mRNA has a linear structure having uracil base instead of thymine, and its secondary structure could be hairpin, stem-loop, etc. ; while tRNA has Cloverleaf structure that carries three specific stem-loops; and rRNA has much complex structure with numerous folds and loops.
mRNA acts as the messenger of DNA; tRNA carries amino acids during protein synthesis; rRNA is the protein producer of the cell. These three RNA plays a vital role in the process of transcription and further in protein synthesis. These are essential factors for every cell as life would not have been possible in their absence.
Ribonucleic acid is abbreviated as RNA, which is the compound active in cellular protein synthesis. It has high molecular weight and acts as a genetic code in some viruses. They have nitrogenous bases as adenine, guanine, cytosine and uracil (replacing thymine of DNA). They are single-stranded biopolymer. The RNA has ribose nucleotides where the nitrogenous bases are attached to the ribose sugar which are attached by the phosphodiester bonds forming the chain or strands of different lengths.
In the year 1965, R.W. Holley described the RNA structure. The essential and significant process of molecular biology is the flow of genetic information in a cell, which is three steps; DNA makes RNA that leads to proteins. Therefore, proteins are regarded as the workhorses of the cell, that play essential roles in the cell.
So, whenever the cell needs any protein it sends signals by activating the that particular protein’s genes and the DNA coding for that protein, produce multiple copies of that part which are further processed, transcribed and translated.
The process of RNA transcription is mediated by the RNA polymerase (enzyme) that construct RNA complement to template DNA. The method of transcription is appropriately controlled by three chief factors, promoter, regulator and inhibitor.
In this context, we will discuss the structural as well as the functional differences between the three types of RNA in eukaryotic cells.
Content: mRNA Vs tRNA Vs rRNA
|BASIS FOR COMPARISON||mRNA||tRNA||rRNA|
|Meaning||mRNA or messenger RNA is the connection between gene and protein, and it is the result of the transcribed gene by RNA polymerase.||tRNA or transfer RNA is a cloverleaf shaped RNA molecule and provides specific amino acids to the ribosomes.||rRNA or ribosomal RNA is used for the formation of the ribosomes.|
|Role||mRNA carries genetic information from the nucleus to ribosomes for the synthesis of proteins.||tRNA carries specific amino acids to the ribosomes to assist the protein biosynthesis.||rRNA these provide the structural framework for the formation of ribosomes.|
|Size||In mammals, the size of the molecules is around 400 to 12, 000 nucleotides (nt).||The size of the molecule of tRNA is 76 to 90 nucleotides (nt).||The size of the molecule of rRNA may vary from the 30S, 40S, 50S and 60S.
|Shape||mRNA is linear in shape.||tRNA is a cloverleaf shape.||rRNA is a sphere shape (complex structure).|
|Comprise of||mRNA is comprised of codons.||tRNA is comprised of anticodons.||rRNA does not have anticodon or codon sequences.
Definition of mRNA
The synthesis of messenger RNA or mRNA takes place in the nucleus (in eukaryotes) as heterogeneous nuclear RNA (hnRNA). Further, the processing of hnRNA releases mRNA. Now, this (mRNA) will enter the cytoplasm to take part in protein synthesis.
mRNA has a short half-life, with high molecular weight. These are said as the link between gene and protein. This form of RNA or the eukaryotic mRNA is exclusively modified (post-transcription modification) just to prevent from hydrolysis by 5′-exonucleases (enzyme). So, these are capped at 5′-terminal ends by 7-methylguanosine triphosphate. This capping also helps in recognition of the mRNA for protein synthesis.
At the 3′-terminal end, of mRNA there is a polymer of adenylate residues (20 – 20 nucleotides) known as poly (A) tail or polyadenosine tails. This tail provides the stability of mRNA and also prevent the attack of 3′-exonucleases.
mRNA molecules also have certain modified bases like 6-methyladenylates in the internal structure; these mRNA also have intron, which is spliced out before the formation of the mature mRNA molecule.
Definition of tRNA
Transfer RNA or tRNA is the soluble RNA, the molecules contain approximately 75 nucleotides and have a molecular weight of 25,000. There are 20 species of tRNAs that corresponds to 20 amino acids present in the protein structure. The structure of the tRNA was first described by Holley.
During protein, translation tRNA is the decoder of the message of the mRNA. The tRNA structure resembles the cloverleaf model. The structure has four arms: the acceptor arm, the anticodon arm, the D arm, the TψC arm and the variable arm.
The acceptor’s arm is capped with CCA sequence (5′ to 3′). The amino acids are attached to the acceptor’s arm. The acceptor’s arm has three specific nucleotides bases (anticodon), which recognise the triplet codon of mRNA. The D arm is named after the presence of dihydrouridine. The TψC arm has the sequence of T, pseudouridine and C. The variable arm is the most variable arm and has two categories which are Class I and Class II tRNAs.
The tRNA is also modified after transcription like inosine, methylguanosine, and pseudouridine. This is done to include nonstandard bases. As the ribosome cannot form protein with the help of mRNA; the anticodon, a sequence of three key bases of tRNA are complementary to the codon of three bases of mRNA.
This is the first chief role of tRNA, and then the process continues as each molecule carries an amino acid that matches the mRNA codon.
Definition of rRNA
Ribosomal RNA or rRNA is the primary factor of ribosomes. These are factories for protein synthesis. The eukaryotic ribosomes are made up of two nucleoproteins complexes – the 60S and 40S subunits. The 60s subunit is further divided into 28S RNA, 5S RNA and 5.8S RNA, whereas 40S RNA has 18S RNA as its subunit.
The function of rRNAs is not clear in ribosomes; it is believed that the RNA molecules travel to the cytoplasm and bind with mRNA and other rRNA molecules and proteins to form ribosomes.
Key Differences Between mRNA, tRNA and rRNA
Given below are the critical points to understand the variations among the mRNA, tRNA and rRNA:
- mRNA or messenger RNA is the connection between gene and protein, which are formed from the transcribed gene by RNA polymerase; tRNA or transfer RNA is cloverleaf shaped RNA molecule, and assist in giving specific amino acids to the ribosomes; rRNA or ribosomal RNA is the used for the formation of the ribosomes.
- mRNA carries genetic information from the nucleus to ribosomes for the synthesis of proteins; while tRNA carries specific amino acids to the ribosomes to assist the protein biosynthesis, and on the other hand, rRNA provides the structural framework for the formation of ribosomes.
- mRNA is synthesised in nucleus, tRNA is synthesised in the cytoplasm, whereas rRNA is synthesised in the ribosome.
- In mammals, size of the molecules of mRNA is around 400 to 12, 000 nucleotides (nt), while the size of a molecule of tRNA is 76 to 90 nucleotides (nt) and that of rRNA may vary from the 30S, 40S, 50S and 60S.
- The mRNA are linear in shape; tRNA has the cloverleaf shape, and rRNA are sphere shape (complex structure).
- mRNA is comprised of codons, whereas tRNA is comprised of anticodons, and rRNA does not have anticodon or codon sequences.
There are three major types of RNA in a cell, which are mRNA, tRNA and rRNA. These play a significant role in protein synthesis. The mRNA are the carriers of the message and thus initiate the protein formation. This process also involves tRNA and rRNA, where tRNA brings the specific amino acids and rRNA play a role in the formation of ribosomes. The whole process takes place from the nucleus to the ribosome.
Alf Beth says
A superb, well-written, clear description of various types of RNA. Thank you.