Protein biosynthesis requires that genes which are coded onto DNA be decoded to the sequence of amino acids which are to be assembled. The DNA is located in the nucleus of the cell, while the amino acids required for the synthesis are located in the cytosol. The encoded sequence for a protein is transcribed onto a messenger RNA (mRNA) which carries the information in the form of a sequence to the cytosol. Here the transcribed “message” from the DNA is translated by the ribosome to produce the protein. Hence protein biosynthesis involves two crucial steps of
- Transcription
- Translation
Translation of mRNA is done by the ribosomes which decodes the mRNA sequence and fits the appropriate transfer RNA (tRNA) which contains the appropriate amino acid attached to it. As the ribosome goes down the mRNA, it keeps fitting appropriate tRNA and forms new peptide bonds making the protein sequence longer. The study of the translation procedure is essential to understanding how a number of antibiotics act, as they block translation in bacteria, causing improper protein biosynthesis which eventually leads to death of the bacteria. The animation below shows how exactly the translation procedure occurs in bacteria.
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The above animations has been supplied by Dr. Gary E. Kaiser from the Community College of Baltimore County– and it illustrates the mechanism of action of how translation of mRNA by tRNA occurs in ribosomes. The transcript of the animation is as follows:
” Slide 1: An initiation complex to begin translation forms when a 30S ribosomal unit attaches to the ribosome binding site (XXXs) located at the 5′ end of the mRNA and a formyl-methionyl (f-met) tRNA with UAC anticodon hydrogen bonds with the start codon AUG on the mRNA.
Slide 2: A 50S ribosomal subunit then joins the initiation complex to form a 70S ribosome.
Slide 3: There are three sites on the ribosome. The “A” site is where new charged tRNA molecules first attach. The “P” site is where the growing peptide chain is held. The “E” site is where the uncharged tRNA molecules leave the ribosome.
Slide 4: A charged tRNA with an anticodon complementary to the next codon of the mRNA , in this case CGC, binds to the “A” site of the ribosome. The codon CGC codes for the amino acid arginine because a tRNA with a complementary GCG anticodon only carries arginine.
Slide 5: The amino acid carried by the tRNA at the “P” site is removed from its tRNA and forms a peptide bond with the amino acid carried by the tRNA at the “A” site.
Slide 6: The ribosome moves down the mRNA the distance of one codon and the tRNA that originally carried f-met is released from the ribosome at the “E” site.
Slide 7: A charged tRNA with an anticodon complementary to the next codon of the mRNA, in this case GGA, binds to the “A” site of the ribosome. The codon GGA codes for the amino acid glycine because a tRNA with a complementary CCU anticodon only carries a glycine.
Slide 8: The amino acids carried by the tRNA at the “P” site are removed from the tRNA and form a peptide bond with amino acid carried by the tRNA at the “A” site.
Slide 9: The ribosome moves down the mRNA the distance of one codon and the tRNA that originally carried Arg is released from the ribosome at the “E” site.
Slide 10: A charged tRNA with an anticodon complementary to the next codon of the mRNA, in this case UCC, binds to the “A” site of the ribosome. The codon UCC codes for the amino acid serine because a tRNA with a complementary AGG anticodon only carries a serine.
Slide 11: The amino acids carried by the tRNA at the “P” site are removed from the tRNA and form a peptide bond with amino acid carried by the tRNA at the “A” site.
Slide 12: The ribosome moves down the mRNA the distance of one codon and the tRNA that originally carried Gly is released from the ribosome at the “E” site.
Slide 13: A charged tRNA with an anticodon complementary to the next codon of the mRNA, in this case CCC, binds to the “A” site of the ribosome. The codon CCC codes for the amino acid proline because a tRNA with a complementary GGG anticodon only carries a proline.
Slide 14: The amino acids carried by the tRNA at the “P” site are removed from the tRNA and form a peptide bond with amino acid carried by the tRNA at the “A” site.
Slide 15: The ribosome moves down the mRNA the distance of one codon and the tRNA that originally carried Ser is released from the ribosome at the “E” site.
Slide 16: A charged tRNA with an anticodon complementary to the next codon of the mRNA, in this case ACC, binds to the “A” site of the ribosome. The codon ACC codes for the amino acid threonine because a tRNA with a complementary UGG anticodon only carries a threonine.
Slide 17: The amino acids carried by the tRNA at the “P” site are removed from the tRNA and form a peptide bond with amino acid carried by the tRNA at the “A” site.
Slide 18: The ribosome moves down the mRNA the distance of one codon and the tRNA that originally carried Pro is released from the ribosome at the “E” site.
Slide 19: There is no corresponding tRNA for the stop codon – UGA in this case. The completed protein is released from the last tRNA and the ribosomal units separate “
References
- David L. Nelson, Michael M. Cox, Lehninger Principles of Biochemistry
- Alberts, Bruce. Molecular Biology of the Cell, 5e. New York: Garland Science, 2008.
