or RNA is one of the three major biological macromolecules that are essential for all known forms of life (along with DNA and proteins). A central tenet of molecular biology states that the flow of genetic information in a cell is from DNA through RNA to proteins: “DNA makes RNA makes protein”. Proteins are the workhorses of the cell; they play leading roles in the cell as enzymes, as structural components, and in cell signalling, to name just a few. DNA(deoxyribonucleic acid) is considered the “blueprint” of the cell; it carries all of the genetic information required for the cell to grow, to take in nutrients, and to propagate. RNA–in this role–is the “DNA photocopy” of the cell. When the cell needs to produce a certain protein, it activates the protein’s gene–the portion of DNA that codes for that protein–and produces multiple copies of that piece of DNA in the form of messenger RNA, or mRNA. The multiple copies of mRNA are then used to translate the genetic code into protein through the action of the cell’s protein manufacturing machinery, the ribosomes. Thus, RNA expands the quantity of a given protein that can be made at one time from one given gene, and it provides an important control point for regulating when and how much protein gets made.
For many years RNA was believed to have only three major roles in the cell–as a DNA photocopy (mRNA), as a coupler between the genetic code and the protein building blocks (tRNA), and as a structural component of ribosomes (rRNA). In recent years, however, we have begun to realize that the roles adopted by RNA are much broader and much more interesting. We now know that RNA can also act as enzymes (called ribozymes) to speed chemical reactions. In a number of clinically important viruses RNA, rather than DNA, carries the viral genetic information. RNA also plays an important role in regulating cellular processes–from cell division, differentiation and growth to cell ageing and death. Defects in certain RNAs or the regulation of RNAs have been implicated in a number of important human diseases, including heart disease, some cancers, stroke and many others.
Types of RNA
There are 4 types of RNA, each encoded by its own type of gene:
- mRNA – Messenger RNA: Encodes the amino acid sequence of a polypeptide.
- tRNA – Transfer RNA: Brings amino acids to ribosomes during translation.
- rRNA – Ribosomal RNA: With ribosomal proteins, makes up the ribosomes, the organelles that translate the mRNA.
- snRNA – Small nuclear RNA: With proteins, forms complexes that are used in RNA processing in eukaryotes. (Not found in prokaryotes.)