Not all primary transcripts are processed into mRNA. Some RNA transcripts have functions of their own, and many of these transcripts are produced by RNA polymerases other than Pol II. These primary transcripts undergo different types of RNA processing, and their processed forms include such important noncoding RNA types as:
Ribosomal RNA (rRNA), found in all ribosomes that aid in translation. In eukaryotic cells, the genes and transcripts for ribosomal RNA are concentrated in the nucleolus, a distinct, dense, non–
Transfer RNA (tRNA) that carries individual amino acids for use in translation.
Small nuclear RNA (snRNA), found in eukaryotes and involved in splicing, polyadenylation, and other processes in the nucleus.
Small, regulatory RNA molecules that can inhibit translation or cause destruction of an RNA transcript. Two major types of small regulatory RNA are known as microRNA (miRNA) and small interfering RNA (siRNA).
66
By far, the most abundant transcripts in mammalian cells are those for ribosomal RNA and transfer RNA. In a typical mammalian cell, about 80% of all of the RNA consists of ribosomal RNA, and another approximately 10% consists of transfer RNA. Why are these types of RNA so abundant? The answer is that they are needed in large amounts to synthesize the proteins encoded in the messenger RNA. The roles of mRNA, tRNA, and rRNA in protein synthesis are discussed in the next chapter.