine-Dalgarno (SD) sequence - In
Escherichia coli is observed
sequence with high percentage of
purine nucleotide bases, known as
the Shine-Dalgarno sequence. This
region is located close to 5’ end of
the mRNA molecule, 6-10 bases
upstream of the initiating codon.
The 16S rRNA component of the
small ribosomal subunit possess a
complementary to the SD sequence
near its 3'-end.

10. Initiating codon (AUG)
• In prokaryotes this event is facilitated by IF-2-GTP, while in eukaryotes by
eIF-2-GTP and additional eIFs.
• The charged initiator transport RNA approaches the P site on the small
ribosomal subunit
• In bacteria (and in mitochondria), a methionine is attached to the initiator
tRNA an subsequently a formyl group is added by the enzyme
transformylase
• in eukaryotes, the initiator transport RNA attaches a non formylated
methionine
• the large ribosomal subunit joins the complex formed by now, and thus a
fully functional ribosome is formed
• This complex has a charged initiating tRNA in the P site, and the A site
empty

11. Translation Elongation
• During the elongation step the polypeptide chain adds amino acids to the
carboxyl end the chain protein grows as the ribosome moves from the 5' -
end to the 3'-end of the mRNA
• In prokaryotes, the delivery of the aminoacyl-tRNA to ribosomal A site is
facilitated by elongation factors EF-Tu-GTP and EF-Ts, and requires GTP
hydrolysis
• The peptidyl-transferase is an important enzyme which catalyzes the
formation of the peptide bonds
• After the peptide bond has been formed between the polypeptide and the
amino acid, the newly formed polypeptide is linked to the tRNA at the A
site
• Once this step is completed, the ribosome moves 3 nucleotides toward the
3'-end of the mRNA. This process is known as translocation

13. Termination of Translation
• Termination happens when the A site of the ribosome reaches one of
the three termination codons (UAA, UAG or UGA)
• In prokaryotes, these codons are recognized by different release
factors (abbreviated with RF)
• When these release factors bind the complex, this cause in hydrolysis
of the bond linking the peptide to the tRNA at the P site and releases
the nascent protein from the ribosome