RNAs (Ribonucleic acids) are an essential component of all living cells. RNAs take part in the protein synthesis. There are three different types of RNAs present in a cell, namely- mRNA or messenger RNA, rRNA or ribosomal RNA and tRNA or transfer RNA. They are named according to the function they perform. Each of the three types of RNAs performs unique functions and have different structures.
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mRNA provides a template for gene coding during protein synthesis, tRNA carries the amino acids to the ribosomes, which has to be added to the polypeptide chain and rRNA forms ribosomes along with proteins.
Difference between mRNA, tRNA and rRNA
The table below shows the main difference between mRNA, tRNA and rRNA.
| mRNA | tRNA | rRNA |
| Linear structure | Cloverleaf shape | Spherical and has a complex structure |
| In mammals, mRNAs are 300 to 12000 nucleotides long | 76 to 90 nucleotides | Prokaryotic and eukaryotic cells contain different rRNA, e.g. 5s, 16s, 23s and 5s, 5.8s, 18s and 28s, respectively in different ribosomal subunits |
| Site of synthesis is the nucleus | Produced in the cytoplasm | Found in ribosomes |
| They provide a template or carry genetic information contained in DNA for protein synthesis | They carry specific amino acids to ribosomes for protein synthesis | They facilitate the association of ribosomes |
| mRNA carries codons for the translation process | They carry anticodons, specific to particular amino acid | Codons or anticodons are absent |
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Messenger RNA (mRNA):
mRNA is formed by transcription process from DNA. They provide a template for protein synthesis. RNA polymerase catalyses the transcription process. In eukaryotic cells, mRNA is formed in the nucleus as hnRNA (heterogenous RNA), which after processing, introns are removed and exons joined together after splicing to form mRNA and move to the cytoplasm to take part in protein synthesis.
mRNA is modified at 5’ and 3’ ends for stability and to prevent hydrolysis by exonucleases. 5’ end is capped by 7-methylguanosine and polyadenylation at 3’ end, i.e. a poly-A tail.
Transfer RNA (tRNA):
tRNAs are known as soluble RNAs. As the name suggests, they are a carrier of specific amino acids for coding to the growing end of the polypeptide chain during protein synthesis. Each amino acid has a specific tRNA.
It has a folded structure, which looks like a cloverleaf. It has three hairpin loops, a small variable loop and the acceptor arm, which carries amino acids. 5’ end has a terminal phosphate group.
- The acceptor arm – It is present at 3’ end and carries amino acids. It has a terminal sequence of CCA (5’ -> 3’)
- The anticodon loop – It has a complementary codon for coding a particular amino acid it carries, during the translation process. And attaches to the codon on mRNA
- D loop – has dihydrouridine
- TψC loop – has pseudouridine
There is a small variable loop present between the anticodon loop and the acceptor arm.
Ribosomal RNA (rRNA)
rRNA associates with certain proteins to form ribosomes. They bind to tRNA and accessory proteins and move along mRNA during translation. Large and small ribosomal subunits contain different rRNAs.
To conclude, all three rRNAs are essential in protein synthesis mechanism. They differ in their structure and function. mRNA has a linear structure and carries genetic information copied from DNA. tRNA has an L shaped 3D structure. It is specific to each amino acid and carries an amino acid to the growing chain of a polypeptide during the translation process. rRNAs are spherical and provide a structural framework for ribosomes. They associate with tRNA and are essential for protein synthesis.
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