How Does tRNA Drive Protein Synthesis During Translation?
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-
Anticodon Loop: It contains 7 bases out of which three bases form the anticodon loop and attaches to the codon of mRNA.
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.
T 𝝭 C Loop: This loop contains two modified nucleotides- pseudouridine and ribothymidine. This loop serves as the attachment site for ribosomes.
AA Binding Site: This site serves as the binding site for amino acid. It contains a CCA- OH group.
Variable Loop: It is generally present between the T𝚿C 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
Transfer of amino acid to tRNA -
AA-AMP-Enzyme complex + transfer RNA 🡪 Amino Acid- tRNA + AMP + Enzyme.
Two sites at ribosome are present that are called A-site and P-site where units of ribosome bind to the cap region of messenger RNA and comparatively smaller units bind to mRNA followed by binding of them with the larger subunits. It makes AUG lie on P-site and methionyl tRNA binds to P-site.
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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FAQs on Translation Protein Synthesis Explained
1. What is translation in biology, and where does it occur in a cell?
Translation is the process in which the genetic information encoded in a messenger RNA (mRNA) molecule is used to create a specific sequence of amino acids, forming a polypeptide chain. This process essentially translates the language of nucleic acids (codons) into the language of proteins (amino acids). In eukaryotic cells, translation occurs in the cytoplasm, specifically on cellular machinery called ribosomes.
2. What is the fundamental difference between transcription and translation?
The fundamental difference lies in the template and the product. Transcription is the synthesis of an RNA molecule from a DNA template, occurring in the nucleus of eukaryotes. In contrast, translation is the synthesis of a protein from an mRNA template, occurring in the cytoplasm. In simple terms, transcription is 'rewriting' the genetic code from DNA to RNA, while translation is 'interpreting' that RNA code to build a protein.
3. What are the three main stages of the translation process?
The process of translation is generally divided into three key stages:
- Initiation: The ribosome assembles around the mRNA to be read. The first transfer RNA (tRNA), carrying the amino acid methionine, attaches to the start codon (AUG).
- Elongation: The ribosome moves along the mRNA, reading one codon at a time. For each codon, the corresponding tRNA brings the correct amino acid, which is added to the growing polypeptide chain.
- Termination: When the ribosome reaches a stop codon (UAA, UAG, or UGA) on the mRNA, the process ends. A release factor binds to the stop codon, and the completed polypeptide chain is released from the ribosome.
4. What are the specific roles of mRNA, tRNA, and rRNA in protein synthesis?
Each type of RNA has a distinct and vital role in translation. mRNA (messenger RNA) acts as the template, carrying the genetic code from the DNA in the nucleus to the ribosome. tRNA (transfer RNA) functions as an adapter molecule; it reads the codons on the mRNA via its anticodon and brings the specific amino acid that corresponds to that codon. rRNA (ribosomal RNA) is a major structural and catalytic component of ribosomes, the site of protein synthesis. It helps align the mRNA and tRNA and catalyses the formation of peptide bonds between amino acids.
5. Why is the genetic code described as 'degenerate' yet 'unambiguous'?
The genetic code has these two important features. It is degenerate because a single amino acid can be coded for by more than one codon. For example, the amino acid Leucine is specified by six different codons. This degeneracy provides a buffer against the harmful effects of mutations. The code is unambiguous because a single codon will only ever code for one specific amino acid. For instance, the codon CCG always codes for Proline and nothing else. This ensures high fidelity during protein synthesis.
6. How does a ribosome know where to start and stop translation on an mRNA molecule?
The ribosome identifies specific signals on the mRNA molecule. Translation begins at the start codon, which is almost universally AUG. The ribosome binds to the mRNA and scans along it until it encounters this codon, setting the correct reading frame. The process continues until the ribosome encounters one of the three stop codons: UAA, UAG, or UGA. These codons do not code for an amino acid; instead, they signal for termination factors to bind and release the newly synthesised polypeptide chain.
7. What happens to the polypeptide chain after the process of translation is complete?
After being released from the ribosome, a newly synthesised polypeptide chain is often not yet functional. It must undergo post-translational modifications to become a mature protein. These modifications include:
- Folding: The linear chain folds into a specific three-dimensional structure, which is crucial for its function.
- Chemical Modification: Groups like phosphates, sugars, or lipids may be added to the amino acids.
- Cleavage: Parts of the polypeptide chain may be cut away.
8. What is the importance of Untranslated Regions (UTRs) on an mRNA strand?
Untranslated Regions (UTRs) are sections of the mRNA located before the start codon (5' UTR) and after the stop codon (3' UTR). Although they are not translated into protein, they are critically important. The 5' UTR plays a key role in the initiation of translation, influencing how efficiently the ribosome binds and begins synthesis. The 3' UTR is crucial for determining the overall stability of the mRNA molecule, its location within the cell, and can contain regulatory sequences that affect whether translation occurs at all.
