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Ribonucleic Acid RNA

Ribonucleic Acid RNA - Like DNA, RNA also consists of chains of ribonucleotides (Base +Ribose+ Phosphate), held together by diphosphoester linkaged except for RNA from some bacteriophages (MS2,Qβ) and fungal plasmids, it appears that most other RNAs are single stranded.

Earlier it was believed that RNA unlike DNA, is found exclusively in the cytoplasm and had no genetic role but only an intermediary role: This appears to be now incorrect since many viruses, bacteriophages and plasmids (fungal) contain RNA as their genetic material.
In microorganisms. at least three types of RNAs are recogonized. These are the messenger RNA (m-RNA), transfer RNA (t-RNA or earlier called soluble RNA), and ribosomal RNA (r-RNA) found in the ribosomes (structures involved in protein synthesis), All three types of  RNAs are copied from DNA in a process called transcription.

These molecules therefore have a base sequence complementary to that of DNA from which it is copied. Thus RNA can form a stable duplex structure or a hybrid structure with complementary single stranded DNA.

m-RNA or messenger RNA as it is called, carries the information for the sequence of amino acids in peptides (see, genetic code)while t-RNA and r-RNA are involved in protein synthesis. All these species of  RNA have different molecular weights  although basically they are chains of ribonucleoside phosphates. m-RNA is characterized by its rapid turnover (degradation). For example, it is esti­mate that m-RNA for E. coli  β-galactosidase has a half life of about three minutes.

The size and molecular weight of the m-RNA varies depending on the amount of information these molecules carry. Sometimes these molecules may carry information for the synthesis of more than one polypeptide (polycistronic). The amount of m-RNA in a cell at any given time is small (about 4-5 per cent of total RNA ).

The r-RNA is found in association with ribosomes which are involved in protein synthesis. Bacterial ribosomes contain three species of r-RNA. (23 S, 16 Sand 5 S). There is evidence to suggest that all these three species of RNA are derived from a larger pro­duct of transcription which is subsequently cleaved before being incorporated into ribosome (r-RNA processing). Like mRNA, the r-RNA can also hybridize with single stranded DNA and it has been found that E.coli about 1.2 per cent of DNA codes for r-RNA The third type of RNA found in cells is t-RNA. This species is involved in protein synthesis.

Holley and his co-workers in 1965 were the first to publish the sequence of nucleotides in yeast alanyl-t-RNA. The primary and secondary structures of several t-RNA is now known. There is great degree of similarity in the primary and secondary structures of t-RNA and the generally accepted structure (clover leaf structure) . Unlike in m-RNA, the number of bases in t-RNA molecules varies only between 75-85. These t-RNAs are copied from the DNA and about 20 species of t-RNA exist in a cell. Each t-RNA molecule has a 3' end with CCA. The 3' OH of the terminal adenine is the acceptor for activated amino acids. Although they are single polynucleotide chains, they show a considerable degree of base pairing between short sequences.

 
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