In addition to rRNA and tRNA genes, there are thousands of additional genes that are transcribed into nonprotein-coding RNAs, some with various known functions and many whose functions are not yet known. For example, small nuclear RNAs (snRNAs) function in RNA splicing, and small nucleolar RNAs (snoRNAs) function in rRNA processing and base modification in the nucleolus. The RNase P RNA functions in tRNA processing, and a large family (~2000 in humans) of short micro-RNAs (miRNAs) regulates the translation and stability of specific mRNAs. The functions of these nonprotein-coding RNAs are discussed in Chapter 10. An RNA found in telomerase (see Figure 8-44) functions in maintaining the sequence at the ends of chromosomes, and the 7SL RNA functions in the transport of secreted proteins and most membrane proteins into the endoplasmic reticulum (see Chapter 13). These and other nonprotein-coding RNAs encoded in the human genome, and their functions when known, are listed in Table 8-2. Recent advances in DNA sequencing have led to the discovery of about 10,000 long noncoding RNAs (lncRNAs) in nuclei of mammalian cells. Some of these have been found to function in regulating the expression of specific protein-coding genes. Pursuing the functions of lncRNAs is currently a highly active area of research.
