Types and Functions of RNA
The three main, most well-known and most commonly studied are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), which are present in almost all organisms. These and other types of RNAs primarily carry out biochemical reactions, similar to enzymes. Some, however, also have complex regulatory functions in cells. Owing to their involvement in many regulatory processes, to their abundance, and to their diverse functions, RNAs play important roles in both normal cellular processes and diseases.
In addition to mRNA, tRNA, and rRNA, RNAs can be broadly divided into coding (cRNA) and noncoding RNA (ncRNA). There are two types of ncRNAs, housekeeping ncRNAs (tRNA and rRNA) and regulatory ncRNAs, which are further classified according to their size. Long ncRNAs (lncRNA) have at least 200 nucleotides, while small ncRNAs have less than 200 nucleotides. Small ncRNAs are subdivided into micro RNA (miRNA), small nucleolar RNA (snoRNA), small nuclear RNA (snRNA), small-interfering RNA (siRNA), and PIWI-interacting RNA (piRNA).
PIWI-interacting RNA (piRNA)
These are of functional significance which are about 26 to 31 nucleotides long and exist in most animals. They regulate the expression of transposons (jumping genes) by keeping the genes from being transcribed in the germ cells (sperm and eggs). Most piRNA are complementary to different transposons and can specifically target those transposons.
Circular RNA (circRNA)
It is unique from other RNA types because its 5′ and 3′ ends are bonded together, creating a loop. The circRNAs are produced from many protein-encoding genes, and some can serve as templates for protein synthesis, similar to mRNA.
Small Nuclear RNA (snRNA)
Small nuclear RNAs (snRNA) are non-coding RNAs that are responsible for splicing introns. The snRNAs join with proteins to form small nuclear ribonucleoproteins (snRNP), which most commonly contain U1, U2, U4, U5, and U6 snRNA molecules. Spliceosome assembly and activity begins once U1 of the snRNP binds a complementary sequence on the 5’ splice site of a pre-mRNA transcript. Introns are then removed from the pre-mRNA transcript by the spliceosome complex and mature mRNA forms.
These are non-coding RNAs mainly involved in gene regulation. They are mostly processed from introns and are transcribed into primary miRNA from the host gene by RNA polymerase II. They are then modified by endonucleases, such as Drosha and Dicer into a mature miRNA. Studies have shown that miRNAs that bind to an untranslated region (3’UTR) on mRNAs suppress translation, while miRNA binding to promoter regions can upregulate transcription. miRNAs can also function similarly to hormones.
Small Interfering RNAs (siRNA)
Small Interfering RNAs (siRNA) are double-stranded, non-coding RNAs that stop gene expression through RNA interference. They interfere with gene expression by degrading mRNA and preventing the translation of proteins. siRNAs form from long double-stranded RNAs with the assistance of Dicer. Once fully formed, siRNA binds to RNA induced silencing complex (RISC) and cleaves mRNA through a catalytic RISC protein, Argonaute. Small interfering RNAs have the potential to be therapeutic agents for diseases due to their potency and ability to knock down genes.
Small Nuclear RNA (snRNA)
snRNA is involved in the processing of pre-messenger RNA (pre-mRNA) into mature mRNA. They are very short, with an average length of only 150 nucleotides.
Transfer-messenger RNA (tmRNA)
It is found in many bacteria and plastids. tmRNA tag the proteins encoded by mRNAs that lack stop codons for degradation, and prevents the ribosome from stalling due to the missing stop codon.
Ribozymes (RNA enzymes)
RNAs are now known to adopt complex tertiary structures and serve as biological catalysts. Such RNA enzymes are known as ribozymes, and they stop many of the features of a classical enzyme, such as an active site, a binding site for a substrate and a binding site for a cofactor, such as a metal ion.
Double-stranded RNA (dsRNA)
This type of RNA has two strands bound together, as with double stranded DNA. dsRNA forms the genetic material of some viruses.