Transfer RNA: Structure and Function (tRNA)
Transfer RNA commonly abbreviated as tRNA are adaptor between codons and amino acids. The heart of protein synthesis is the “translation” of nucleotide sequence information into amino acids. Each codon in the mRNA sequence represents a particular amino acid, and each codon is recognized by a specific tRNA. The tRNA molecules accomplish this, serving as the physical link between the mRNA and the amino acid sequence of proteins. They are formerly referred to as sRNA, for soluble RNA. In this article, we will learn about the tRNA structure and function.
Transfer or tRNA
Transfer or tRNA has an important role in transcription as it helps decode the messenger RNA that specifies the protein product of the gene from which the mRNA is transcribed. Therefore it specifies the sequence from the genetic code that corresponds to a particular amino acid. There are many types of tRNA molecules, but each is attached to a specific amino acid, and each recognizes a particular codon, or codons, in the mRNA. Therefore after recognizing the codon, the tRNA binds to its corresponding codon in the ribosome, and the tRNA transfers the appropriate amino acid to the end of the growing amino acid chain. The amino acid is obtained from the cytoplasmic pool. Then they are bonded together to form proteins. Further, the tRNA continues to decode the mRNA until the entire mRNA is translated into a protein.
Another striking aspect of tRNA is the presence of several unusual bases in their primary structure. These are the enzymatic modifications of normal bases in the polynucleotide chain. For example,
- It is derived from uridine by an isomerization site of attachment of the uracil base to the ribose and is switched from the nitrogen at ring position 1 to the carbon at ring position 5.
- The ψU loop is named so because of the characteristic presence of the unusual base ψU in the loop. These modified bases are often found within the sequence 5’-TψUCG-3’.
- It is derived from uridine by enzymatic reduction of the double bond between the carbons at positions 5 and 6. Other unusual bases found in tRNA include hypoxanthine, thymine, and methylguanine.
- The D loop gets its name from the characteristic presence of dihydrouridines in the loop.
These modified bases are not essential for tRNA function, but cells lacking these modified bases show reduced rates of growth.
Structure of Transfer or tRNA
A typical eukaryotic tRNA is composed of 76 to 90 nucleotides in length. It, therefore, has a distinctive folded structure with three hairpin-like loops. On its primary structure, it can be easily decomposed and its secondary structure is usually visualized as a cloverleaf structure. Whereas the tertiary structure of the tRNA is an L-shaped 3D structure that allows them to fit into the P & A site of the ribosome. It is basically a string of RNA that is folded into a series of loops. Its structural components are :
- A phosphate group at the 5′ terminal.
- The CCA tail is at the 3′ end of the tRNA molecule. It stands for the cytosine-cytosine-adenine sequence. The loaded amino acid forms a covalent bond with the 3′- hydroxyl group on the CCA tail.
- The acceptor stem is present next to the CCA til on the 3′ end of the tRNA. These are generally referred to as genomic tags.
- A variable loop. It helps in the recognition of the tRNA. As the name implies it varies in size from 3 to 21 bases.
- The T-arm is a 4-5 bp stem ending in the loop containing the sequence TΨC, where Ψ stands for pseudouridine.
- The D-arm which is a 4-6 bp stem ending in the loop often contains dihydrouridine.
- The anticodon end contains complementary codons to the mRNA. It reads in totally reverse order since 3′-5′ directionality is required to read the mRNA from the 5′ to 3′ end.
Function of Transfer or tRNA
It is very important to understand tRNA structure and function, in order to understand the biology of translation.
- As discussed earlier the prime role of the transfer or tRNA is in translation. It transfers the amino acids to form the right sequence of polypeptides. Thus it assists in protein synthesis.
- The mRNA is decoded until the stop codon is reached and is not identified by any of the tRNAs.
- Every amino acid is identified by particular tRNAs only. Therefore the transfer of amino acids shall not be possible without those amino acids.
- tRNAs predominantly act as adaptor molecules in linking specific amino acids to their respective codons in the mRNA molecules.
- The tRNA transfers the amino acids to three binding sites of tRNA on ribosome namely aminoacyl (A), peptidyl (P) and exit (E) which is needed for growing the polypeptide chain.
- Aminoacylation is performed by tRNA without which translation would not begin.
- Researchers have reported evidence suggesting that a preliminary form of tRNA could have been a replicator molecule in the very early development of life.
To summarize, transfer RNA or tRNA has its folded structure due to hydrogen bonding in its complementary bases. Thus, the tRNA structure and function are correlated. This, therefore, keeps on increasing the folds in the tRNA structure. It undergoes many changes in conformations as it transits to different sites on the ribosome. Being the mRNA decode it stands out with the major responsibility of protein synthesis. Therefore it makes out the reason for the biological importance of tRNA as protein makes up major compositions of the human cells and thus the body.
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