Protein Synthesis

Aminoacylation Attachment of an amino acid to its cognate (matching) transfer RNA (tRNA), catalyzed by the cognate aminoacyl-tRNA synthetase (AARS). Anticodon The trinucleotide sequence at the end of one arm of tRNA that base pairs with a complementary messenger RNA (mRNA) codon. Codon The trinucleotide sequence of an mRNA that specifies which amino acid will […]

Protein Synthesis

PROTEINS are polymers of amino acids joined by peptide bonds (proteins are therefore also known as polypeptides). The number and order of the amino acids contained in a particular protein are prescribed by the DNA sequence of that protein’s gene. The mechanism by which a genetic message is translated from its nucleic acid form to […]


Many cellular components play a role in protein synthesis (Fig. 1). Even in relatively simple bacteria, translation of a single polypeptide from its genetic message requires dozens of participants—proteins, RNAs, and nucleotides—working together as carriers, catalysts, energy sources, and cofactors. Peptide bond formation takes place rapidly at the ribosome, with as many as 40 amino […]

TRANSFER RNAs (Protein Synthesis)

Although an mRNA nucleotide sequence dictates the polypeptide sequence to be made, mRNAs do not directly recognize amino acids. Amino acids are instead linked to transfer RNA (tRNA) "adaptor" molecules, which serve as reading heads to decipher the codons of mRNA through base-pairing complementarity (Fig. 2). FIGURE 2 Transfer RNA folding. The tRNA cloverleaf secondary […]


Decoding of the protein message occurs at the ribo-some, after prior attachment of amino acids to the tRNA molecules. Aminoacyl-tRNA synthetases (AARSs) are the family of enzymes responsible for covalent attachment of each amino acid to its correct, or cognate, tRNA molecule. This first step in protein synthesis is responsible for establishing the rules of […]


Translation of an mRNA message into its polypeptide product on the ribosome is a polymerization reaction, and can be divided into three phases: initiation, elongation, and termination. Initiation requires the assembly of the translational machinery from its individual components to form a complex that is primed for peptide bond formation. Formation of this initiation complex […]


Initiation of protein synthesis requires assembly of the ribosomal subunits, messenger RNA, and initiator tRNA at the start codon. This organization of translational components is facilitated by protein initiation factors (Fig. 5). FIGURE 5 Translation initiation. In prokaryotes, three initiation factors are responsible for assembling the initiation complex prior to decoding of a message. The […]

ELONGATION (Protein Synthesis)

The heart of protein biosynthesis lies in the elongation cycle, with its sequential decoding of mRNA codons to assemble the useful portion of the polypeptide. Elongation can be further broken down into three phases— aminoacyl-tRNA decoding, peptide bond formation, and translocation of the new peptidyl-tRNA (Fig. 7). A. Decoding According to Base Pairing Comparison of […]

TERMINATION (Protein Synthesis)

When the mRNA stop codon is reached, the fully synthesized protein does not simply fall off the ribosome. Release factors (RFs) are the protein assistants that recognize the presence of a stop codon in the ribosomal A-site and trigger cleavage of the polypeptide from the P-site tRNA (Fig. 9). In prokaryotes, RF1 hydrolyzes the protein […]

RIBOSOME RECYCLING (Protein Synthesis)

Following release of the synthesized protein, the ribo-some contains an empty tRNA in the P-site or E-site, and mRNA is still bound with the stop codon in the A-site. This arrangement of components is the post-termination complex. A protein known as the ribosome recycling factor (RRF) promotes the dissociation of this complex in preparation for […]