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Protein synthesis



After reading this, you should be able to:
1 Outline the process of protein synthesis.
2. Explain the importance of protein synthesis and give some examples of the proteins synthesized by humans.
The information necessary for a particular protein lies in a segment of the DNA molecule called gene. This information is given to another molecule called messenger RNA (mRNA) in a process called transcription; the enzyme polymerase catalyses this process.

Transcription of mRNA occurs when a particular segment of DNA (gene) unwinds or unzips, exposing a sequence of bases. As the DNA strands constituting the gene separate, one strand begins to function as the template for the base pairing of complementary ribonucleotides.

The enzyme RNA polymerase mtalyses linkages of the assembled rlbonucleotides; the result is the formation of a strand of an mRNA molecule. The transcribed mRNA is of opposite polarity to its DNA template strand.

Through the action of enzymes, nucleotide sequences called introns, which are not used in protein synthesis, are cut out from the transcribed mRNA; and the remaining nucleotide sequences called exons, which are used in protein synthesis, are spliced together.

After the process of cutting and splicing. the mRNA molecule leaves the nucleus through a nuclear pore and moves into cytoplasm; the sequences of three nucleotide bases, on the mRNA, that code for speciiic amino acids of a particular protein are called codons.

Meanwhile, the unzipped segment 0f DNA zips up again. Other RNAs that participate in protein synthesis are transfer RNA [tRNA] and ribosomal RNA (rRNA): like the mRNA, pass into the cytoplasm through pores In the nuclear membrane.

In the cytoplasm, rRNA combines with existing protein molecules to form ribosomes.
   There are two types of ribosomes; the small ribosome subunit and the large ribosome subunit. The ribosomes provide a physical site for protein synthesis.

Also in the cytoplasm, tRNA which already has a sequence of three bases called the anticodon at one end, combines with an amino acid at its other end.

It must be noted that anticodon is the source of attraction between a tRNA and an mm; and that anticodon is a sequence of three bases, at a portion of a tRNA, that are complementary to a specific codon.

The translation of the information being married by the mRNA into a particular protein starts when the mRNA. on entering the cytoplasm attaches itself by means of the initiator codon AUG, to a special site on a small ribosome subunit. A small ribosome subunit has two of such special sites.

The initiator tRNA, which has the amino acid methronine at one end, pairs with the initiator codon AUG by means of its anticodon UAC A large ribosome subunit then attaches to the mRNA; a small ribosome pair is formed in the process. A large ribosome subunit has the P site or peptide site and the A site or amino site. The initiator tRNA binds to the P site; the A site is vacant.

The second special site on the small ribosome subunit is lined up with the codon next to the initiator codon. A second tRNA then binds to the codon next to the initiator codon, and It pairs with this codon using is anticodon.

A peptide bond is then formed between the amino acids assembled by the two tRNAs. At the same time. the bond between the iirst tRNA and its amino acid (methranine) breaks; and the first tRNA is released from the P site.

The second tRNA together with its amino acid engaged in a peptide bond with methionine, leaves the A site and binds to the P site. The A site is left vacant as a result. The small ribosome large rlbosome pair shifts along the mRNA, by one codon. The vacant A site is now opposite this third codon: a third tRNA binds to the vamntA site.

The anticodon of the third tRNA pairs with the third codon. A second peptide bond is formed between the third and the second amino acids. and the bond between the second amino acid and its tRNA breaks. releasing the tRNA in the process. (The growing peptide chain is always attached to the RNA that is moving from the A site to the P site; the incoming tRNA bearing the next amino acid always binds to the A site].

This series of shifting of ribosome pair along mRNA by one codon, binding of a tRNA with codon, formulation of peptide bond between adjacent amino acid, continues until the ribosome pair shifts to any of the termination codons or step codons. These codons are UGA, UAG, and UAA. There is no tRNA with an anticodon complementary to any of the stop codons.

The ribosome pair, on shifting to a termination codon, stops moving along the mRNA. it comes off the mRNA and separates into its two subunits. Also, the last +RNA attached to the growing polypeptide chain breaks from its amino acid. The synthesis of a Particular protein is then completed.

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