RNA is a single stranded polynucleotide and simpler than DNA. It is proved that viruses evolved prior to complex organisms and most of viruses contain RNA as genetic material.
The RNA molecules synthesized in mammalian cells are often very different from those made in prokaryotic organisms, particularly the m RNA encoding transcripts. Prokaryotic m RNA can be translated as it is being synthesized, whereas, in mammalian cells, most RNAs are made as precursor molecules that have to be processed into mature, active RNA. Erroneous processing and splicing of m RNA transcripts are a cause of disease.
The process of synthesizing RNA from a DNA template has been characterized best in prokaryotes. Although in mammalian cells the regulation of RNA synthesis and the processing of the RNA transcripts are different from that in prokaryotes, the process of RNA synthesis per se is quite similar in these two classes of organisms. Therefore, the description of RNA synthesis in prokaryotes is applicable to eukaryotes even though the enzymes involved and the regulatory signals are different.
Bacterial DNA dependent RNA polymerase is multisubunit enzymes which in mammals are of various types and these polymerases are much more complex than prokaryotic RNA polymerases. They all have two large subunits and a number of smaller subunits (as a many as 14 in the case of RNA pol II).
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The eukaryotic RNA polymerases have extensive amino acid homologies with prokaryotic RNA polymerases. Bacterial promoters are relatively simple and eukaryotic promoters are more complex.
In prokaryotic organisms, the primary transcripts of m RNA-encoding genes begin to serve as translation templates even before their transcription has been completed. This is presumably because the site of transcription is not compartmentalized into a nucleus as it is in eukaryotic organisms.
Thus, transcription and translation are coupled in prokaryotic cells. Consequently, prokaryotic m RNAs are subjected to little processing prior to carrying out their intended function in protein synthesis. Prokaryotic /RNA and RNA molecules are transcribed in units considerably longer than the ultimate molecule.
In fact, many of the t RNA transcription units contain more than one molecule. Thus, in prokaryotes the processing of these r RNA and t RNA precursor molecules in required for the generation of mature functional molecules.
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Nearby all eukaryotic RNA primary transcripts undergo extensive processing between the time they are synthesized and time at which they serve their ultimate function, whether it is m RNA or r RNA. The process includes nucleolytic cleavage to smaller molecules and coupled nucleolytic and ligation reactions (splicing of exons).
In addition to the catalytic action served by the sn RNA in the formation of m RNA, several other enzymatic functions have been attributed to RNA. Ribosome’s are RNA molecules with catalytic activity.
They are generally involved transesterification reactions and most are concerned with RNA metabolism (splicing and endoribonuclease). Recently, a ribosomal RNA component was noted to hydrolyze an aminoacyl ester and, thus, to play a central role in peptide bond function.
Their observations made in organelles from plants, yeasts, viruses and higher eukaryotic cells, show that RNA can act as an enzyme. This has revolutionized thinking about enzyme action and the origin of life itself.