- RNA is a single-stranded biopolymer having a complex structure with high molecular weight and was described by R.W. Holley in 1965.
- Its structure is similar to that of DNA of which DNA is long and double-stranded but RNA is short and contains only one strand with ribose nucleotides attached by a phosphodiester bond.
- Due to its single-stranded structure, it does not hold genetic information as much as DNA does but has a capacity to function as a genetic code in some viruses that do not contain DNA molecules.
- Although RNA is typically a single-stranded molecule, there are double-stranded RNA present in some of the viruses. Like Rhinoviruses, influenza viruses, coronaviruses, and Ebola viruses are single-stranded RNA viruses while Rotaviruses and retro-viruses are double-stranded RNA viruses.
- Double-stranded RNA viruses are not common in eukaryotic cells therefore, its infection can cause severe viral diseases.
- RNA molecules are mainly involved in the protein synthesis processes.
Structure of RNA
The structure of the RNA molecule consists of ribonucleotide linked by a phosphodiester bond.
A ribonucleotide comprises of:
- Nitrogenous bases
- Ribose sugar and
- A phosphate group
1. Nitrogenous bases:
- RNA has four nitrogen bases – Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).
- We can now observe that why DNA differs from RNA as one nitrogen base is different in both the molecules.
- Adenine and Guanine are considered as the major building blocks in both RNA and DNA molecules.
2. Ribose sugar:
- Ribose sugar is a cyclic structure of five carbon rings and one oxygen. It contains a hydroxyl (-OH) group attached to the second carbon making it prone to hydrolysis which is another difference from DNA sugar molecule.
- Because of the lack of hydroxyl group in DNA molecules, it is believed to have stability and genetically evolved making it carrier of genetic information in most organisms.
3. Phosphate group:
- The phosphate group of RNA is similar to that of DNA with a phosphorous atom bonded with four oxygen atom with 3 single bonded and one double bonded.
- The phosphate group also acts as a backbone for RNA strands.
Functions of RNA
- The main function of RNA is it synthesizes proteins through the process of translation. Three different types of RNA are involved in this process: mRNA, rRNA, and tRNA.
- Its type, mRNA serves as a messenger between DNA and ribosomes and promotes ribosomes in choosing exact aminoacids.
- RNA transfers genetic information in some of the viruses.
- RNA also helps in catalyzing biological reactions and controls gene expression.
Types of RNA and their functions
RNA is self complementing that base pairs with its own sequences by folding its strand into complex structural forms making bulges and helices.
There are three most well studied types of RNA:
- Messenger RNA (mRNA)
- Ribosomal RNA (rRNA)
- Transfer RNA (tRNA)
Messenger RNA (mRNA)
- mRNA is the most heterogeneous type of RNA in terms of base sequence and size.
- mRNA is responsible for carrying genetic information from DNA and controls all the cellular activities. Then, it interacts with the ribosomes and synthesizes proteins required by the body cells during the protein translation.
- A sequence of three nitrogenous bases of the mRNA determines the amino acid sequence in the produced protein.
- mRNA is unstable and short-lived RNA, especially in prokaryotic cells.
- The main function of mRNA is that it acts as an intermediate between DNA and protein which is used by ribosomes during protein synthesis.
Ribosomal RNA (rRNA)
- rRNA is the component of ribosomes composed of about 60% of the mass of the ribosome and is located in the cytoplasm where ribosomes are present.
- Its major function is to synthesize and translate mRNA into proteins that occur in the cell cytoplasm and transport it to the nucleolus.
- The encoded rRNA differ in size in which each ribosome have one large rRNA and one small rRNA.
- Another function of the rRNA is that it catalyzes the peptidyl transferase reaction during protein synthesis.
Transfer RNA (tRNA):
- tRNA is the smallest RNA among all and is one of the essential components of the protein translation process.
- It functions as transferring of correct amino acid to the site of protein synthesis and also acts as an adapter in the mRNA sequence during protein translation. Therefore, they are also referred to as an adapter molecule.
- The structure of tRNA is like a cloverleaf which is stabilized by a hydrogen bond within the nucleotides.
- The sequence contains four usual nitrogenous bases but also contains some unusual bases like methyl guanine and methylcytosine. These both are formed by the methylation of the usual bases.
- Any mutation in the sequence of rRNA and tRNA results in lethal mutagenesis as both are important for proper protein synthesis.