The synthesis of protein at the ribosomes form mRNA is called translation.
- Role of tRNA in Translation
Transfer RNA (tRNA) plays an important role in translation. It reads codes on the mRNA and brings different amino acids. Thus a
polypeptide chain is formed. The tRNA has complementary pairing of bases. But tRNA has some unusual bases. These bases disrupt the normal base pairing and forms loops in the molecule. The center loop is called anticodon loop. It has a sequence of three unpaired bases called the anticodon. The mRNA codon form pairing with its complementary anticodon of tRNA. The tRNA carries amino acids.
- Role of ribosomes in protein synthesis
Ribosomes are the sites of protein synthesis. They consist of large and small subunits. These subunits organize the pairing between the codon and the anticodon. Ribosome has several binding sites for mRNA and tRNA. Ribosome has three sites:
(a)P site (peptidyl site): It is the site of the ribosome where peptide bonds are formed.
(b)A site (aminoacyl site): This site contains successive (next) amino acid with its rRNA.
(c)E site (exit site): It is a site of ribosome where empty tRNA is present.
Steps of protein synthesis
There are following stePs of protein synthesis:
The mRNA binds to a small ribosomal subunit. The initiation codon on mRNA is AUG. The initiation code of mRNA aligns with the P (peptidyl) site of the ribosome. A tRNA with methionine binds to the mRNA on initiation codon. The large subunit joins this complex. It forms a complete ribosome.
The A (aminoacyl) site is present next to the P site. It has second codon. A second tRNA attaches on this codon with its anticodon. Two tRNA molcules with their amino acids are now side-by-side in A sites. This step requires enzyme and energy. Guanine triphosphate (GTP) provides energy to this step. An enzyme peptidyl transferase breaks the bond between the amino acid and tRNA in the P site. It also catalyzes the formation of a peptide bond between that amino acid and the amino acid in the A site.
The mRNA strand then moves along the ribosome. It exposes new codon. The tRNA with their two amino acids moves to P site. A third tRNA can now enter the exposed A site. This process continues and all the entire mRNA has been translated.
The ribosome reaches at the end of the mRNA. The polypeptide chain has been synthesized. Termination codon like UAA comes and translation ends.
- Export of protein
Protein synthesis occurs on ribosomes. Most of the ribosomes are attached on the surface of the rough endoplasmic reticulum. The newly synthesized protein moves into the ER. The protein then moves into the Golgi apparatus. It is packed into a secretary vesicle or a lysosome.