RNA (Ribonucleic Acid)
RNA (Ribonucleic Acid)
STRUCTURE OF RNA
- RNA is a polymer of ribonucleotides held together by 3′,5-phosphodiester bridges.
- Although RNA has certain similarities with DNA structure, they need specific differences
- Pentose: The sugar in RNA is ribose in contrast to deoxyribose in DNA.
- Pyrimidine RNA contains the pyrimidine uracil in place of thymine (in DNA).
- Single strand RNA is usually a single-stranded polynucleotide. However, this strand may fold at certain places to offer a double-stranded structure, if complementary base pairs are in close proximity.
- Chargaff’s rule-not obeyed: Due to the single-stranded nature, there is no specific relation between purine and pyrimidine contents.
- Thus the guanine content is not equal to cytosine (as is the case in DNA).
- Susceptibility to alkali hydrolysis: Alkali can hydrolyse RNA to 2′,3-cyclic diesters.
- This is possible due to the presence of a hydroxyl group at 2 position. DNA cannot be subjected to alkali hydrolysis due to lack of this group.
- Orcinol colour reaction: RNAs can be histologically identified by orcinol reaction due to the presence of ribose.
TYPES OF RNA
- The three major sorts of RNAs with their respective cellular composition are given below
- Messenger RNA (mRNA) 5-10%
- Transfer RNA (IRNA):10-20%
- Ribosomal RNA (tRNA)
- 50-80%Besides the three RNAS referred above, other RNAs are also present in the cells.
- These include heterogeneous nuclear RNA (hnRNA), small nuclear RNA (snRNA), small nucleolar RNA (snoRNA) and small cytoplasmic RNA (scRNA).
- The major functions of these RNAs are given in The RNAs are synthesized from DNA, and are primarily involved in the process of protein biosynthesis The RNAs vary in their structure and function. A brief description of the major RNAs is given.
MESSENGER RNA (mRNA)
- The mRNA is synthesized within the nucleus (in eukaryotes) as heterogeneous nuclear RNA (hnRNA). hnRNA, on processing, liberates the functional mRNA which enters the cytoplasm to participate in protein synthesis. mRNA has a high molecular weight with a short half-life.
- The eukaryotic mRNA is capped at the 7 methyl guanosine triphosphate.
- It is believed that this cap helps to prevent the hydrolysis of mRNA by 5′-exo-nucleases. Further, the cap may be also involved in the recognition of mRNA for protein synthesis.
- The 3′-terminal end of mRNA contains a polymer of adenylate residues (20-250 5′-terminal nucleotides) which is known as poly (A) tail.
- This tail may provide stability to mRNA, besides preventing it from the attack of 3′-exonucleases.
- mRNA molecules often contain certain modified bases like 6-methyladenylates within the internal structure.
TRANSFER RNA (tRNA)
- Transfer RNA (soluble RNA) molecule contains 71-80 nucleotides (mostly 75) with a molecular weight of about 25,000. There are at least 20 species of tRNAs, corresponding to 20 amino acids present in protein structure.
- The structure of tRNA (for alanine) was first elucidated by Holley.
- The structure of tRNA, depicted in resembles that of a cloverleaf. tRNA contains mainly four arms, each arm with a base-paired stem.
- The acceptor arm: This arm is capped with a sequence CCA (5′ to 3). The amino acid is attached to the acceptor arm.
- The anticodon arm: This arm, with the three specific nucleotide bases (anticodon), is responsible for the recognition of triplet codon of mRNA. The codon and anticodon are complementary to each other.
RIBOSOMAL RNA (rRNA)
- The ribosomes are the factories of proteinate synthesis. The eukaryotic ribosomes composed of two major nucleoprotein complexes-60S subunit and 405 subunits.
- The 60S subunit contains 28S rRNA, 5S rRNA and 5.8S rRNA while the 40S subunit contains 18S rRNA.
- The function of rRNAs in ribosomes isn’t clearly known. It is believed that they play a significant role in the binding of mRNA to ribosomes and protein synthesis.