PROTEIN SYNTHESIS

STEP 1: Transcription of mRNA from a DNA gene 

• mRNA stands for messenger RNA. 
• Protein synthesis begins with the transcription of mRNA from a DNA gene in the nucleus.
•  The RNAs then migrate from the nucleus into the cytoplasm. 
• Prior to the beginning of the protein synthesis, all of the component parts are assembled in the ribosomes. 

STEP 2: Initiation

• In the cytoplasm, protein synthesis is actually initiated by the AUG codon on mRNA. 
• The AUG codon signals both the interaction of the ribosome with m-RNA and also the tRNA with the anticodons (UAC). The AUG codon is known as the start codon. This is the codon for the amino acid, methionine.
• The ribosome moves along the mRNA to read the next codon. Once this is done, a tRNA (transfer RNA) with an anticodon approaches the mRNA. This tRNA carries along with it the corresponding amino acid. 
• The next process is to start growing peptide chain by having amine bond to the carboxyl acid group of methionine in order to elongate the peptide.

STEP 3: Elongation:

Elongation of the peptide begins as various tRNA's read the next codon. The tRNA with the anticodon brings along the corresponding amino acid and a peptide bond is formed between the amino acids. The previous tRNA then returns into the cytoplasm to be reused. The whole process is then repeated.

STEP 4: Elongation and Termination:

• When the stop signal on mRNA is reached, the protein synthesis is terminated. The stop codons are UAA, UAG, and UGA. The last amino acid is hydrolyzed from its t-RNA.
• The peptide chain leaves the ribosome. The N-formyl-methionine that was used to initiate the protein synthesis is also hydrolyzed from the completed peptide at this time.
• The ribosome is now ready to repeat the synthesis several more times.