What is tRNA?
Transfer RNA or tRNA is a short RNA molecule with the function of assisting in the protein synthesis in a cell. It has two important sections, a region that attaches with particular amino acid and a trinucleotide which is commonly called an anticodon. During a translation process, whenever an amino acid adds to a growing protein chain, a transfer RNA molecule forms the base pairs to add its complementary sequence to the messenger RNA (mRNA) in order to ensure that the right amino acid attaches to the protein molecule.
The tRNA full form is transfer RNA. As per its structural features, it forms the link between the transcription and the translation of the RNA during protein synthesis. Its anticodon end matches with the respective codon in the mRNA molecules. During this process, the tRNA carries the specific amino acids that are supposed to attach according to the codon encoding.
Structure and Functions of tRNA
There are different types of RNA molecules present in a cell with distinct functions. One such kind is tRNA. It helps in the synthesis of protein by assisting mRNA in the process. Previously, it was known as sRNA or soluble RNA. It has several functions that we will discuss in this article along with its structural significance.
Structure of tRNA
A tRNA molecule contains 76-90 nucleotide molecules. As per the tRNA structure, each of them defines a specific amino acid. It means that a type of this RNA will carry only a specific amino acid to attach with the codon part of mRNA for protein synthesis.
The stop codons present in the mRNA will not be attached to the tRNA for amino acid transfer. Only this codon signals the stopping of a polypeptide chain synthesis and the release of the synthesized polypeptide in the cellular system. The rest of the codons have specific anticodons. These anticodons are trinucleotide regions of the tRNA that attaches with the particular codons for amino acid transfer.
(Image will be uploaded soon)
The tRNA structure is of two types. Its secondary structure resembles a cloverleaf whereas the tertiary structure resembles the L letter in inverted format. The folds in the structures occur due to the hydrogen bonds occurring between the complementary bases present in the molecule. Let us discuss the structures in an elaborate way.
The secondary shape of the tRNA structure is made of three hairpin-resembling loops. This is why it looks like a cloverleaf. The prime constituents of a tRNA molecule are:
Acceptor Arm
The acceptor arm is formed from the base pairing of 7 to 9 nucleotides 3’ and 5’ terminals. The 3’ end has a specific CCA sequence that forms the CCA tail. The 5’ terminal comprises a phosphate group.
The aminoacylation of tRNA happens in the 3’ terminal right at the hydroxyl group. It is also called the first step or the initiation of the translation process. The catalysis of this reaction occurs due to the influence of the aminoacyl tRNA synthetase enzyme.
This process is also called the charging of tRNA. It refers to the attachment of a unit of amino acid to the tRNA molecule using the aminoacyl transferase enzyme. During this binding process, adenosine triphosphate (ATP) is bonded with an amino acid molecule to form AMP-amino acid and PP. The latter is formed as a byproduct and is used later. It is then the aminoacyl transferase enzyme binds the AMP-amino acid with the 3’ terminal hydroxyl group of tRNA.
DHU Loop
This is the D-shaped loop containing dihydrouridine (DHU) in the form of a modified nucleotide. It is the D-arm with 3 to 4 base pairs present in every tRNA molecule.
Anticodon Loop
It is a loop made of 5 nucleotide base pairs. The long stem contains a complementary sequence made of 3 nucleotides that match the specific mRNA codons. This codon sequence signifies what amino acid needs to be carried by the tRNA. It is these unpaired anticodon bases that pair with the specific bases of a codon in mRNA. This is why every tRNA has a unique identification decided by the respective mRNA.
TΨC Loop
The TΨC Loop is the T-arm stem consisting of 4 to 5 base pairs creating a loop. It also contains pseudouridine, another type of modified uridine. This loop remains nearest to the 3’ terminal. It is thought that this loop interacts with the ribosomal RNA.
Variable Loop
This variable loop is present right between the anticodon loop and the TΨC loop. The size of this variable loop varies from 3 to 21 base pairs. This part of the tRNA molecule helps in recognizing its pattern and type.
Function of tRNA
The prime function of tRNA is to transfer amino acids to form the right sequence of the polypeptides. Hence, it assists protein synthesis.
It also functions as an adapter molecule to link specific amino acids to the respective codons located in the mRNA molecules.
Every amino acid defines a particular tRNA. It means that the carriage and transfer of amino acids will not possible without those tRNAs.
The decoding process of the mRNA continues until the stop codon is reached and not recognized by any of the tRNAs.
This is what you need to know about the structure of this RNA and the tRNA translation process. Understand the complex structure of this transfer RNA molecule and learn how it assists in adding amino acids to the polypeptide chain to build specific proteins. Consider referring to a tRNA diagram to understand the significance of its different sections and base-pair sequences.
FAQs on Transfer RNA
1. What is Transfer RNA (tRNA) and what is its primary function in a cell?
Transfer RNA (tRNA) is a small RNA molecule that acts as a physical link between the nucleotide sequence of nucleic acids (like mRNA) and the amino acid sequence of proteins. Its primary function is to carry a specific amino acid to the ribosome during protein synthesis (translation), ensuring that the amino acid matches the corresponding codon on the messenger RNA (mRNA) strand.
2. What is the importance of the clover-leaf structure of tRNA?
The two-dimensional clover-leaf model is crucial as it clearly shows the distinct functional regions or arms of the tRNA molecule. These regions include:
- The Acceptor Arm: This is where the specific amino acid attaches.
- The Anticodon Loop: Contains a three-base sequence called the anticodon, which is complementary to an mRNA codon.
- The TΨC Loop: Helps in binding the tRNA to the ribosome.
- The DHU Loop: Believed to play a role in the recognition of the tRNA by the correct enzyme for amino acid attachment.
This structure ensures that tRNA can simultaneously bind to an amino acid, the mRNA, and the ribosome.
3. How does tRNA differ from mRNA and rRNA in their roles during protein synthesis?
While all three RNAs are essential for protein synthesis, their roles are distinct:
- tRNA (Transfer RNA): Acts as an 'adapter' that reads the mRNA codon and transfers the corresponding amino acid to the ribosome.
- mRNA (Messenger RNA): Carries the genetic code transcribed from DNA out of the nucleus to the ribosome. It serves as the template for building the protein.
- rRNA (Ribosomal RNA): Is a major structural component of ribosomes. It catalyses the formation of peptide bonds between amino acids to form a polypeptide chain.
4. Why is tRNA often described as an 'adapter molecule'?
tRNA is called an adapter molecule because it translates the genetic information from one 'language' to another. It adapts the language of nucleic acids (the sequence of codons on mRNA) into the language of proteins (the sequence of amino acids). By binding a specific amino acid at one end and recognising the corresponding mRNA codon at the other, it bridges the gap between the genetic code and the protein being built.
5. What is the significance of the process called 'aminoacylation' or 'charging' of tRNA?
Aminoacylation, or the 'charging' of tRNA, is the process of attaching the correct amino acid to its corresponding tRNA molecule. This reaction is catalysed by a specific enzyme called aminoacyl-tRNA synthetase. This step is critically important for the accuracy of translation. If the wrong amino acid is attached, it will be incorrectly incorporated into the growing protein, potentially leading to a non-functional or harmful protein.
6. How does a single tRNA molecule recognise the correct codon on an mRNA strand?
A tRNA molecule recognises the correct codon on an mRNA strand through its anticodon loop. This loop contains a sequence of three nucleotides, known as the anticodon, which is complementary to a specific codon on the mRNA. During translation within the ribosome, the anticodon of the tRNA binds to the mRNA codon through complementary base pairing (A with U, and G with C), ensuring the correct amino acid is added to the polypeptide chain.
7. Why are there fewer types of tRNA molecules than the 61 codons that specify amino acids?
This phenomenon is explained by the 'wobble hypothesis'. It states that the base pairing between the third base of the mRNA codon and the first base of the tRNA anticodon does not have to be as strict as the first two positions. This 'wobble' allows a single tRNA anticodon to recognise and bind to multiple different codons that code for the same amino acid. This makes the translation process more efficient without compromising accuracy.
8. What would be the consequence if a cell's aminoacyl-tRNA synthetase enzymes became non-functional?
If the aminoacyl-tRNA synthetase enzymes were non-functional, the 'charging' of tRNA molecules would cease. This means that tRNA molecules would not be able to attach to their specific amino acids. As a result, the entire process of protein synthesis would halt, as there would be no mechanism to deliver the required amino acids to the ribosome for building polypeptide chains. This would be lethal for the cell.
