List of 11 Other Types of RNA
Many people know of the three most commonly studied types of RNA (ribonucleic acid) found in all organisms: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). Messenger RNA is transcribed from DNA, the genetic code of life, and is translated into protein outside of the nucleus. Transfer RNA is involved in translation, where the tRNA carry amino acids for protein elongation. Ribosomal RNA codes for ribosomes. Ribosome protein subunits are created in the nucleolus; fully assembled ribosomes are found in the cytoplasm and are key regulators of translation. However, there are many more types of RNA that are found within different types of organisms. Listed below are eleven "other" types of RNA (along with their basic descriptions) that may not have been introduced in basic biology courses:
- snRNA (small nuclear RNA): It is involved with intron splicing and is found in eukaryotes and archaebacteria. They also regulate transcription factors (proteins that help control DNA transcription) and RNA polymerase II (the polymerase associated with binding to DNA for transcription), and help regulate the length of telomeres. snRNA perform these tasks by forming complexes with certain proteins.
- snoRNA (small nucleolar RNA): It helps in the modification of RNA (a majority being rRNA, tRNA, and snRNA) and is found in eukaryotes and archaebacteria. A majority of these snoRNA can be divided into two groups: those involved in methylation (adding methane groups) or those involved in pseudouridylation (it is an isomer of uridine, where uridine is a nucleoside that is composed of a complex of uracil and a ribose sugar).
- miRNA (micro RNA): It regulates gene expression and is found in all eukaryotes except for marine plants, algae, and fungi. They regulate gene expression by base-pairing with certain mRNA. By doing so, they can control the mRNA’s stability and their efficiency of translation.
- siRNA (short interfering RNA): It regulates gene expression. They are foreign double-stranded RNA that are cut into small pieces, and are then incorporated in RISC (RNA-induced silencing complex). The RISC then uses one strand of siRNA to find other complementary (and perhaps foreign) RNA to destroy, thereby protecting the cell from potentially dangerous RNA.
- piRNA (piwi-interacting RNA): It regulates the expression of transposons and is found in most animals. They silence transposons, retrotransposons, and some other genetic elements in the germ cells by keeping the genes from being transcribed. Most piRNA are the antisense of different transposon sequences, which means that they may target these transposons in particular.
- SRP RNA (signal recognition particle RNA): It helps in transporting proteins through the lipid bilayer membrane of a cell and is found in all cells. However, they only can control the movement of proteins within the cell, as they are not secreted with the proteins that are transported out of the cell. It also helps sort proteins after translation.
- tmRNA (transfer-messenger RNA): It forms complexes with proteins that bind to the bacteria’s stalled ribosomes (ribosomes that have for some reason stopped translation, which includes cases where an mRNA accidentally does not have a stop codon). The stalled ribosome is then “saved” and reused again for other mRNA and the tmRNA marks the bad mRNA and unfinished polypeptide for degradation.
- Telomerase RNA: It is found in the telomerase of eukaryotes. Recall that for telomerase to extend the telomeres at the ends of chromosomes, the telomerase uses an RNA template that is integrated in itself to attach complementary DNA bases to the chromosome’s ends. The RNA template of telomerase is telomerase RNA.
- lncRNA (long noncoding RNA): They are strands of RNA that are more than 200 nucleotides long that do not code for proteins. There are more lncRNA than there are mRNAs that code for proteins in mammals. However, the exact purpose of having lncRNA is unknown, much like how the purpose of having introns is still ambiguous.
- crRNA (CRISPR RNA): It allows bacterial or archaebacterial cells to fend off parasites. Similar to RNAi in eukaryotes, CRISPR is repeated throughout the bacteria’s (or archaebacteria) genome and short harmless pieces of the potentially dangerous foreign DNA is incorporated between these CRISPR sequences, which allows the bacteria recognize this type of foreign DNA within the cell (by sense-antisense pairing) and destroy it.
- Guide RNA: They exclusive to kinetoplastid protists and help in “editing” the RNA of these organisms. The RNA that is “edited” is the mRNA in the mitochondria of these organisms, where they “guide” the insertion and deletion of uridine residues (see snoRNA for “uridine” definition).
Of course, there are many other types of RNA out there that still may not yet have been discovered. As biologists continue researching the contents within a cell, their understanding of the importance of RNA increases. With numerous roles, such as protection of the cell from foreign genetic material and regulation of gene expression, RNA is no longer considered just a "product of DNA." It is in fact, much more.