Types and Location of Various RNAs

There are three major classes of RNA in cells messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Of these, the latter two are termed stable RNAs, as they have a longer half-life than that of mRNA [1], Ribosomal RNA is the most abundant class of RNA in a cell. In a typical eukaryotic cell (yeast, plant, and animal), there are other RNAs, such as organelle RNA and small RNAs in nuclei (snRNAs) or in the cytoplasm (7S RNA). In eukaryotic cells, most RNAs are synthesized as larger precursor molecules and are then processed into smaller mature RNAs. Total RNA in a human cell may range from 10 to 30 pg, with most of it in the cytoplasm (about 85%), while the rest is in the nucleus. 

Splicing machinery removes introns and joins exons. The spliced transcript (mRNA) is modified post-transcriptionally at 3 end by addition of poly A+ tail and at the 5 end by the addition of rn7GpppNm(cap) 

Abundance Number of Different RNA Species/Cell Copies/Cell 

Ribosomal RNAs (rRNAs) play an active structural role in ribosomes that are essential components of the cellular protein synthesis machinery. rRNAs are also believed to participate in tRNA binding, ribosomal subunit association, and antibiotic interactions. In a typical eukaryotic cell, there are four types of rRNA (28S, 18S, 5.8S, and 5S) that vary in size and sequence, 

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