Structure of Ribonucleic Acid (RNA)
The RNA is usually single stranded except some viruses such as TMV, yellow mosaic virus, influenza virus, foot and mouth disease virus, reovirus, wound tumour virus, etc. which have dsDNA. The single strand of the RNA is folded either at certain regions or entirely to form hairpin shaped structure. In the hairpin shaped structures the complementary bases are linked by hydrogen bonds which give stability to the molecules. However, no complementary bases are found in the unfolded region. The RNA does not possess equal purine-pyrimidine ratio, as it is found in the DNA.
Like DNA, the RNA is also the polymer of four nucleotides—each one contains D-ribose, phosphoric acid and a nitrogenous base. The bases are two purines (A,G) and two pyrimidines (C, U). Thyamine is not found in RNA. Pairing between bases occurs as A-U and G-C. The nucleotides formed by the four bases are adenosine monophosphate. (AMP), guanosine monophosphate (GMP), cytosine monophosphate (CMP), and uridine monophosphate (UMP). These are found freely in nucleoplasm but in the form of triphosphates e.g.
ATP, GTP, UTP and CTP. As a result of polymerization the ribonucleotides form a polynucleotide chain of RNA.
If the RNA is involved in genetic mechanism, it is called genetic RNA
as found in r)lant, animal and bacterial viruses. The DNA acts as genetic material and RNA follows the order of DNA. In such cells the RNA does not have genetic role. Therefore, it is called non-genetic RNA.
The non-genetic RNA is of three types : (i)
ribosomal RNA (rRNA), (ii)
transfer RNA (tRNA) or soluble RNA (sRNA), and (iii)
messenger RNA (mRNA) or template RNA. These three types of RNA differ from each other in structure, site of synthesis in eukaryotic cell and function.
The mRNA and tRNA are synthesized on DNA template, whereas rRNA is delivered from nucleolar DNA. These RNAs are synthesized during different stages. During cleavage most of mRNA is synthesized , whereas tRNA is synthesized at the end of cleavage. Synthesis of rRNA occurs during gastrulation. The total population of rRNA is about 90% of all RNAs.
Like DNA, the RNA is not self-replicating but it has to depend on DNA. Therefore, replication of non-genetic RNA is known as DNA dependent RNA replication. Moreover, the genetic RNA of viruses is self-replicating i.e.
it can form its own several replica copies. Differences between the DNA and RNA molecules are given in Table 2.3.
Table 2.3. Differences between RNA and DNA.
|RNA is more primitive than DNA
||DNA originated after RNA
|RNA is the genetic material of some plant, animal and bacterial viruses.
||DNA is the genetic material of almost all living organisms
|Except some viruses (e.g. reovirus) most cellular RNA is single stranded.
||Except a few viruses (e.g. ΦX174), most DNA is double stranded.
|Pentose sugar is ribose.
||Pentose sugar is deoxyribose.
|The bases are adenine, guanine, cytosine and uracil.
||The bases are adenine, guanine, cytosine and thymine.
|Base pairing occurs between adenine and uracil (A-U), and guanine and cytosine (G-C).
||Base pairs are A-T and G-C.
|Base pairing is seen only in hairpin structure and helical region.
||Base pairing occurs throughout the length of DNA molecule.
|RNA contains a few (about 12,000) nucleotides
||DNA contains millions of nucleotides e.g. over 4 millions.
|The RNA molecules are of three types: rRNA, mRNA, tRNA.
||DNA is only of one type
|The mRNA is found in nucleolus, tRNA and rRNA are found in cytoplasm. They are formed on the DNA.
||DNA is found in chromosomes.
DNA is also found in mitochondria and chloroplasts.
|RNAs translate the transcripts of DNA into proteins.
||DNA encodes the genetic masses in a forms the transcripts.
|Genetic RNA uses the enzyme reverse transcriptase during replication.
||This enzyme is not required by DNA. DNA after replication forms DNA and after transcription forms RNA.