[Biology Class Notes] on Translation Protein Synthesis Pdf for Exam

Protein synthesis is completed by the process of protein translation. DNA segments are transcribed into messenger RNA molecules via transcription and the messenger RNA is translated for protein synthesis via translation. In the process of translation, messenger RNA works together with the transfer RNA i.e. tRNA and ribosome for the synthesis of proteins. The whole process of transcription and translation is called gene expression. Protein synthesis can be defined as the process in which the molecules of amino acids are arranged as a single line into proteins by involving ribosomal RNA, transfer RNA, messenger RNA, and other enzymes.

Translation Process in Protein Synthesis

The translation is a process of protein synthesis from mRNA with the help of ribosomes. Translational unit of mRNA from 5’ to 3` includes start codon, region coded polypeptide, a stop codon, and untranslated regions (UTRs) at 5`end & 3`end both for more efficiency of the process.

The ribosome is the place where the whole machinery of translation is present. Each eukaryotic ribosome has two parts: a smaller 40S subunit and a larger 60S subunit. The smallest unit has a point for attachment of mRNA. Along with the largest subunit, it forms a P-site or peptidyl transfer (Donor site).

There are binding sites for initiation factors, elongation factors, translocation, etc.

Structure and Role of tRNA in Protein Synthesis 

The transfer RNA(tRNA) is a family of about 60 small sized ribonucleic acids that can recognize the codon of mRNA and exhibit a higher affinity for 21 activated amino acids which combine with them and carry them to the site of protein synthesis. tRNA molecules have been variously termed as soluble RNA or supernatant RNA or adapted RNA of the cell.

Structurally, tRNA looks like a cloverleaf or inverted L shaped molecule which on one end has an amino acid receptor end and on the other end has an anticodon loop. The L shape results due to the modification in the nucleotides of tRNA such as pseudouridine, dihydrouridine(DHU), inosine and ribothymidine. The bent in the chain of each tRNA molecule contains a definite sequence of three nitrogenous bases that constitute the anticodon. It recognizes the codon on mRNA. The main constituents of tRNA are-

1. Anticodon Loop: It contains 7 bases out of which three bases form the anticodon loop and attaches to the codon of mRNA.

2. DHU Loop: This loop serves as the binding site for aminoacyl synthetase enzyme and it contains around 8-12 bases. The D arm contains the modified nucleotide called dihydrouridine. 

3. T ? C Loop: This loop contains two modified nucleotides- pseudouridine and ribothymidine. This loop serves as the attachment site for ribosomes.

4. AA Binding Site: This site serves as the binding site for amino acid. It contains a CCA- OH group.

5. Variable Loop: It is generally present between the T?C loop and anticodon loop.

The function of tRNA is specific in protein synthesis as they pick up specific amino acids from the amino acid pool and carry over the mRNA strand.

Protein Synthesis Steps Involved

The three stages of translation are-

• Initiation involves assembling ribosomes around mRNA and activating amino acid and delivering it to the transfer RNA.

• Elongation is the process in which the RNA strand gets longer by adding amino acids. 

• The termination process only involves releasing a polypeptide chain.

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Explanation of Steps of Translation

1.Initiation

• Initiation in prokaryotes requires large and small ribosome subunits, the mRNA, initiating transfer RNA, and 3 initiation factors (IFs).

• Amino acids are activated by binding with the enzyme called aminoacyl tRNA synthetase in presence of ATP forming an enzyme complex and P site.

Amino acid and ATP in the presence of aminoacyl transfer RNA synthetase ?  Pi + AA-AMP-Enzyme complex

AA-AMP-Enzyme complex + transfer RNA ? Amino Acid- tRNA + AMP + Enzyme.

2.Elongation of the Polypeptide Chain

• At the 2nd codon, other aminoacyl transfer RNA complexes that are charged initiate binding at A-site.

• At P-site- peptide bond between the carboxyl molecule and the amino molecule is observed whereas at A-site bond between amino molecule and amino acid is formed through the enzyme named as a peptidyl transferase.

• Sliding of ribosome over messenger RNA from one codon to its alternate codon in the direction of 5’ to 3`.

• A polypeptide chain is formed by the attachment of amino acids to one alternate to another in a chain formed by the peptide bond, and the attachment is based in accordance with the sequence of codons resulting in elongation of the protein chain.

3. Termination of Polypeptide

• Reaching the A-site of the ribosome at a termination codon which is present, not coding for any amino acid, no charged transfer RNA binds to the A-site of ribosome.

• A polypeptide is now not associated with the ribosome and dissociates and is catalyzed by a “release factor”, a factor that releases 3 termination codons called UGA, UAG, and UAA.

Protein Synthesis in Diagram

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