Classes of RNA
Ribonucleic acids (RNA) are categorized into two separate classes. The first class of RNA, denoted as messenger RNA (mRNA) encodes the information necessary to make proteins (polypeptide chains). Messenger RNA serves as an intermediate for information passed between DNA and protein. The steps in which a gene influences phenotype are known as gene expression. For most genes, the RNA transcript of the specific gene is only necessary for the synthesis of the protein product, which is the ultimate functional product that impacts the phenotype.
The second class is functional RNA, which does not encode information to synthesize proteins because the functional RNA itself is the final functional product. Functional RNA has various roles in the cell such as transferring information from DNA to protein, processing of other RNA’s, and regulate RNA and protein levels in the cell. Transfer RNA (tRNA) and ribosomal RNA (rRNA) are found both in prokaryotes and eukaryotes. Transfer RNA molecules transport the correct amino acid to mRNA during translation. Ribosomal RNA constitutes the majority of the ribosome, a large macromolecule that monitors the assembly of the amino acid chain by mRNA and tRNA. Despite that all tRNA and rRNA are encoded by a small amount of genes (a few tens to a few hundreds at most), rRNA’s account for a vast percentage of the RNA in a cell due to their stability and constant transcription into multiple copies.
The following functional RNA is only found in the processing of RNA in eukaryotes. Small nuclear RNA (snRNA) is part of a system that processes RNA transcripts even further. Some will combine with multiple protein subunits in order to form the ribonucleoprotein processing complex known as the spliceosome. The spliceosome removes introns from eukaryotic mRNA.
Another group of functional RNA (specific to eukaryotes) suppresses the expression of genes and maintains genome stability. MicroRNA (miRNA) was recently discovered by scientists to have an influential role in regulating the amount of protein produced in many eukaryotic genes. Small interfering RNA (siRNA) and piwi-interacting RNA (piRNA) help protect the structure/integrity of plant and animal genomes respectively, by physically restraining the transposable elements from spreading to chromosomal loci. Small interfering RNA also inhibits virus productivity.
Long noncoding RNA (ncRNA) was recently discovered to be transcribed from the majority of regions of genomes of humans, animals, and plants. Though a few take part in dosage compensation, the full function is still currently unknown.
Protein synthesis and mRNA processing occurs throughout the entire lifetime of a cell, necessitating the presence of tRNA, rRNA, and snRNA. Their transcription is categorized as constitutive because they are continuously transcribed. In contrast, miRNA, siRNA, piRNA, and ncRNA are transcribed and/or processed from more complex transcripts intermittently; only when they are necessary to fulfill roles in protecting the genome and regulating gene expression.